sql - 如何用非相关子查询替换相关子查询？

Question

由于相关子查询，我有一个性能较差的查询，我想替换它 非相关子查询。如何做到这一点:

我的查询:

select a.emp_num, a.name , b.cont_date 
from main_emp a INNER JOIN main_cont b
ON a.emp_num = b.emp_num AND a.calc_year = b.calc_year 

join 
(
    select emp_num,calc_year, max(bb.cont_date) AS max_date from main_cont bb
    GROUP BY emp_num,calc_year
)   bb_max
on a.emp_num =  bb_max.emp_num and a.calc_year = bb_max.calc_year and b.cont_date = bb_max.max_date

where
( 0 = ( select count(*) from main_serv x where x.emp_num = a.emp_num and x.calc_year = a.calc_year ) 
    or b.cont_date >  ( select max(y.ser_date) from main_serv y where y.emp_num = a.emp_num and y.calc_year = a.calc_year) ) -- The problem here
and a.calc_year = 2015
order by 1

---

现在我想转换这个子查询:

  ( 0 = ( select count(*) from main_serv x where x.emp_num = a.emp_num and x.calc_year = a.calc_year ) 
        or b.cont_date >  ( select max(y.ser_date) from main_serv y where y.emp_num = a.emp_num and y.calc_year = a.calc_year) ) 

至

像这样加入:

 join 
    (
        select emp_num,calc_year, max(bb.cont_date) AS max_date from main_cont bb
        GROUP BY emp_num,calc_year
    )   bb_max
    on a.emp_num =  bb_max.emp_num and a.calc_year = bb_max.calc_year and b.cont_date = bb_max.max_date

但我不知道如何执行此操作，因为我有 ((0 =(subquery) OR (subquery))

Answer 1

我相信你的 WHERE 子句的这一部分

( 0 = ( select count(*) from main_serv x where x.emp_num = a.emp_num and x.calc_year = a.calc_year ) 
    or b.cont_date >  ( select max(y.ser_date) from main_serv y where y.emp_num = a.emp_num and y.calc_year = a.calc_year) )

可以呈现为

There are no rows in main_serv
OR
There are no rows in main_serv where ser_date is equal to or greater than b.cont_date.

我相信析取的第二部分涵盖了完整的条件，因为当 main_serv 中没有行时，肯定不会有与条件的第二部分匹配的行，并且如果有main_serv 中的任何行，它将是确定结果的条件的第二部分。因此，我将重写整个析取部分，如下所示:

not exists (
  select *
  from main_serv as x
  where x.emp_num = a.emp_num
    and x.calc_year = a.calc_year
    and x.ser_date >= b.cont_date
)

此子查询仍然与主查询相关，但它不会执行任何聚合，并且总体上可能比您的版本有所改进。