Question

我有两个表使用相同的复合键，我想用左连接加入它们。我的第一个查询是否以正确的方式执行此操作，或者第二次加入对抗B，因为在我的第二个示例中更好/更高效？我还应该提一下，这些表存在于数据库链接的两侧，所以我认为较少的连接会更好......

表A：

key1 | key2 | field
--------------------
1    | 1    | frog
1    | 2    | fish
3    | 2    | erik

表B：

key1 | key2 | otherField
---------------------------
1    | 1    | "three times"
1    | 3    | "rarely"
2    | 2    | "once"

期望的结果：

key1 | key2 | field | otherField
---------------------------------
1    | 1    | frog  | "three times"
1    | 2    | fish  | null
3    | 2    | erik  | null

方法1：

select * 
from A
left join B on (A.key1 = B.key1)
where (B.key2 = A.key2 or (B.key1 is null and B.key2 is null));

方法2：

select *
from A
left join B b1 on (A.key1 = b1.key1)
left join B b2 on (A.key2 = b2.key2)

Answer 1

只需在两个键上保持联接：

SELECT A.key1, A.key2, A.field, B.otherfield
FROM A LEFT JOIN B 
  ON (A.key1 = B.key1) 
 AND (A.key2 = B.key2);

两个SQL Double连接的最佳性能

1 个答案: