Question

我有这个选择，我想优化它。

我想知道我是否可以使用所有这UNION ALL只用一个where而不是一直重复这个，使mysql扫描表4次而不是1次

select id from sells
where user_id in (select fv from favorite where user =?)
union all
select id from likes
where user_id in (select fv from favorite where user =?)
union all
select id from favorites
where user_id in (select fv from favorite where user =?)
union all
select id from comments
where user_id in (select fv from favorite where user =?)

有可能吗？我怎么能改变它？

Answer 1

select id,user_id from(
    select id,user_id from sells
    union all
    select id,user_id from likes
    union all
    select id,user_id from favorites
    union all
    select id,user_id from comments
) as t
where user_id in (select fv from favorite where user =?)

Answer 2

你可以这样做：

select user_id
from (select user_id from sells union all
      select user_id from likes union all
      select user_id from favorites union all
      select user_id from comments
     ) x
where user_id in (select fv from favirote where user = ?);

然而，由于表现，我会劝阻这一点。有两个点击。首先，子查询已实现，这会减慢处理速度。更重要的是，子查询不利用索引，进一步降低查询速度。

您的版本可能是最合理的，假设您有适当的索引（在所有user_id列和fv上）。

此外，如果您不想重复，请使用union代替union all。我通常提倡union all，但这似乎是需要重复删除的情况。

最有效的方法可能就是：

select f.fv
from favorites f
where f.user = ? and
      (exists (select 1 from sells s where s.user_id = f.fv) or
       exists (select 1 from likes l where l.user_id = f.fv) or
       exists (select 1 from favorites f where s.user_id = f.fv) or
       exists (select 1 from comments c where s.user_id = c.fv) 
      );

这可以使用索引，不需要额外的开销。

只使用一个UNION ALL选择的地方

2 个答案: