sql - 嵌套查询性能替代方案

Question

来自 性能透视 , 这是编写有关嵌套查询的以下查询的最佳方法吗:

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

你可以试试这个——

SELECT meg, currency
FROM
(
SELECT a.meg,a.currency, 
dense_rank() over (PARTITION BY a.emp_num ORDER BY a.sal_year desc) year_rank,
dense_rank() over (PARTITION BY a.emp_num ORDER BY a.sal_mon desc) mon_rank
FROM alt6sal a 
WHERE  a.meg_code IN (1,2)
)
WHERE year_rank = 1
AND mon_rank = 1;