Question

我想知道是否有办法将自我子查询转换为自联接这是自我子查询

SELECT a, 
       b 
FROM   c AS t1 
WHERE  ( b IN (SELECT b 
               FROM   c AS t2 
               WHERE  ( t1.b = b ) 
                      AND ( t1.e <> e )) )

Answer 1

如果您只想找到重复项，EXIST可能会更快：

SELECT a,b FROM c WHERE EXISTS(SELECT NULL FROM c c2 WHERE c2.b=c.b AND c2.e<>c.e)

如果您希望将每个记录与其副本一起加入，但每个记录只能获得一条记录：

select  t1.a
,       t1.b
,       t1.e as t1e
,       t2.e as t2e
from    c as t1
inner join c as t2
on      t1.b = t2.b 
        and t1.e > t2.e

（请注意，我使用了>代替<>）

Answer 2

由于e是主键，另一种接近此方法的方法是

SELECT a, 
       b 
FROM   (SELECT a, 
               b, 
               COUNT(*) OVER (PARTITION BY b) AS Cnt 
        FROM   c) T1 
WHERE  Cnt > 1

Answer 3

SELECT t1.a, t2.b
FROM c as t1
join c as t2 on t1.b=t2.b
WHERE t1.e <> t2.e

Answer 4

select  t1.a
,       t1.b
from    c as t1
join    c as t2
on      t1.b = t2.b 
        and t1.e <> t2.e

将自子查询转换为自联接

4 个答案: