The DROP statement in SQL is used to remove or delete an existing database object such as a table, index, view, or procedure. Whenever we use DROP statement with any of the database objects, it will remove them permanently along with their associated data.

And when that database object is an index, the DROP INDEX statement in SQL is used.

Dropping an SQL Index

An SQL Index can be dropped from a database table using the DROP INDEX statement.

It is important to understand that dropping an index can have a significant impact on the performance of your database queries. Therefore, only try to remove an index if you are sure that it is no longer required.

Note − We cannot delete the indexes created by PRIMARY KEY or UNIQUE constraints. In order to delete them, you need to drop the constraints entirely using ALTER TABLE statement.

Syntax

Following is the syntax of the DROP INDEX command in SQL −

DROP INDEX index_name ON table_name;

Here,

• index_name is the name of the index that you want to drop.
• table_name is the name of the table that the index is associated with.

Example

In this example, we will learn how to drop an index on a table named CUSTOMERS, which can be created using the following query −

CREATE TABLE CUSTOMERS(
   ID INT NOT NULL,
   NAME VARCHAR(15) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS VARCHAR(25),
   SALARY DECIMAL(10, 4),
   PRIMARY KEY(ID));
);

Now, insert some values into the above created table using the following query −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', ''32'', ''Ahmedabad'', 2000),
(2, ''Khilan'', ''25'', ''Delhi'', 1500),
(3, ''Kaushik'', ''23'', ''Kota'', 2000),
(4, ''Chaitali'', ''25'', ''Mumbai'', 6500),
(5, ''Hardik'',''27'', ''Bhopal'', 8500),
(6, ''Komal'', ''22'', ''Hyderabad'', 9000),
(7, ''Muffy'', ''24'', ''Indore'', 5500);

Once the table is created, create an index on the column NAME in the CUSTOMERS table using the following query −

CREATE INDEX INDEX_NAME on CUSTOMERS(NAME);

Now, verify if the index is created on the CUSTOMERS table using the following SHOW INDEX query −

SHOW INDEX FROM CUSTOMERS;

On executing the above query, the index list is displayed as follows −

Then, drop the same index INDEX_NAME in the CUSTOMERS table using the following DROP INDEX statement −

DROP INDEX INDEX_NAME ON CUSTOMERS;

Output

If we compile and run the above query, the result is produced as follows −

Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

Verify if the index for the column NAME is dropped using the following query −

SHOW INDEX FROM CUSTOMERS;

In the following list of indexes, you can observe that name of the column Name is missing.

DROP INDEX with IF EXISTS

The DROP INDEX IF EXISTS statement in SQL is used to drop an index only if it exists in the table. This statement is specifically useful when you want to drop an index, but you are not sure if the index exists. This clause is not supported by MySQL.

The IF EXISTS clause ensures that the statement only removes the index if it exists. If the index does not exist, it simply terminates the execution.

Syntax

Following is the syntax of the DROP INDEX IF EXISTS in SQL −

DROP INDEX IF EXISTS index_name
ON table_name;

Here,

• index_name is the name of the index that you want to drop.
• table_name is the name of the table that the index is associated with.

Example

In this example, let us try to drop an index in the SQL Server database.

Let us consider the previously created table CUSTOMERS and let us create an index for the NAME column in the table using the following query −

CREATE INDEX INDEX_NAME on CUSTOMERS(NAME);

Then, let us drop it using the following query −

DROP INDEX IF EXISTS INDEX_NAME ON CUSTOMERS;

Output

When we execute the above query, the output is obtained as follows −

Commands completed successfully.

Verification

Let”s verify whether the index for the NAME is dropped or not using the following query −

EXEC sys.sp_helpindex @objname = N''CUSTOMERS

As you observe, the column NAME is deleted from the list of indexes.

Example

Now, let us delete an index that doesn”t exist in the CUSTOMERS table using the following query −

DROP INDEX IF EXISTS INDEX_NAME ON CUSTOMERS;

Output

Since no indexes with the specified name exist in the database, so the above query simply terminates the execution without giving any error.

Commands completed successfully.

Removing indexes created by PRIMARY KEY or UNIQUE

The DROP INDEX statement does not drop indexes created by PRIMARY KEY or UNIQUE constraints. To drop indexes associated with them, we need to drop these constraints entirely. And it is done using the ALTER TABLE… DROP CONSTRAINT statement.

Syntax

Following is the syntax of the ALTER TABLE… DROP CONSTRAINT statement in SQL −

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

Here,

• table_name is the name of the table that contains the PRIMARY KEY constraint.
• constraint_name is the name of the PRIMARY KEY constraint that you want to drop.

Example

Assume the previously created table (CUSTOMERS) and let us first list all the indexes that are created on the table using the following query −

EXEC sys.sp_helpindex @objname = N''CUSTOMERS

The list is displayed as follows −

Here, the PK__CUSTOMER__3214EC27CB063BB7 is the name of the PRIMARY KEY constraint that was created on the ID column of the CUSTOMERS table.

Now, let us drop the index created by the PRIMARY KEY constraint.

ALTER TABLE customers
DROP CONSTRAINT PK__CUSTOMER__3214EC27CB063BB7;

Output

When we execute the above query, the output is obtained as follows −

Commands completed successfully.

