有人可以帮我优化这个查询吗？

Question

我有三张桌子A, B, C。 我必须从A rollNumber = 11获取SName 和来自Bname的{​​{1}} B 然后BCode = 10

我写了这个查询：

select C.SName,C.Bname, C.ID from C where SName = "one obtained from A table" and BName = "one obtained from B table"

Answer 1

SELECT C.SName, C.BName, C.ID
FROM C
INNER JOIN (SELECT * FROM A WHERE rollNumber='11') as t1
ON t1.SName = C.SName
INNER JOIN (SELECT * FROM B WHERE Bcode='10') as t2
ON t2.BName = C.Bname;

您可以执行的一项优化是在执行连接之前先过滤到较小的临时表。这将导致JOIN中的行数少得多。另一个优化是索引JOIN ... ON子句和WHERE子句中涉及的所有列。