首先准备四张表A、B、C、D,
-------------------------------- A | B | C | D a b | a c | a d | d e 1 1 | 1 1 | 1 A | A 1 2 2 | 1 3 | 2 B | B 2 3 4 | 1 4 | 3 C | C 3 4 6 | 2 1 | 4 D | D 4 --------------------------------
下面我们需要用一条sql语句将A表所有的列,B表对与A表a字段相关联的列求count,通过C表,将D表与A表关联起来,得到的结果如下:
1 1 3 1 2 2 1 2 3 4 0 3 4 6 0 4
Mysql语句解决:
select A.a,A.b,IFNULL(c,0) as c,D.a from (select 1 as a,2 as b union allselect 2 as a,2 as b union allselect 3 as a,4 as b union allselect 4 as a,6 as b) as A left outer join (select a,count(a) as c from(select 1 as a,3 as c union allselect 1 as a,3 as c union allselect 1 as a,3 as c union allselect 2 as a,3 as c ) B group by a) as Bon A.a=B.a join (select 1 as a,'A' as d union allselect 2 as a,'B' as d union allselect 3 as a,'C' as d union allselect 4 as a,'D' as d ) as C on A.a=C.a join(select 1 as a,'A' as d union allselect 2 as a,'B' as d union allselect 3 as a,'C' as d union allselect 4 as a,'D' as d ) as Don D.d=C.d
每个人都需要一台速度更快、更稳定的 PC。随着时间的推移,垃圾文件、旧注册表数据和不必要的后台进程会占用资源并降低性能。幸运的是,许多工具可以让 Windows 保持平稳运行。
Copyright 2014-2025 https://www.php.cn/ All Rights Reserved | php.cn | 湘ICP备2023035733号