Verification

Verify whether it is dropped or not by listing the existing indexes using the following query −

EXEC sys.sp_helpindex @objname = N''CUSTOMERS

The following error is displayed because the list of indexes is empty.

The object ''CUSTOMERS'' does not have any indexes, or you do not have permissions.

An index is an effective way to quickly retrieve data from an SQL database. It is a database object that references the data stored in a table, which significantly improves query and application performance of a database.

The process of indexing in SQL is similar to that of an index in a book: it is a database object in the form of a table, contains details of data location, and holds a separate storage space.

Even though indexes help accelerate search queries, users are not able to directly see these indexes in action.

What is SQL Index?

An SQL index is a special lookup table that helps in efficiently searching or querying database tables to retrieve required data. For example, when we try to retrieve data from multiple tables using joins, indexes improve the query performance.

Indexes are used to optimize the query performance of any Relational Database Management System (RDBMS) as data volumes grow. Hence, they are preferred to be used on frequently queried large database tables.

Creating an SQL Index

An index can be created on one or more columns of a table in an SQL database using the CREATE INDEX statement.

Syntax

Following is the syntax of CREATE INDEX statement in SQL −

CREATE INDEX index_name
ON table_name (column_name1, column_name2,... column_nameN);

Here,

• index_name This specifies the name of the index that you want to create.
• table_name This specifies the name of the table on which you want to create the index.
• (column_name1, column_name2…column_nameN) are the names of one or more columns on which the index is being created.

Example

To create an index on a database table, we first need to create a table. So, in this example, we are creating a table named CUSTOMERS using the following query −

CREATE TABLE CUSTOMERS(
   ID INT NOT NULL,
   NAME VARCHAR(15) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS VARCHAR(25),
   SALARY DECIMAL(10, 4),
   PRIMARY KEY(ID));
);

Then, insert some values into the CUSTOMERS table using the following query −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', ''32'', ''Ahmedabad'', 2000),
(2, ''Khilan'', ''25'', ''Delhi'', 1500),
(3, ''Kaushik'', ''23'', ''Kota'', 2000),
(4, ''Chaitali'', ''25'', ''Mumbai'', 6500),
(5, ''Hardik'',''27'', ''Bhopal'', 8500),
(6, ''Komal'', ''22'', ''Hyderabad'', 9000),
(7, ''Muffy'', ''24'', ''Indore'', 5500);

Once the table is created, create an index for the column named NAME in the CUSTOMERS table using the following query −

CREATE INDEX index_name ON CUSTOMERS(NAME);

Output

When we execute the above query, the output is obtained as follows −

Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

The following SHOW INDEX query is used to display all the indexes created on an existing table.

SHOW INDEX FROM CUSTOMERS;

In the list obtained, you can find the column name NAME, along with the ID in the list of indexes.

Creating an Index on Multiple Fields

We can also create an index on multiple fields (or columns) of a table using the CREATE INDEX statement. To do so, you just need to pass the name of the columns (you need to create the index on).

Example

Instead of creating a new table, let us consider the previously created CUSTOMERS table. Here, we are creating an index on the columns NAME and AGE using the following query −

CREATE INDEX mult_index_data on CUSTOMERS(NAME, AGE);

Output

When we execute the above query, the output is obtained as follows −

Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

Now, let us list all the indexes that are created on the CUSTOMERS table using the following SHOW INDEX query −

SHOW INDEX FROM CUSTOMERS;

As you observe, you can find the column names NAME, and AGE along with ID (PRIMARY KEY), in the list of indexes.

The SQL Indexes

SQL Indexes are special lookup tables that are used to speed up the process of data retrieval. They hold pointers that refer to the data stored in a database, which makes it easier to locate the required data records in a database table.

SQL Indexes work similar to the index of a book or a journal.

While an index speeds up the performance of data retrieval queries (SELECT statement), it slows down the performance of data input queries (UPDATE and INSERT statements). However, these indexes do not have any effect on the data.

SQL Indexes need their own storage space within the database. Despite that, the users cannot view them physically as they are just performance tools.

The CREATE INDEX Statement

An index in SQL can be created using the CREATE INDEX statement. This statement allows you to name the index, to specify the table and which column or columns to index, and to indicate whether the index is in an ascending or descending order.

Preferably, an index must be created on column(s) of a large table that are frequently queried for data retrieval.

Syntax

The basic syntax of a CREATE INDEX is as follows −

CREATE INDEX index_name ON table_name;

Types of Indexes

There are various types of indexes that can be created using the CREATE INDEX statement. They are:

• Unique Index

• Single-Column Index

• Composite Index

• Implicit Index

Unique Indexes

Unique indexes are used not only for performance, but also for data integrity. A unique index does not allow any duplicate values to be inserted into the table. It is automatically created by PRIMARY and UNIQUE constraints when they are applied on a database table, in order to prevent the user from inserting duplicate values into the indexed table column(s). The basic syntax is as follows.

CREATE UNIQUE INDEX index_name
on table_name (column_name);

Single-Column Indexes

A single-column index is created only on one table column. The syntax is as follows.

CREATE INDEX index_name
ON table_name (column_name);

Composite Indexes

A composite index is an index that can be created on two or more columns of a table. Its basic syntax is as follows.

CREATE INDEX index_name
on table_name (column1, column2);

Implicit Indexes

Implicit indexes are indexes that are automatically created by the database server when an object is created. For example, indexes are automatically created when primary key and unique constraints are created on a table in MySQL database.

The DROP INDEX Statement

An index can be dropped using SQL DROP command. Dropping an index can effect the query performance in a database. Thus, an index needs to be dropped only when it is absolutely necessary.

The basic syntax is as follows −

DROP INDEX index_name;

When should indexes be avoided?

Although indexes are intended to enhance a database”s performance, there are times when they should be avoided.

The following guidelines indicate when the use of an index should be reconsidered.

• Indexes should not be used on small tables.

• They should not be used on tables that have frequent, large batch updates or insert operations.

• Indexes should not be used on columns that contain a high number of NULL values.

• Columns that are frequently manipulated should not be indexed.

The SQL Alternate Key

SQL Alternate Keys in a database table are candidate keys that are not currently selected as a primary key. They can be used to uniquely identify a tuple(or a record) in a table.

There is no specific query or syntax to set the alternate key in a table. It is just a column that is a close second candidate which could be selected as a primary key. Hence, they are also called secondary candidate keys.

If a database table consists of only one candidate key, that is treated as the primary key of the table, then there is no alternate key in that table.

Let us understand the concept of alternate key with an example. Suppose we have a table named CUSTOMERS with various fields like ID, NAME, AGE, AADHAAR_ID, MOBILE_NO and SALARY as shown below.

The details like id, mobile number and aadhaar number of a customer are unique, and we can identify the records from the CUSTOMERS table uniquely using their respective fields; ID, AADHAAR_ID and MOBILE_NO. Therefore, these three fields can be treated as candidate keys.

And among them, if one is declared as the primary key of the CUSTOMERS table then the remaining two would be alternate keys.

Features of Alternate Keys

Following are some important properties/features of alternate keys −

• The alternate key does not allow duplicate values.
• A table can have more than one alternate keys.
• The alternate key can contain NULL values unless the NOT NULL constraint is set explicitly.
• All alternate keys can be candidate keys, but all candidate keys can not be alternate keys.
• The primary key, which is also a candidate key, can not be considered as an alternate key.

Example

For a better understanding, let us create the above discussed table demonstrating the usage of the various keys and illustrating the fields that can be considered as alternate keys.

CREATE TABLE CUSTOMERS(
   ID INT,
   NAME VARCHAR (20),
   AGE INT,
   AADHAAR_ID BIGINT,
   MOBILE_NO BIGINT,
   SALARY DECIMAL (18, 2),
   PRIMARY KEY(ID)
);

Now, insert some records into the CUSTOMERS table using the INSERT statement as shown below −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 32, 90123498456, 9023456789, 22000.00 ),
(2, ''Khilan'', 25, 91123249545, 9032456578, 24500.34 ),
(3, ''Kaushik'', 23, 91223242546, 9012436789, 20000.12 );

The table will be created as −

Keys in a table

As a summary lets revisit all the keys in a database table −

Candidate Key

A Candidate key is a subset of super keys that is used to uniquely identify records of a table. It can be a single field or multiple fields. The primary keys, alternate keys, foreign keys in a table are all types of candidate key.

A Primary Key is a main key that is used to retrieve records from a table. It is a single column or field in a table that uniquely identifies each record in a database table.

It can be set using the PRIMARY KEY keyword while creating a table using the CREATE TABLE statement. Following is the basic syntax to create primary key constraint on a column in a table −

CREATE TABLE table_name(
   column1 datatype,
   column2 datatype,
   column3 datatype,
   .....
   columnN datatype,
   PRIMARY KEY(column_name)
);

The Primary key of one table will be the Foreign key in another table. While inserting values into these tables, values in the primary key field must match the values in the foreign key field; otherwise, the foreign key column will not accept the INSERT query and throws an error.

In SQL server, the syntax to set a foreign key field in a table is −

CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...
    CONSTRAINT fk_name
	FOREIGN KEY (column_name)
	REFERENCES referenced_table(referenced_column)
);

Alternate Key

An Alternate key is a candidate key that could be a primary key but is not. Like primary key, it also uniquely identifies the records in a field of a table to retrieve row tuples from the said table. There can be a single or multiple fields identifying as alternate keys in a table.

The SQL Composite Key

An SQL Composite Key is a key that can be defined on two or more columns in a table to uniquely identify any record. It can also be described as a Primary Key created on multiple columns.

Composite Keys are necessary in scenarios where a database table does not have a single column that can uniquely identify each row from the table. In such cases, we might need to use the combination of columns to ensure that each record in the table is distinct and identifiable.

Let us understand the composite keys with an example. Suppose if we have a table named CUSTOMERS with various fields like ID, NAME, AGE, AADHAAR_ID, MOBILE_NO and SALARY as shown below −

We can select the two columns AADHAAR_ID and MOBILE_NO and define a Composite key on them, and it can be used to fetch the records of the CUSTOMERS table uniquely.

Features of Composite Keys

Following are some important features of the SQL Composite Key −

• A Composite Key can be created by combining more than one Candidate Key.
• Each Candidate Key (or column) that makes up a Composite Key may or may not be a Foreign Key. However, if all the columns of the Composite Key are Foreign Keys in their own right, then the Composite Key is known as a Compound Key.
• A Composite Key cannot be NULL; i.e. any column of the Composite Key must not contain NULL values.
• The individual columns making up the Composite Key can contain duplicate values, but, the combination of these columns must be unique across the database table.

Syntax

Following is the syntax to create an SQL Composite Key while creating a table −

CREATE TABLE table_name(
   column1 datatype,
   column2 datatype,
   column3 datatype,
   .....
   columnN datatype,
   CONSTRAINT composite_key_name,
   PRIMARY KEY(column_name)
);

Here, the composite_key_name is the optional placeholder which holds the name of a Composite Key in a table. It is used while dropping the constraint from a table in some databases.

Example

In the following example, we are creating a table named CUSTOMERS with multiple columns. The Composite Key is created when a PRIMARY KEY is defined on ID and NAME columns together. Look at the query below −

CREATE TABLE CUSTOMERS(
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),
   CONSTRAINT ck_customers
   PRIMARY KEY (ID, NAME)
);

Where, ck_customers is the name of a composite key of this table.

Output

Following is the output of the above statement −

Query OK, 0 rows affected (0.02 sec)

Verification

As we have created a Composite Key on the columns ID and NAME of the CUSTOMERS table, the combination of values in these columns can not be duplicated. To verify it, let us insert two records with same values in these columns into the CUSTOMERS table −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 32, ''Ahmedabad'', 2000.00 ),
(1, ''Ramesh'', 25, ''Delhi'', 1500.00 );

You can observe that the second INSERT statement generates an error message saying “Duplicate entry” as shown below −

ERROR 1062 (23000): Duplicate entry ''1-Ramesh'' for key ''customers.PRIMARY''

Dropping a Composite Key in MySQL

You can drop the composite key from a table in MySQL database using the ALTER TABLE… DROP statement.

Syntax

Following is the syntax to drop the Composite Key in MySQL −

ALTER TABLE table_name DROP PRIMARY KEY;

Example

Using the following SQL statement, we can drop the Composite Key constraint from the CUSTOMERS table −

ALTER TABLE CUSTOMERS DROP PRIMARY KEY;

Output

The above SQL statement produces the following output −

Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

Verification

Since, we have dropped the composite from the CUSTOMERS table, so now you can insert the duplicate values in the columns ID and NAME.

Let us insert two records with the same ID and NAME into the CUSTOMERS table −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 25, ''Delhi'', 1500.00 ),
(1, ''Ramesh'', 23, ''Kota'', 2000.00 );

If you retrieve the contents the CUSTOMERS table you can find the records with same ID and NAME as −

Dropping a Composite Key in SQL Server

In SQL Server, we have a different syntax to drop a composite key of a table. The syntax is almost similar, but we just need to specify the composite key name in order to drop it, rather than the keyword PRIMARY KEY.

Syntax

Following is the syntax to drop a composite key in SQL Server −

ALTER TABLE table_name DROP composite_key_name;

Example

Assuming that a composite key “ck_customers” is created on ID and NAME columns of the CUSTOMERS table, we will use the following query to drop it −

ALTER TABLE CUSTOMERS DROP ck_customers;

Output

When we execute the above query, the composite key will be dropped.

Commands completed successfully.

Verification

To verify whether we have removed the composite key from the CUSTOMERS table or not, insert duplicate values into the ID and NAME columns using the following query −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 25, ''Delhi'', 1500.00 ),
(1, ''Ramesh'', 23, ''Kota'', 2000.00 );

As we can see in the table below, both the customers have the same ID and NAME −

The SQL Primary Key

The SQL Primary Key is a column (or combination of columns) that uniquely identifies each record in a database table. The Primary Key also speeds up data access and is used to establish a relationship between tables.

Even though a table can only have one Primary Key, it can be defined on one or more fields. When a primary key is created on multiple fields of a table, it is called a .

Let us say, you are developing an application called “Customer Management System” to handle all the customer data of a member-only resort. This data can include their personal details, assigned member IDs, other details of the membership they opted, etc. And in all the tables created within this database, the member ID is used to distinguish the customers from each other. So, this field will be the Primary Key.

Following is the diagram of a CUSTOMERS table that holds the personal details of the customers. And as we can observe, the primary key is defined on the CUST_ID column. Using this primary key, we can retrieve a unique record of any customer.

Points to Remember

Here are some key points of the PRIMARY KEY −

• It contains only a unique value.

• It can not be null.

• One table can have only one Primary Key.

• A primary key length cannot be more than 900 bytes.

Creating an SQL Primary Key

While creating a table using the CREATE TABLE statement, you can add the primary key constraint on a particular column of the table just by to specifying the name of the column along with the keyword “PRIMARY KEY”.

Syntax

Following is the syntax to define a column of a table as a primary key −

CREATE TABLE table_name(
   column1 datatype,
   column2 datatype,
   column3 datatype,
   .....
   columnN datatype,
   PRIMARY KEY(column_name)
);

Example

In the following example, we are trying to create a table with the name CUSTOMERS with various fields in an SQL database. While creating the table, we will add the constraint “PRIMARY KEY” on the column named ID.

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),
   PRIMARY KEY (ID)
);

Output

Following is the output of the above SQL statement −

Query OK, 0 rows affected (0.03 sec)

Verification

As we know primary key value must be unique, so you can not insert the record with the same ID. Here, we will verify the constraint created on the ID column, by inserting records with duplicate ID values.

First of all, let”s insert a record into the CUSTOMERS table −

INSERT INTO CUSTOMERS VALUES
(3, ''Kaushik'', 23, ''Kota'', 2000.00);

Now, let”s insert one more record with same ID −

INSERT INTO CUSTOMERS VALUES
(3, ''Chaitali'', 25, ''Mumbai'', 6500.00);

As we have mentioned above, if any field/column is defined as Primary Key in a database table, two records can not have the same value in that column/field. Therefore, the second insert statement generates the following error −

ERROR 1062 (23000): Duplicate entry ''3'' for key ''customers.PRIMARY''

Similarly, a primary key column cannot contain null values. Here, using the INSERT statement we are passing a NULL value to the primary key column (ID).

INSERT INTO CUSTOMERS VALUES
(NULL, ''Komal'', 22, ''Hyderabad'', 4500.00);

This statement generates the following error −

ERROR 1048 (23000): Column ''ID'' cannot be null

Creating Primary Key on an Existing Column

We can also add the PRIMARY KEY constraint on an existing column of a table using the ALTER TABLE statement.

Syntax

Following is the syntax to create a primary constraint on existing columns of a table −

ALTER TABLE table_name ADD CONSTRAINT PRIMARY KEY (column_name);

Example

In this example, we are adding the PRIMARY KEY constraint on the NAME column of the existing CUSTOMERS table −

ALTER TABLE CUSTOMERS ADD CONSTRAINT PRIMARY KEY(NAME);

Output

Following is the output of the above statement −

Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

Dropping an SQL Primary Key

If you can add a Primary Key Constraint to a column in the table, you can drop it as well. This is done by using the ALTER TABLE… DROP statement.

Syntax

Following is the syntax of the ALTER TABLE statement to can drop the Primary key constraints from the column of a table −

ALTER TABLE table_name DROP PRIMARY KEY;

Example

Let us consider the CUSTOMERS table where we have created a primary key constraint on a column named ID. You can drop this constraint from the column ID by executing the following statement −

ALTER TABLE CUSTOMERS DROP PRIMARY KEY;

Output

The above SQL query produces the following output −

Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0

Verification

As we have dropped the Primary key from the column named ID, we can insert multiple records with the same ID. Following statement inserts four records with the same ID −

INSERT INTO CUSTOMERS VALUES
(3, ''Chaitali'', 25, ''Mumbai'', 6500.00 ),
(3, ''Hardik'', 27, ''Bhopal'', 8500.00 ),
(3, ''Komal'', 22, ''Hyderabad'', 4500.00 ),
(3, ''Muffy'', 24, ''Indore'', 10000.00 );

If you verify the content of this table, you can find multiple records with same ID −

SELECT * FROM CUSTOMERS;

The table will be displayed as −

The SQL Foreign Key

In SQL, a Foreign Key is a column in one table that matches a Primary Key in another table, allowing the two tables to be connected together.

A foreign key also maintains referential integrity between two tables, making it impossible to drop the table containing the primary key (preserving the connection between the tables).

The foreign key can reference the unique fields of any table in the database. The table that has the primary key is known as the parent table and the key with the foreign key is known as the child table.

Let”s consider an example scenario, assume we have two tables namely CUSTOMERS (ID, NAME, AGE, ADDRES, SALARY) and ORDERS (ID, DATE, CUSTOMER_ID, AMOUNT). Here the id of the customer is primary key (ID) in the CUSTOMERS table and foreign key in the ORDERS (CUSTOMER_ID) table observe the following diagram −

Features of Foreign Key

Following is the of features of Foreign Key −

• A Foreign Key is used to reduce the redundancy (or duplicates) in the table.

• It helps to normalize (or organize the data in a database) the data in multiple tables.

Syntax

Following is the basic syntax to add Foreign Key constraints on a column of a table in MySQL database −

CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...
    CONSTRAINT fk_name
	FOREIGN KEY (column_name)
	REFERENCES referenced_table(referenced_column)
);

Example

Let us create two tables with the names CUSTOMERS and ORDERS. The following query creates a table with the name CUSTOMERS −

CREATE TABLE CUSTOMERS(
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25) ,
   SALARY DECIMAL (18, 2),
   PRIMARY KEY (ID)
);

Output

Following is the output of the above SQL statement −

Query OK, 0 rows affected (0.02 sec)

Now, let us create the ORDERS table. While doing so, we add the foreign key constraint on column CUSTOMER_ID reference on column ID of the CUSTOMERS table as shown in the statement below −

CREATE TABLE ORDERS (
   ID INT NOT NULL,
   DATE DATETIME,
   CUSTOMER_ID INT,
   CONSTRAINT FK_CUSTOMER
   FOREIGN KEY(CUSTOMER_ID)
   REFERENCES CUSTOMERS(ID),
   AMOUNT DECIMAL,
   PRIMARY KEY (ID)
);

Output

The above statement produces the following output −

Query OK, 0 rows affected (0.04 sec)

Verification

We have created a Foreign Key Constraint on a column named CUSTOMER_ID in the ORDERS table that references the column named ID of the CUSTOMERS table; so you can”t drop table1 (CUSTOMERS) before dropping the table2 (ORDERS).

