Question

这是有效的声明：

SELECT foo FROM table1 WHERE
  column1 IN (SELECT bar FROM table2) AND
  column2 IN (SELECT bar FROM table2)

但是有什么方法可以避免重复执行SELECT bar FROM table2语句以使其更简单，更经济？

我的意思是这样的：

SELECT foo FROM table1 WHERE
  «column1 and also column2» IN (SELECT bar FROM table2)

Answer 1

您可以与EXISTS运算符一起使用相关的子查询

select *
from table1 t1
where exists (select *
              from table2 t2
              where t2.bar in (t1.column1, t1.column2));

如果两列都应与table2.bar中的值匹配，则可以使用以下内容：

select *
from table1 t1
where exists (select *
              from table2 t2
              where t2.bar = t1.column1
                and t2.bar = t1.column2);

Answer 2

您可以使用聚合：

select *
from table1
where exists (
    select 1
    from table2
    where table2.bar in (table1.column1, table1.column2)
    having count(distinct bar) = case when table1.column1 = table1.column2 then 1 else 2 end
);

因此，如果table1包含对(foo, bar)，而table2包含值(foo), (baz)，则不会匹配。

IN条件左侧的多列

2 个答案: