Question

我理解在使用外连接时JOIN子句和WHERE子句中的过滤器是不同的。让我们说我有这两张桌子。

table1
id | value
---+------
 1 |    11
 2 |    12

table2
id | value
---+------
 1 |   101

现在，如果我查询

select a.id as id1, a.value as value1, b.value as value2
from table1 as a
left join table2 on a.id=b.id and a.value=11

结果就是这样，一个额外的行，其值为1 = 12

id1 | value1 | value2
----+--------+--------
 1  |     11 |     101
 2  |     12 |    NULL

但是，如果我将过滤器放在where子句中，它会给我我想要的东西。问题是它为什么会这样？

Answer 1

左连接示例中使用的第二个条件限制将考虑连接哪些行。

select f1.id as id1, t1.value as value1, t2.value as value2
from t1
left join t2 on t1.id=t2.id AND T2.VALUE=11

t1
id | value
---+------
 1 |    11 ONLY join on this row because t1.value=11
 2 |    12

t2
id | value
---+------
 1 |   101 this has t1.id=t2.id, so it does get joined

which would produce this final result:

id1 | value1 | value 2
----+--------+--------
 1  |     11 |     101
 2  |     12 |    NULL

将谓词T2.VALUE=11移动到where子句有一系列不同的事件，如下所示：

select f1.id as id1, t1.value as value1, t2.value as value2
from t1
left join t2 on t1.id=t2.id
WHERE T2.VALUE=11

t1
id | value
---+------
 1 |    11 this row does meet t1.id=t2.id, so it gets joined
 2 |    12 this row does NOT meet t1.id=t2.id, FAILS to join

t2
id | value
---+------
 1 |   101 this row does meet t1.id=t2.id, so it gets joined

which would produce this INTERIM result:

id1 | value1 | value 2
----+--------+--------
 1  |     11 |     101
 2  |     12 |    NULL

NOW the where clause is considered

id1 | value1 | value 2
----+--------+--------
 1  |     11 |     101 T2.VALUE does equal 11 so this row will be returned
 2  |     12 |    NULL T2.VALUE does NOT = 11 so this row is NOT returned

Thus the final result is:

id1 | value1 | value 2
----+--------+--------
 1  |     11 |     101

外连接中过滤器的行为

1 个答案: