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标题: left Join与right join详解 [打印本页]

作者: 流星陨落 时间: 2009-07-11 14:26
标题: left Join与right join详解

示例表如下：
select * from emp;
+----+-----------+
| id | name    |
+----+-----------+
|  1 | Dennis-1  |
|  2 | Dennis-2  |
|  3 | Dennis-3  |
|  4 | Dennis-4  |
|  5 | Dennis-5  |
|  6 | Dennis-6  |
|  7 | Dennis-7  |
|  8 | Dennis-8  |
|  9 | Dennis-9  |
| 10 | Dennis-10 |
select * from emp_dept;
+---------+--------+----------+
| dept_id | emp_id | emp_name |
+---------+--------+----------+
| R&D    |    1 | Dennis-1 |
| DEv    |    2 | Dennis-2 |
| R&D    |    3 | Dennis-3 |
| Test |    4 | Dennis-4 |
| Test |    5 | Dennis-5 |
+---------+--------+----------+
1. left join：left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的. 换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录B表记录不足的地方均为NULL
select a.id,a.name,b.dept_id
  from emp a left join emp_dept b on (a.id=b.emp_id);
  # 挑出左边的 table emp 中的所有资料,即使 emp_dept 中没有的资料也挑出来,没有的就用 NULL 来显示,
  # 也即显示资料是以左边的 table emp 中的资料为基础
  mysql> select a.id,a.name,b.dept_id
  -> from emp a left join emp_dept b on (a.id=b.emp_id);
  +----+-----------+---------+
  | id | name    | dept_id |
  +----+-----------+---------+
  |  1 | Dennis-1  | R&D    |
  |  2 | Dennis-2  | DEv    |
  |  3 | Dennis-3  | R&D    |
  |  4 | Dennis-4  | Test |
  |  5 | Dennis-5  | Test |
  |  6 | Dennis-6  | NULL |
  |  7 | Dennis-7  | NULL |
  |  8 | Dennis-8  | NULL |
  |  9 | Dennis-9  | NULL |
  | 10 | Dennis-10 | NULL |
  +----+-----------+---------+
  # 挑出 table emp 中有而 table emp_dept 中没有的人员资料
  select a.id,a.name,b.dept_id
  from emp a left join emp_dept b on (a.id=b.emp_id)
  where b.dept_id IS NULL;
  mysql> select a.id,a.name,b.dept_id
  -> from emp a left join emp_dept b on (a.id=b.emp_id)
  -> where b.dept_id IS NULL;
  +----+-----------+---------+
  | id | name    | dept_id |
  +----+-----------+---------+
  |  6 | Dennis-6  | NULL |
  |  7 | Dennis-7  | NULL |
  |  8 | Dennis-8  | NULL |
  |  9 | Dennis-9  | NULL |
  | 10 | Dennis-10 | NULL |
  +----+-----------+---------+
  # 把 table emp_dept 放在左边的情形(当然以 emp_dept 中的数据为基础来显示资料,emp 中比emp_dept 中多的资料也就不会显示出来了):
  select a.id,a.name,b.dept_id
  from emp_dept b left join emp a on (a.id=b.emp_id);
  mysql> select a.id,a.name,b.dept_id
  -> from emp_dept b left join emp a on (a.id=b.emp_id);
  +------+----------+---------+
  | id | name    | dept_id |
  +------+----------+---------+
  | 2 | Dennis-2 | DEv    |
  | 1 | Dennis-1 | R&D    |
  | 3 | Dennis-3 | R&D    |
  | 4 | Dennis-4 | Test |
  | 5 | Dennis-5 | Test |
  +------+----------+---------+
  2.right join
  ---------------
  select a.id,a.name,b.dept_id
  from emp a right join emp_dept b on (a.id=b.emp_id);
  # 挑资料时以右边 table emp_dept 中的资料为基础来显示资料
  mysql> select a.id,a.name,b.dept_id
  -> from emp a right join emp_dept b on (a.id=b.emp_id);
  +------+----------+---------+
  | id | name    | dept_id |
  +------+----------+---------+
  | 2 | Dennis-2 | DEv    |
  | 1 | Dennis-1 | R&D    |
  | 3 | Dennis-3 | R&D    |
  | 4 | Dennis-4 | Test |
  | 5 | Dennis-5 | Test |
  +------+----------+---------+
  5 rows in set (0.00 sec)
  # 我们再把 table 的位置交换一下,再用 right join 试试
  select a.id,a.name,b.dept_id
  from emp_dept b right join emp a on (a.id=b.emp_id);
  mysql> select a.id,a.name,b.dept_id
  -> from emp_dept b right join emp a on (a.id=b.emp_id);
  +----+-----------+---------+
  | id | name    | dept_id |
  +----+-----------+---------+
  |  1 | Dennis-1  | R&D    |
  |  2 | Dennis-2  | DEv    |
  |  3 | Dennis-3  | R&D    |
  |  4 | Dennis-4  | Test |
  |  5 | Dennis-5  | Test |
  |  6 | Dennis-6  | NULL |
  |  7 | Dennis-7  | NULL |
  |  8 | Dennis-8  | NULL |
  |  9 | Dennis-9  | NULL |
  | 10 | Dennis-10 | NULL |
  +----+-----------+---------+
  # 是不是和 left join 一样了?
  # 如果用right join 同不用 Join 直接挑资料是相同的,它等介於以下的指令
  select a.id,a.name,b.dept_id
  from emp a ,emp_dept b
  where a.id=b.emp_id;
  mysql> select a.id,a.name,b.dept_id
  -> from emp a ,emp_dept b
  -> where a.id=b.emp_id;
  +----+----------+---------+
  | id | name    | dept_id |
  +----+----------+---------+
  |  2 | Dennis-2 | DEv    |
  |  1 | Dennis-1 | R&D    |
  |  3 | Dennis-3 | R&D    |
  |  4 | Dennis-4 | Test |
  |  5 | Dennis-5 | Test |
  +----+----------+---------+
  怎样,弄明白了吗?
  Enjoy it!

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