MySQL8.0的WITH查询详情

关于MySQL8的WITH查询学习

前言：

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。
可以查看官方文档【点击跳转】

1、示例

官方第一个示例，可以看出该查询语句创建了cte1,cte2,cte3,cte4这4个临时表，后面的临时表依赖前面的临时表数据。
最后一行为最终查询结果，实际ct4因为ct3结果包含3行数据，但是使用MAX,MIN得到一行结果。

WITH cte1(txt) AS (SELECT "This "),     cte2(txt) AS (SELECT CONCAT(cte1.txt,"is a ") FROM cte1),     cte3(txt) AS (SELECT "nice query" UNION                   SELECT "query that rocks" UNION                   SELECT "query"),     cte4(txt) AS (SELECT concat(cte2.txt, cte3.txt) FROM cte2, cte3)SELECT MAX(txt), MIN(txt) FROM cte4; +----------------------------+----------------------+| MAX(txt)                   | MIN(txt)             |+----------------------------+----------------------+| This is a query that rocks | This is a nice query |+----------------------------+----------------------+1 row in set (0,00 sec)

WITH RECURSIVE my_cte AS(  SELECT 1 AS n  UNION ALL  SELECT 1+n FROM my_cte WHERE n<10)SELECT * FROM my_cte; +------+| n    |+------+|    1 ||    2 ||    3 ||    4 ||    5 ||    6 ||    7 ||    8 ||    9 ||   10 |+------+10 rows in set (0,00 sec)

这个是将临时表列名指定在第一行WITH RECURSIVE my_cte(a,b,c) AS(  SELECT 1,1,1  UNION ALL  SELECT 1+a,2+b,3+c FROM my_cte WHERE a<10)SELECT * FROM my_cte; 这个第一行没有指定列名，然后列名由第一个查询返回结果确定WITH RECURSIVE my_cte AS(  SELECT 1 AS a,1 AS b,1 AS c  UNION ALL  SELECT 1+a,2+b,3+c FROM my_cte WHERE a<10)SELECT * FROM my_cte;

WITH RECURSIVE cte_name [list of column names ] AS(  SELECT ...      <-- specifies initial set  UNION ALL  SELECT ...      <-- specifies initial set  UNION ALL  ...  SELECT ...      <-- specifies how to derive new rows  UNION ALL  SELECT ...      <-- specifies how to derive new rows  ...)[, any number of other CTE definitions ]

3、练习

关于递归的练习主要用于表里面包含父节点id之类的，详情可以参考下面的练习。
定义下面这样的表，存储每个区域（省、市、区）的id，名字及上级区域的pid

 CREATE TABLE tb(id VARCHAR(3), pid VARCHAR(3), name VARCHAR(64)); INSERT INTO tb VALUES('002', 0, '浙江省');INSERT INTO tb VALUES('001', 0, '广东省');INSERT INTO tb VALUES('003', '002', '衢州市');INSERT INTO tb VALUES('004', '002', '杭州市');INSERT INTO tb VALUES('005', '002', '湖州市');INSERT INTO tb VALUES('006', '002', '嘉兴市');INSERT INTO tb VALUES('007', '002', '宁波市');INSERT INTO tb VALUES('008', '002', '绍兴市');INSERT INTO tb VALUES('009', '002', '台州市');INSERT INTO tb VALUES('010', '002', '温州市');INSERT INTO tb VALUES('011', '002', '丽水市');INSERT INTO tb VALUES('012', '002', '金华市');INSERT INTO tb VALUES('013', '002', '舟山市');INSERT INTO tb VALUES('014', '004', '上城区');INSERT INTO tb VALUES('015', '004', '下城区');INSERT INTO tb VALUES('016', '004', '拱墅区');INSERT INTO tb VALUES('017', '004', '余杭区');INSERT INTO tb VALUES('018', '011', '金东区');INSERT INTO tb VALUES('019', '001', '广州市');INSERT INTO tb VALUES('020', '001', '深圳市'); WITH RECURSIVE cte AS ( SELECT id,name FROM tb WHERE id='002' UNION ALL SELECT k.id, CONCAT(c.name,'->',k.name) AS name FROM tb k INNER JOIN cte c ON c.id = k.pid) SELECT * FROM cte;

分析结果包含第一行SELECT id,name FROM tb WHERE id='002'的数据，此时表中只有一行数据
然后连表查询SELECT k.id, CONCAT(c.name,'->',k.name) AS name FROM tb k INNER JOIN cte c ON c.id = k.pid，递归的将父节点数据放入临时表
最终查询出来的就是递归的结果。

关于MySQL8的WITH查询学习

1、示例

3、练习

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