First of all, let”s drop the CUSTOMERS table without dropping the ORDERS table using the DROP TABLE statement −

DROP TABLE CUSTOMERS;

If you verify the error message below, you will observe that it says that the table can not be dropped because it is referenced by a FOREIGN KEY constraint −

ERROR 3730 (HY000): Cannot drop table ''customers'' referenced by a foreign key constraint ''FK_CUSTOMER'' on table ''orders''.

Foreign Key Constraint on an Existing Column

We can also create a Foreign key constraint on a column of an existing table. This is useful when you forget to add a Foreign Key constraint on a column while creating a table, or when you want to add this constraint on another column even if one Foreign Key column exists in a table.

Syntax

Using the ALTER TABLE statement we can add a Foreign Key constraint on an existing column in a table in MySQL database as shown below −

ALTER TABLE TABLE2
ADD CONSTRAINT[symbol]
FOREIGN KEY(column_name)
REFERENCES TABLE1(column_name);

Here, FK_ORDERS is the name of the foreign key constraint. It is optional to specify the name of a constraint but it comes in handy while dropping the constraint.

Example

Assume the CUSTOMERS and ORDERS tables have already been created in the SQL database. Now, we will add a Foreign Key Constraint on the ID column of the ORDERS table.

Following is the SQL query to add the foreign key constraint on an the column of an existing table −

ALTER TABLE ORDERS
ADD CONSTRAINT FK_ORDERS
FOREIGN KEY(ID)
REFERENCES CUSTOMERS(ID);

Output

Following is the output of the above program −

Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

We have created a Foreign Key Constraint on a column named CUSTOMER_ID in the ORDERS table that references the column name ID of the CUSTOMERS table. So, you can”t drop table1 (CUSTOMERS) before dropping the table2 (ORDERS).

First of all, let us drop the CUSTOMERS table without dropping the ORDERS table by executing the following statement −

DROP TABLE CUSTOMERS;

This generates an error message saying that the table can not be dropped because it is referenced by a FOREIGN KEY constraint −

ERROR 3730 (HY000): Cannot drop table ''customers'' referenced by a foreign key constraint ''FK_CUSTOMER'' on table ''orders''.

Dropping a FOREIGN KEY

You can drop the foreign key from a table, without dropping that entire table, using the ALTER TABLE statement.

Syntax

Following is the syntax to drop the FOREIGN key constraint from the column of the table using the ALTER TABLE statement−

ALTER TABLE table_name DROP FOREIGN KEY (constraint symbol);

Where, FK_NAME is the name of the foreign key constraint you need to drop.

Example

The SQL query to drop the foreign key constraint from the column of a table is as follows −

ALTER TABLE ORDERS DROP FOREIGN KEY FK_ORDERS;

Output

Following is the output of the above SQL query −

Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

Since we have dropped the Foreign key constraint from the ORDERS table, you can now directly drop the CUSTOMERS table without dropping the ORDERS table, as shown below −

DROP TABLE CUSTOMERS;

If you verify the below status code thrown by the above SQL command, you observe that the CUSTOMERS table has dropped.

Query OK, 0 rows affected (0.02 sec)

Primary Key vs Foreign Key

Even though both the primary key and foreign key refer to the same column, there are many differences to be observed in the way they work. They are listed below −

The main difference between the Left Join and Right Join can be observed in the way tables are joined.

They are both types of Outer Joins; that is, they retain unmatched rows in one table and discard the unmatched rows of another. Left Join preserves the unmatched rows of left table while Right join preserves the unmatched rows of right table.

Working of Left Join

Left Join or Left Outer Join in SQL combines two or more tables, where the first table is returned as it is; but, only the record(s) that have counterparts in first table are returned from consequent tables.

If the ON clause matches zero records in consequent tables with the rows in first table, left join will still return these rows of first table in the result, but with NULL in each column from the right table.

Syntax

Following is the basic syntax of Left Join in SQL −

SELECT table1.column1, table2.column2...
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

Example

The example below demonstrates the Left Join operation on two relative tables. Here, the first table contains the salary information while the second table contains marital status information. Since Alex”s status is unknown, it is not recorded in the table.

When both tables are joined using the Left Join query, since there is no record matching Alex”s Status, the value is recorded as NULL in the final table.

Working of Right Join

Right Join or Right Outer Join query in SQL returns all rows from the right table, even if there are no matches in the left table. This means that if the ON clause matches 0 (zero) records in left table with the records in right table; right join will still return the rows of right table in the result, but with a NULL value in each column of the left table.

Syntax

Following is the basic syntax of a Right Join in SQL −

SELECT table1.column1, table2.column2...
FROM table1
RIGHT JOIN table2
ON table1.column_name = table2.column_name;

Example

Now in this example, the Right Join operation is performed on the same tables. Here, we are starting the join from the right table; since, the right table does not contain the record value matching Alex”s row, the row is discarded from the final table.

The final table only consists of two rows as the right table consists of two rows only.

Left Join Vs Right Join

Let us summarize all the differences between the Left Join and Right Join in the table below −

As we can observe from the summary, there aren”t wide range of differences between the Left and Right joins. Every difference between them zeroes down to the way the tables are joined and the join point of view.

The SQL Unique Key

The SQL Unique Key (or, Unique constraint) does not allow duplicate values in a column of a table. It prevents two records from having same values in a column.

Unique Key is just an alternative to the Primary Key; as both Unique and Primary Key constraints ensure uniqueness in a column of the table.

Suppose we have a table named CUSTOMERS to store the customer records in a Bank and if one of the column names is MOBILE_NO then, we can create a UNIQUE constraint on this column to prevent the entry of multiple records with the same mobile number.

Features of Unique Keys

Following is the list of some key features of the Unique Key in an SQL database −

• The unique key is similar to the primary key in a table, but it can accept NULL values, whereas the primary key does not.

• It accepts only one NULL value.

• It cannot have duplicate values.

• It can also be used as a foreign key in another table.

• A table can have more than one Unique column.

Creating SQL Unique Key

You can create a Unique Key on a database table using the UNIQUE keyword in SQL. While creating a database table, specify this SQL keyword along with the column (where this key needs to be defined on).

Syntax

Following is the syntax to create a UNIQUE key constraint on a column in a table −

CREATE TABLE table_name(
   column1 datatype UNIQUE KEY,
   column2 datatype,
   .....
   .....
   columnN datatype
);

Example

Using the following SQL query, we are creating a table named CUSTOMERS with five fields ID, NAME, AGE, ADDRESS, and SALARY in it. Here, we are creating a Unique Key on the ID column.

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL UNIQUE KEY,
   NAME VARCHAR(20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2)
);

Output

Following is the output of the above SQL statement −

Query OK, 0 rows affected (0.03 sec)

Verification

Since we have created a UNIQUE constraint on the column named ID, we cannot insert duplicate values in it. Let us verify by inserting the following records with duplicate ID values into the CUSTOMERS table −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 32, ''Ahmedabad'', 2000.00 ),
(1, ''Khilan'', 25, ''Delhi'', 1500.00 );

On execution, following error is displayed proving that the UNIQUE constraint is indeed defined on the ID column −

ERROR 1062 (23000): Duplicate entry ''1'' for key ''customers.ID''

Multiple Unique Keys

We can create one or more Unique Keys on one or more columns in an SQL table.

Syntax

Following is the syntax to create unique key constraints on multiple columns in a table −

CREATE TABLE table_name(
   column1 datatype UNIQUE KEY,
   column2 datatype UNIQUE KEY,
   .....
   .....
   columnN datatype
);

Example

Assume we have created a table with the name CUSTOMERS in the SQL database using CREATE TABLE statement. A Unique key is defined on columns ID and NAME using the UNIQUE keyword as shown below −

CREATE TABLE BUYERS (
   ID INT NOT NULL UNIQUE KEY,
   NAME VARCHAR(20) NOT NULL UNIQUE KEY,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2)
);

Output

Following is the output of the above SQL statement −

Query OK, 0 rows affected (0.03 sec)

Verification

Since we have created a UNIQUE constraint on the column named ID and NAME, we cannot insert duplicate values in it. Let us verify by inserting duplicate records into the BUYERS table using the following INSERT statement −

INSERT INTO BUYERS VALUES
(1, ''Ramesh'', 32, ''Ahmedabad'', 2000.00 ),
(1, ''Rajesh'', 25, ''Delhi'', 1500.00 );

Following error is displayed −

ERROR 1062 (23000): Duplicate entry ''1'' for key ''customers.ID''

In the same way if you try to insert the another record with duplicate value for the column NAME as −

INSERT INTO BUYERS VALUES (2, ''Ramesh'', 36, ''Chennai'', 1700.00 );

Following error is generated −

ERROR 1062 (23000): Duplicate entry ''Ramesh'' for key ''buyers.NAME''

Unique Key on an Existing Column

Until now, we have only seen how to define a Unique Key on a column while creating a new table. But, we can also add a unique key on an existing column of a table. This is done using the ALTER TABLE… ADD CONSTRAINT statement.

Syntax

Following is the syntax to create a unique constraint on existing columns of a table −

ALTER TABLE table_name ADD CONSTRAINT
UNIQUE_KEY_NAME UNIQUE (column_name);

Note − Here the UNIQUE_KEY_NAME is just the name of the UNIQUE KEY. It is optional to specify and is used to drop the constraint from the column in a table.

