SQL LEFT JOIN
SQL左联接返回左表中的所有值,并且还包括右表中的匹配值,如果没有匹配的联接值,则返回NULL
左联接的基本语法:
SELECT table1.column1, table2.column2....
FROM table1
LEFTJOIN table2
ON table1.column_field = table2.column_field;
在本示例中,让我们采用两个表来详细说明所有事情:
客户表:
ID |
NAME |
AGE |
SALARY |
1 |
ARYAN |
51 |
56000 |
2 |
AROHI |
21 |
25000 |
3 |
VINEET |
24 |
31000 |
4 |
AJEET |
23 |
32000 |
5 |
RAVI |
23 |
42000 |
这是第二张表
ORDER TABLE:
O_ID |
DATE |
CUSTOMER_ID |
AMOUNT |
001 |
20-01-2012 |
2 |
3000 |
002 |
12-02-2012 |
2 |
2000 |
003 |
22-03-2012 |
3 |
4000 |
004 |
11-04-2012 |
4 |
5000 |
使用LEFT JOIN将这两个表结合在一起:
SQL SELECT ID, NAME, AMOUNT,DATE
FROM CUSTOMER
LEFT JOIN ORDER
ON CUSTOMER.ID = ORDER.CUSTOMER_ID;
这将产生以下结果:
ID |
NAME |
AMOUNT |
DATE |
1 |
ARYAN |
NULL |
NULL |
2 |
AROHI |
3000 |
20-01-2012 |
2 |
AROHI |
2000 |
12-02-2012 |
3 |
VINEET |
4000 |
22-03-2012 |
4 |
AJEET |
5000 |
11-04-2012 |
5 |
RAVI |
NULL |
NULL |