SQLite Joins clause is used to combine records from two or more tables in a database. A JOIN is a means for combining fields from two tables by using values common to each.

SQL defines three major types of joins −

• The CROSS JOIN
• The INNER JOIN
• The OUTER JOIN

Before we proceed, let”s consider two tables COMPANY and DEPARTMENT. We already have seen INSERT statements to populate COMPANY table. So just let”s assume the list of records available in COMPANY table −

ID          NAME        AGE         ADDRESS     SALARY
----------  ----------  ----------  ----------  ----------
1           Paul        32          California  20000.0
2           Allen       25          Texas       15000.0
3           Teddy       23          Norway      20000.0
4           Mark        25          Rich-Mond   65000.0
5           David       27          Texas       85000.0
6           Kim         22          South-Hall  45000.0
7           James       24          Houston     10000.0

Another table is DEPARTMENT with the following definition −

CREATE TABLE DEPARTMENT(
   ID INT PRIMARY KEY      NOT NULL,
   DEPT           CHAR(50) NOT NULL,
   EMP_ID         INT      NOT NULL
);

Here is the list of INSERT statements to populate DEPARTMENT table −

INSERT INTO DEPARTMENT (ID, DEPT, EMP_ID)
VALUES (1, ''IT Billing'', 1 );

INSERT INTO DEPARTMENT (ID, DEPT, EMP_ID)
VALUES (2, ''Engineering'', 2 );

INSERT INTO DEPARTMENT (ID, DEPT, EMP_ID)
VALUES (3, ''Finance'', 7 );

Finally, we have the following list of records available in DEPARTMENT table −

ID          DEPT        EMP_ID
----------  ----------  ----------
1           IT Billing  1
2           Engineering 2
3           Finance     7

The CROSS JOIN

CROSS JOIN matches every row of the first table with every row of the second table. If the input tables have x and y row, respectively, the resulting table will have x*y row. Because CROSS JOINs have the potential to generate extremely large tables, care must be taken to only use them when appropriate.

Following is the syntax of CROSS JOIN −

SELECT ... FROM table1 CROSS JOIN table2 ...

Based on the above tables, you can write a CROSS JOIN as follows −

sqlite> SELECT EMP_ID, NAME, DEPT FROM COMPANY CROSS JOIN DEPARTMENT;

The above query will produce the following result −

EMP_ID      NAME        DEPT
----------  ----------  ----------
1           Paul        IT Billing
2           Paul        Engineering
7           Paul        Finance
1           Allen       IT Billing
2           Allen       Engineering
7           Allen       Finance
1           Teddy       IT Billing
2           Teddy       Engineering
7           Teddy       Finance
1           Mark        IT Billing
2           Mark        Engineering
7           Mark        Finance
1           David       IT Billing
2           David       Engineering
7           David       Finance
1           Kim         IT Billing
2           Kim         Engineering
7           Kim         Finance
1           James       IT Billing
2           James       Engineering
7           James       Finance

The INNER JOIN

INNER JOIN creates a new result table by combining column values of two tables (table1 and table2) based upon the join-predicate. The query compares each row of table1 with each row of table2 to find all pairs of rows which satisfy the join-predicate. When the join-predicate is satisfied, the column values for each matched pair of rows of A and B are combined into a result row.

An INNER JOIN is the most common and default type of join. You can use INNER keyword optionally.

Following is the syntax of INNER JOIN −

SELECT ... FROM table1 [INNER] JOIN table2 ON conditional_expression ...

To avoid redundancy and keep the phrasing shorter, INNER JOIN conditions can be declared with a USING expression. This expression specifies a list of one or more columns.

SELECT ... FROM table1 JOIN table2 USING ( column1 ,... ) ...

A NATURAL JOIN is similar to a JOIN…USING, only it automatically tests for equality between the values of every column that exists in both tables −

SELECT ... FROM table1 NATURAL JOIN table2...

Based on the above tables, you can write an INNER JOIN as follows −

sqlite> SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT
   ON COMPANY.ID = DEPARTMENT.EMP_ID;

The above query will produce the following result −

EMP_ID      NAME        DEPT
----------  ----------  ----------
1           Paul        IT Billing
2           Allen       Engineering
7           James       Finance

The OUTER JOIN

OUTER JOIN is an extension of INNER JOIN. Though SQL standard defines three types of OUTER JOINs: LEFT, RIGHT, and FULL, SQLite only supports the LEFT OUTER JOIN.

OUTER JOINs have a condition that is identical to INNER JOINs, expressed using an ON, USING, or NATURAL keyword. The initial results table is calculated the same way. Once the primary JOIN is calculated, an OUTER JOIN will take any unjoined rows from one or both tables, pad them out with NULLs, and append them to the resulting table.

Following is the syntax of LEFT OUTER JOIN −

SELECT ... FROM table1 LEFT OUTER JOIN table2 ON conditional_expression ...

To avoid redundancy and keep the phrasing shorter, OUTER JOIN conditions can be declared with a USING expression. This expression specifies a list of one or more columns.

SELECT ... FROM table1 LEFT OUTER JOIN table2 USING ( column1 ,... ) ...

Based on the above tables, you can write an outer join as follows −

sqlite> SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT
   ON COMPANY.ID = DEPARTMENT.EMP_ID;

The above query will produce the following result −

EMP_ID      NAME        DEPT
----------  ----------  ----------
1           Paul        IT Billing
2           Allen       Engineering
            Teddy
            Mark
            David
            Kim
7           James       Finance