外連接中的過濾器的行爲

Question

我瞭解JOIN子句中的過濾器，並且在使用外連接時WHERE子句中的過濾器不同。假設我有這兩張桌子。

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

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

現在，如果我查詢

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 

結果是這樣的，一個額外的一行值1 = 12

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

但是，如果我把過濾器where子句中，它給我我想要的。問題是爲什麼它的行爲如此？

Answer 1

左連接示例中使用的第二個條件限制哪些行將被考慮加入。

 
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 

移動謂詞T2.VALUE=11 where子句具有不同的一系列事件，如下：

 
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