Example

In this example, we add a Unique Key on the ADDRESS column of the existing CUSTOMERS table −

ALTER TABLE CUSTOMERS ADD CONSTRAINT
UNIQUE_ADDRESS UNIQUE(ADDRESS);

Output

Following is the output of the above statement −

Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

Dropping an SQL Unique Key

If you have already created a unique key on a column, you can drop it whenever it is not needed. To drop the Unique Key from the column of a table, you need to use the ALTER TABLE statement.

Syntax

Following is the SQL query to drop the UNIQUE constraint from the column of a table −

ALTER TABLE table_name DROP CONSTRAINT UNIQUE_KEY_NAME;

Example

Consider the CUSTOMERS table created above, we have created the UNIQUE constraints on three columns named ID, NAME and ADDRESS; drop the UNIQUE constraints from the column ADDRESS by executing the following SQL query −

ALTER TABLE CUSTOMERS DROP CONSTRAINT UNIQUE_ADDRESS;

Output

Following is the output of the above statement −

Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

Now, let us insert two duplicate records of column ADDRESS −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 32, ''Ahmedabad'', 2000.00 ),
(2, ''Khilan'', 25, ''Ahmedabad'', 1500.00 );

If you verify the contents of the table, you can observe that both the records have the same ADDRESS as shown below −

SQL provides various relational operators to handle data that is spread across multiple tables in a relational database. Out of them, UNION and JOIN queries are fundamentally used to combine data from multiple tables.

Even though they are both used for the same purpose, i.e. to combine tables, there are many differences between the working of these operators. The major difference is that the UNION operator combines data from multiple similar tables irrespective of the data relativity, whereas, the JOIN operator is only used to combine relative data from multiple tables.

Working of UNION

UNION is a type of operator/clause in SQL, that works similar to the union operator in relational algebra. It does nothing more than just combining information from multiple tables that are union compatible.

The tables are said to be union compatible if they follow the conditions given below −

• The tables to be combined must have same number of columns with the same datatype.
• The number of rows need not be same.

Once these criteria are met, UNION operator returns all the rows from multiple tables, after eliminating duplicate rows, as a resultant table.

Note − Column names of first table will become column names of resultant table, and contents of second table will be merged into resultant columns of same data type.

Syntax

Following is the syntax of the SQL UNION operator −

SELECT * FROM table1
UNION
SELECT * FROM table2;

Example

Let us first create two table “COURSES_PICKED” and “EXTRA_COURSES_PICKED” with the same number of columns having same data types.

Create table COURSES_PICKED using the following query −

CREATE TABLE COURSES_PICKED(
   STUDENT_ID INT NOT NULL,
   STUDENT_NAME VARCHAR(30) NOT NULL,
   COURSE_NAME VARCHAR(30) NOT NULL
);

Insert values into the COURSES_PICKED table with the help of the query given below −

INSERT INTO COURSES_PICKED VALUES
(1, ''JOHN'', ''ENGLISH''),
(2, ''ROBERT'', ''COMPUTER SCIENCE''),
(3, ''SASHA'', ''COMMUNICATIONS''),
(4, ''JULIAN'', ''MATHEMATICS'');

Create table EXTRA_COURSES_PICKED using the following query −

CREATE TABLE EXTRA_COURSES_PICKED(
   STUDENT_ID INT NOT NULL,
   STUDENT_NAME VARCHAR(30) NOT NULL,
   EXTRA_COURSE_NAME VARCHAR(30) NOT NULL
);

Following is the query to insert values into the EXTRA_COURSES_PICKED table −

INSERT INTO EXTRA_COURSES_PICKED VALUES
(1, ''JOHN'', ''PHYSICAL EDUCATION''),
(2, ''ROBERT'', ''GYM''),
(3, ''SASHA'', ''FILM''),
(4, ''JULIAN'', ''PHOTOGRAPHY'');

Now, let us combine the tables COURSES_PICKED and EXTRA_COURSES_PICKED, using the UNION query as follows −

SELECT * FROM COURSES_PICKED
UNION
SELECT * FROM EXTRA_COURSES_PICKED;

Output

The resultant table obtained after performing the UNION operation is −

Working of JOIN

The Join operation is used to combine information from multiple related tables into one, based on their common fields. This operation can be used with various clauses like ON, WHERE, ORDER BY, GROUP BY etc.

There are two types of Joins −

• Inner Join
• Outer Join

The basic type of join is an Inner Join, which only retrieves the matching values of common columns. It is a default join.

The result table of the Outer join includes both matched and unmatched rows from the first table. It is divided into subtypes like Left Join, Right Join, and Full Join.

Syntax

Following is the basic syntax of a Join operation in SQL −

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name = table2.column_name;

Example

In the following example, we will join the same tables we created above, i.e., COURSES_PICKED and EXTRA_COURSES_PICKED, using the query below –

SELECT c.STUDENT_ID, c.STUDENT_NAME, COURSE_NAME, COURSES_PICKED
FROM COURSES_PICKED c
JOIN EXTRA_COURSES_PICKED e
ON c.STUDENT_ID = e.STUDENT_ID;

Output

The resultant table will be displayed as follows −

UNION Vs JOIN

As we saw in the examples given above, the UNION operator is only executable on tables that are union compatible, whereas, the JOIN operator joins two tables that need not be compatible but should be related.

Let us summarize all the difference between these queries below −