sql - 自连接查询

Question

考虑下表:

mysql> select * from phone_numbers;
+-------------+------+-----------+
| number      | type | person_id |
+-------------+------+-----------+
| 17182225465 | home |         1 |
| 19172225465 | cell |         1 |
| 12129876543 | home |         2 |
| 13049876543 | cell |         2 |
| 15064223454 | home |         3 |
| 15064223454 | cell |         3 |
| 18724356798 | home |         4 |
| 19174335465 | cell |         5 |
+-------------+------+-----------+

我正试图找到那些有家庭电话但没有手机的人。

此查询有效:

mysql> select h.*
    -> from phone_numbers h
    -> left join phone_numbers c
    -> on h.person_id = c.person_id
    -> and c.type = 'cell'
    -> where h.type = 'home'
    -> and c.number is null;
+-------------+------+-----------+
| number      | type | person_id |
+-------------+------+-----------+
| 18724356798 | home |         4 |
+-------------+------+-----------+

但这个不是:

mysql> select h.*
    -> from phone_numbers h
    -> left join phone_numbers c
    -> on h.person_id = c.person_id
    -> and h.type = 'home'
    -> and c.type = 'cell'
    -> where c.number is null;
+-------------+------+-----------+
| number      | type | person_id |
+-------------+------+-----------+
| 19172225465 | cell |         1 |
| 13049876543 | cell |         2 |
| 15064223454 | cell |         3 |
| 18724356798 | home |         4 |
| 19174335465 | cell |         5 |
+-------------+------+-----------+

两者之间的唯一区别是 h.type = 'home' 条件的位置 - 第一个是在 where 子句中，第二个是on 子句的一部分。

为什么第二个查询返回的结果与第一个不同？

Answer 1

在第二个 SQL 中，条件 h.type = 'home' 是外连接条件的一部分，而不是对结果的筛选。对于 h.type='cell' 的所有记录，条件 h.type = 'home' 为 FALSE，因此未找到“匹配”c 行 - 因此 c.number 为空，这是您唯一的过滤(WHERE)条件.

在伪代码中，您的第二个 SQL 是这样工作的:

for each row in phone_numbers h /* Note this is ALL home AND cell phones */
   select c.number from phone_numbers c
   where h.person_id = c.person_id
   and h.type = 'home'
   and c.type = 'cell';
   if c.number is null (i.e. no row found)
     display h.*
   end if
end loop;