mysql> select * from employee_tbl;
+----+--------+---------------------+--------+
| id | name | date | singin |
+----+--------+---------------------+--------+
| 1 | 小明 | 2016-04-22 15:25:33 | 1 |
| 2 | 小王 | 2016-04-20 15:25:47 | 3 |
| 3 | 小丽 | 2016-04-19 15:26:02 | 2 |
| 4 | 小王 | 2016-04-07 15:26:14 | 4 |
| 5 | 小明 | 2016-04-11 15:26:40 | 4 |
| 6 | 小明 | 2016-04-04 15:26:54 | 2 |
+----+--------+---------------------+--------+
6 rows in set (0.00 sec)
mysql> select *from students;
+----+--------+-----+------+--------+
| id | name | sex | age | singin |
+----+--------+-----+------+--------+
| 1 | 小明 | 男 | 12 | 1 |
| 2 | 小丽 | 女 | 15 | 0 |
| 3 | 王芳 | 女 | 20 | 3 |
+----+--------+-----+------+--------+
3 rows in set (0.00 sec)
1、左表查询，以左边的表为准，没有数据的使用NULL 表示
给表取别名：
mysql> select a.id,a.name,b.singin from students a left join employee_tbl b on a.name = b.name;
或者不给表取别名：
mysql>select students.id,students.name,employee_tbl.singin from students left jion employee on students.name = employee.name;
+----+--------+--------+
| id | name | singin |
+----+--------+--------+
| 1 | 小明 | 1 |
| 2 | 小丽 | 2 |
| 1 | 小明 | 4 |
| 1 | 小明 | 2 |
| 3 | 王芳 | NULL |
+----+--------+--------+