nested query performance alternatives

Question

From performance perspective , is this the best way to write the following query concerning the nested query :

---

SELECT a.meg,a.currency
FROM alt6sal a 
WHERE  a.meg_code IN (1,2)
AND a.sal_year = (SELECT MAX(ia.sal_year) FROM alt6sal ia WHERE a.emp_num = ia.emp_num )
AND a.sal_mon = (SELECT  MAX(ia.sal_mon) FROM alt6sal ia  WHERE a.emp_num = ia.emp_num AND a.sal_year = ia.sal_year)

Answer 1

如果可以避免相关子查询，性能就更好，非相关子查询的示例：

SELECT a.meg,a.currency
FROM alt6sal a 

join 
(
    select ia.emp_num, max(ia.sal_year) as sal_year_max
    from alt6sal ia
    group by ia.emp_num
) the_year_max
on a.emp_num =  the_year_max.emp_num and a.sal_year = the_year_max.sal_year_max

join 
(
    select ia.emp_num, ia.sal_year, max(ia.sal_mon) as sal_mon_max
    from alt6sal ia
    group by ia.emp_num, ia.sal_year
) the_month_max
on a.emp_num = the_month_max.emp_num and a.sal_year = the_month_max.sal_year
and a.sal_mon = the_month_max.sal_mon_max

WHERE  a.meg_code IN (1,2)

类似的非相关连接用于 OR 而不是 AND，使用 LEFT JOIN 然后过滤非空

SELECT a.meg,a.currency
FROM alt6sal a 

left join 
(
    select ia.emp_num, max(ia.sal_year) as sal_year_max
    from alt6sal ia
    group by ia.emp_num
) the_year_max
on a.emp_num =  the_year_max.emp_num and a.sal_year = the_year_max.sal_year_max

left join 
(
    select ia.emp_num, ia.sal_year, max(ia.sal_mon) as sal_mon_max
    from alt6sal ia
    group by ia.emp_num, ia.sal_year
) the_month_max
on a.emp_num = the_month_max.emp_num and a.sal_year = the_month_max.sal_year
and a.sal_mon = the_month_max.sal_mon_max

WHERE  a.meg_code IN (1,2)
       and 
       (the_year_max.ia_emp_num is not null 
        or the_month_max.ia_emp_num is not null)