【mysql】关于子查询的一个例子
欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入 假设表my_tbl包含三个字段a,b,c;现在需要查询表中列a的每个不同值下的列b为最小值的记录量。 比如表记录为: a b c 1 3 'cd' 2 3 'nhd' 1 5 'bg' 2 6 'cds' 1 7 'kiy' 3 7 'vsd' 3 8 'ndf' 希望得到
欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入
假设表my_tbl包含三个字段a,b,c;现在需要查询表中列a的每个不同值下的列b为最小值的记录量。
比如表记录为:
a b c
1 3 'cd'
2 3 'nhd'
1 5 'bg'
2 6 'cds'
1 7 'kiy'
3 7 'vsd'
3 8 'ndf'
希望得到结果为:
a b c
1 3 'cd'
2 3 'nhd'
3 7 'vsd'
(1)
其中一个做法:先查出每个a值下的b最小值,然后根据这些最小值去查询符合要求的所有记录。
查询符合最小b值的sql写法如下:
select A.* from my_tbl as A where A.b=(select min(b) from my_tbl as B where B.a=A.a);
由于是嵌套查询和取交集,80万条记录情况下竟然用一个小时也没把中间结果算出来(我真怀疑是自己哪里写错了);后面求记录量就免谈了。
(2)
上面的方法是个灾难, 只能弃用了。
具体逻辑为:先按列a,b分组,然后选择每组中列b值最小的记录,生成结果集。
sql语句写法如下:
select a,b,c,count(a) from (select a,b,c from my_tbl group by a,b) as A group by a;
执行查询后,时间竟只用了1.1秒。
再一次证明,sql的查询策略的不同能直接导致性能上的巨大差异。

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