oracle交叉口有无明显

Question

我有以下查询:

select id from t1
intersect
select id from t2
intersect
select id from t3

id在某些表中可能不是唯一的,所以我需要使用distinct.

总的来说更好的是:

select distinct id from (
select id from t1
intersect
select id from t2
intersect
select id from t3)

要么

select distinct id from t1
intersect
select id from t2 -- here id is unique
intersect
select distinct id from t3

Answer 1

没有必要DISTINCT.的INTERSECT操作者自动生成一组不同的值.正如您在此示例中所看到的,x有两行,其中一ID行为1,y只有一ID行为1 INTERSECTION,而两行中只有一行

SQL> ed
Wrote file afiedt.buf

  1  with x as (select 1 id from dual union all select 1 from dual),
  2       y as (select 1 id from dual)
  3  select id
  4    from x
  5  intersect
  6  select id
  7*   from y
SQL> /

        ID
----------
         1

即使你自己使用INTERSECT两行表,你仍然会在输出中得到一行

SQL> ed
Wrote file afiedt.buf

  1  with x as (select 1 id from dual union all select 1 from dual)
  2  select id
  3    from x
  4  intersect
  5  select id
  6*   from x
SQL> /

        ID
----------
         1