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