SQL 中的 SELF JOIN函数

SQL SELF JOIN 用于将一个表连接到自身，就好像该表是两个表一样，在SQL语句中临时重命名至少一个表。

SELF JOIN - 语法

SELF JOIN的基本语法如下-

SELECT a.column_name, b.column_name...
FROM table1 a, table1 b
WHERE a.common_field=b.common_field;

SELF JOIN - 示例

请考虑下表。

链接：https://www.learnfk.comhttps://www.learnfk.com/sql/sql-self-joins.html

来源：LearnFk无涯教程网

客户表(CUSTOMERS)如下。

+----+----------+-----+-----------+----------+
| 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 | Learnfk    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

现在，让无涯教程使用SELF JOIN联接此表，如下所示:

SQL> SELECT  a.ID, b.NAME, a.SALARY
   FROM CUSTOMERS a, CUSTOMERS b
   WHERE a.SALARY < b.SALARY;

这将产生以下输出-

+----+----------+---------+
| 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 | Learnfk    | 2000.00 |
|  2 | Learnfk    | 1500.00 |
|  3 | Learnfk    | 2000.00 |
|  4 | Learnfk    | 6500.00 |
|  5 | Learnfk    | 8500.00 |
|  6 | Learnfk    | 4500.00 |
+----+----------+---------+