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SELF JOIN
The SQL SELF JOIN is used to join a table to itself as if the table were two tables; temporarily renaming at least one table in the SQL statement.
A self join is a regular join, but the table is joined with itself.
SYNTAX
SELECT column_name(s)
FROM table T1, table T2
WHERE condition;
Here T1 and T2 are the different table aliases for the same table.
EXAMPLE
Consider the following Customers table.
| ID | NAME | AGE | ADDRESS | SALARY |
|---|---|---|---|---|
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
Lets apply SELF JOIN as:
SELECT a.id, b.NAME, a.SALARY
FROM CUSTOMERS a, CUSTOMERS b
WHERE a.SALARY < b.SALARY
RESULT:
| ID | NAME | SALARY |
|---|---|---|
| 2 | Ramesh | 1500.00 |
| 2 | kaushik | 1500.00 |
| 1 | Chaitali | 2000.00 |
| 2 | Chaitali | 1500.00 |
| 3 | Chaitali | 2000.00 |
| 6 | Chaitali | 4500.00 |
| 1 | Hardik | 2000.00 |
| 2 | Hardik | 1500.00 |
| 3 | Hardik | 2000.00 |
| 4 | Hardik | 6500.00 |
| 6 | Hardik | 4500.00 |
| 1 | Komal | 2000.00 |
| 2 | Komal | 1500.00 |
| 3 | Komal | 2000.00 |
| 1 | Muffy | 2000.00 |
| 2 | Muffy | 1500.00 |
| 3 | Muffy | 2000.00 |
| 4 | Muffy | 6500.00 |
| 5 | Muffy | 8500.00 |
| 6 | Muffy | 4500.00 |
WHY SELF JOIN ?
A self join allows you to join a table to itself. It is useful for querying hierarchical data or comparing rows within the same table.
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