如何优化这条sql

Question

写了一条sql，但是这条sql性能好差。

select *
  from (select t1.*,
               t2.*,
               t3.*,
               (select t4.b from t4 where t1.a = t4.a),
               '1' haha
          from t1, t2, t3
        
         where t1.c = t2.c
           and t1.c = t3.c
           and exists (select * from t3 where t3.c = t1.c)
           and exists (select * from t2 where t2.c = t1.c)
        
        union
        select t1.*,
               t2.*,
               t3.*,
               (select t4.b from t4 where t1.a = t4.a),
               '2' haha
          from t1, t2, t3
        
         where t1.c = t2.c
           and t1.c = t3.c
           and not exists (select * from t3 where t3.c = t1.c)
           and exists (select * from t2 where t2.c = t1.c)
        
        union
        select t1.*,
               t2.*,
               t3.*,
               (select t4.b from t4 where t1.a = t4.a),
               '3' haha
          from t1, t2, t3
        
         where t1.c = t2.c
           and t1.c = t3.c
           and exists (select * from t3 where t3.c = t1.c)
           and not exists (select * from t2 where t2.c = t1.c))t

请教，这个sql还有更好的写法吗？

Answer 1

看不懂你要查什么。多表查询 用关联查询，inner join left join 什么的。如果想判断某表有没有值，可以用这个表 left join 主表，如果关联不到就是没有