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MySQL with语句讲解

作者：只是甲

这篇文章主要介绍了MySQL with语句小结,对于逻辑复杂的sql，with可以大大减少临时表的数量，提升代码的可读性、可维护性，对mysql with语句相关知识感兴趣的朋友一起看看吧

备注:测试数据库版本为MySQL 8.0

今天我们来聊聊MySQL的with语句
对于逻辑复杂的sql，with可以大大减少临时表的数量，提升代码的可读性、可维护性

MySQL 8.0终于开始支持with语句了，对于复杂查询，可以不用写那么多的临时表了。

如需要scott用户下建表及录入数据语句，可参考:
scott建表及录入数据sql脚本

语句结构:

with subquery_name1 as (subquery_body1),
        subquery_name2 as (subquery_body2)
...
select * from subquery_name1 a, subquery_name2 b
where a.col = b.col
...

优势

一.提升代码的可读性和可维护性

需求:求每个部门的平均工资,以及剔除薪资低于1000的实习人员之后的平均工资

-- 求每个部门的平均工资,以及剔除薪资低于1000的实习人员之后的平均工资
-- 主查询的from后面跟了2个临时表，程序可读性不佳
select d.deptno, tmp1.avg_sal avg_sal1, tmp2.avg_sal avg_sal2
  from dept d
  left join (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
               from emp e1
              group by e1.deptno) tmp1
    on d.deptno = tmp1.deptno
  left join (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
               from emp e1
              where e1.sal > 1000
              group by e1.deptno) tmp2
    on d.deptno = tmp2.deptno;
    
    
-- 求每个部门的平均工资,以及剔除薪资低于1000的实习人员之后的平均工资
-- 2个临时表的定时语句通过with封装成子查询了，程序可读性增强
with tmp1 as
 (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    from emp e1
   group by e1.deptno),
tmp2 as
 (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    from emp e1
   where e1.sal > 1000
   group by e1.deptno)
select d.deptno, tmp1.avg_sal avg_sal1, tmp2.avg_sal avg_sal2
  from dept d
  left join tmp1
    on d.deptno = tmp1.deptno
  left join tmp2
    on d.deptno = tmp2.deptno;
mysql> -- 求每个部门的平均工资,以及剔除薪资低于1000的实习人员之后的平均工资
mysql> -- 主查询的from后面跟了2个临时表，程序可读性不佳
mysql> select d.deptno, tmp1.avg_sal avg_sal1, tmp2.avg_sal avg_sal2
    ->   from dept d
    ->   left join (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    ->                from emp e1
    ->               group by e1.deptno) tmp1
    ->     on d.deptno = tmp1.deptno
    ->   left join (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    ->                from emp e1
    ->               where e1.sal > 1000
    ->               group by e1.deptno) tmp2
    ->     on d.deptno = tmp2.deptno;
+--------+----------+----------+
| deptno | avg_sal1 | avg_sal2 |
+--------+----------+----------+
|     10 |  2916.67 |  2916.67 |
|     20 |  2175.00 |  2518.75 |
|     30 |  1566.67 |  1690.00 |
|     40 |     NULL |     NULL |
+--------+----------+----------+
4 rows in set (0.00 sec)

mysql>
mysql>
mysql> -- 求每个部门的平均工资,以及剔除薪资低于1000的实习人员之后的平均工资
mysql> -- 2个临时表的定时语句通过with封装成子查询了，程序可读性增强
mysql> with tmp1 as
    ->  (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    ->     from emp e1
    ->    group by e1.deptno),
    -> tmp2 as
    ->  (select e1.deptno, round(avg(ifnull(e1.sal, 0)), 2) avg_sal
    ->     from emp e1
    ->    where e1.sal > 1000
    ->    group by e1.deptno)
    -> select d.deptno, tmp1.avg_sal avg_sal1, tmp2.avg_sal avg_sal2
    ->   from dept d
    ->   left join tmp1
    ->     on d.deptno = tmp1.deptno
    ->   left join tmp2
    ->     on d.deptno = tmp2.deptno;
+--------+----------+----------+
| deptno | avg_sal1 | avg_sal2 |
+--------+----------+----------+
|     10 |  2916.67 |  2916.67 |
|     20 |  2175.00 |  2518.75 |
|     30 |  1566.67 |  1690.00 |
|     40 |     NULL |     NULL |
+--------+----------+----------+
4 rows in set (0.00 sec)

mysql>

二.with递归

用with递归构造数列

-- 用with递归构造1-10的数据
with RECURSIVE c(n) as
 (select 1   union all select n + 1 from c where n < 10)
select n from c;
-- 用with递归构造1-10的数据
mysql> with RECURSIVE c(n) as
    ->  (select 1   union all select n + 1 from c where n < 10)
    -> select n from c;
+------+
| n    |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
|    6 |
|    7 |
|    8 |
|    9 |
|   10 |
+------+
10 rows in set (0.00 sec)

用with递归构造级联关系

with RECURSIVE emp2(ename,empno,mgr,lvl)
  as
   (select ename, empno, mgr, 1 lvl from emp where mgr is null
    union all
    select emp.ename, emp.empno, emp.mgr, e2.lvl+1
      from emp, emp2 e2
     where emp.mgr = e2.empno
   )
select lvl,
      concat(repeat('**',lvl),ename) nm
  from emp2
 order by lvl,ename
;
mysql> with RECURSIVE emp2(ename,empno,mgr,lvl)
    ->   as
    ->    (select ename, empno, mgr, 1 lvl from emp where mgr is null
    ->     union all
    ->     select emp.ename, emp.empno, emp.mgr, e2.lvl+1
    ->       from emp, emp2 e2
    ->      where emp.mgr = e2.empno
    ->    )
    -> select lvl,
    ->       concat(repeat('**',lvl),ename) nm
    ->   from emp2
    ->  order by lvl,ename
    -> ;
+------+---------------+
| lvl  | nm            |
+------+---------------+
|    1 | **KING        |
|    2 | ****BLAKE     |
|    2 | ****CLARK     |
|    2 | ****JONES     |
|    3 | ******ALLEN   |
|    3 | ******FORD    |
|    3 | ******JAMES   |
|    3 | ******MARTIN  |
|    3 | ******MILLER  |
|    3 | ******SCOTT   |
|    3 | ******TURNER  |
|    3 | ******WARD    |
|    4 | ********ADAMS |
|    4 | ********SMITH |
+------+---------------+
14 rows in set (0.00 sec)

到此这篇关于MySQL with语句小结的文章就介绍到这了,更多相关mysql with语句内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！

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