SQL − ALTER TABLE Statement

The SQL ALTER TABLE command is a part of Data Definition Language (DDL) and modifies the structure of a table. The ALTER TABLE command can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself.

The ALTER TABLE command can also change characteristics of a table such as the storage engine used for the table. We will make use of the following table in our examples

Syntax

Following is the basic syntax of an ALTER TABLE command −

ALTER TABLE table_name [alter_option ...];

Where, the alter_option depends on the type of operation to be performed on a table. This article will discuss such important operations one by one.

ALTER TABLE − ADD Column

If you need to add a new column to a table, you should use the ADD COLUMN option along with ALTER TABLE statement as shown below −

ALTER TABLE table_name ADD column_name datatype;

Example

Following is the example to ADD a New Column to an existing table −

ALTER TABLE CUSTOMERS ADD SEX char(1);

Output

Executing the query above will produce the following output −

Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

To verify whether the CUSTOMERS table is altered by adding a new column SEX, use the SELECT statement to retrieve the records of the table −

SELECT * FROM CUSTOMERS;

Now, the CUSTOMERS table will be displayed as follows −

ALTER TABLE − DROP COLUMN

If you need to drop an existing column from a table, you should use the DROP COLUMN option along with ALTER TABLE statement as shown below.

ALTER TABLE table_name DROP COLUMN column_name;

Example

Following is the example to DROP sex column from the existing table.

ALTER TABLE CUSTOMERS DROP COLUMN SEX;

Output

Executing the query above will produce the following output −

Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

Verification

To verify whether the CUSTOMERS table is altered by dropping an existing column SEX, use the SELECT statement to retrieve the records of the table −

SELECT * FROM CUSTOMERS;

Now, the CUSTOMERS table is changed and following would be the output from the SELECT statement.

ALTER TABLE − ADD INDEX

You can add index to an existing column of a table using the ADD INDEX statement along with the ALTER statement −

ALTER TABLE table_name 
ADD INDEX index_name [index_type] 

Example

Following query adds an index on the column NAME of CUSTOMERS table −

ALTER TABLE CUSTOMERS ADD INDEX name_index (NAME);

Output

The output will be displayed as −

Query OK, 0 rows affected (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 0

ALTER TABLE − DROP INDEX

You can drop an existing index from a table using the DROP INDEX statement along with the ALTER statement −

ALTER TABLE table_name DROP INDEX index_name;

Example

Following query adds an index on the column NAME of CUSTOMERS table −

ALTER TABLE CUSTOMERS DROP INDEX name_index;

Output

The output will be displayed as −

Query OK, 0 rows affected (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 0

ALTER TABLE − ADD PRIMARY KEY

Following is the syntax to add a primary key in an existing table of a database −

ALTER TABLE table_name 
ADD CONSTRAINT constraint_name
PRIMARY KEY (column1, column2...);

Example

Before we add a primary key to an existing table, first let”s create a new table called EMPLOYEES as follows:

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

Following query adds primary key constraint on the column ID of EMPLOYEES table −

ALTER TABLE EMPLOYEES 
ADD CONSTRAINT MyPrimaryKey 
PRIMARY KEY(ID);

This will produce the following output −

Query OK, 0 rows affected, 1 warning (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 1

Verification

To verify the above query if you describe the table using the DESC EMPLOYEES command −

DESC EMPLOYEES;

This will display the structure of the table created: column names, their respective data types, constraints (if any) etc.

ALTER TABLE − DROP PRIMARY KEY

Following is the syntax to delete a primary key from an existing table of a database −

ALTER TABLE table_name DROP PRIMARY KEY;

Example

Following query deletes primary key constraint from the column ID of EMPLOYEES table −

ALTER TABLE EMPLOYEES DROP PRIMARY KEY;

This will produce the following output −

Query OK, 0 rows affected, 1 warning (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 1

ALTER TABLE − ADD CONSTRAINT

Following is the syntax to add a unique constraint to a column of an existing table −

ALTER TABLE table_name 
ADD CONSTRAINT constraint_name 
UNIQUE(column1, column2...);

Example

Following query adds UNIQUE constraint to the table CUSTOMERS −

ALTER TABLE EMPLOYEES ADD CONSTRAINT CONST UNIQUE(NAME);

This will produce the following output −

Query OK, 0 rows affected (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 0

ALTER TABLE − DROP CONSTRAINT

Following is the syntax to drop a unique constraint from an existing table −

ALTER TABLE table_name DROP CONSTRAINT constraint_name;

Example

Following query adds UNIQUE constraint to the table CUSTOMERS −

ALTER TABLE EMPLOYEES DROP CONSTRAINT CONST;

This will produce the following output −

Query OK, 0 rows affected (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 0

ALTER TABLE − RENAME COLUMN

Following is the syntax to rename a column name of an existing table −

ALTER TABLE table_name 
RENAME COLUMN old_column_name to new_column_name;

Example

Following query renames NAME column in table CUSTOMERS −

ALTER TABLE CUSTOMERS RENAME COLUMN name to full_name;

This will produce the following output −

Query OK, 0 rows affected (0.002 sec)
Records: 0  Duplicates: 0  Warnings: 0

ALTER TABLE − MODIFY DATATYPE

Following is the syntax to change the data type of any column in MySQL, MS Server and Oracle.

SQL Server/MS Access Syntax

ALTER TABLE table_name ALTER COLUMN column_name datatype;

MySQL Syntax

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

Oracle Syntax

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

Example

Following query modifies datatype of SALARY column in MySQL CUSTOMERS table −

ALTER TABLE CUSTOMERS MODIFY COLUMN ID DECIMAL(18, 4);

This will produce the following output −

Query OK, 0 rows affected (0.003 sec)
Records: 0  Duplicates: 0  Warnings: 0