mysql 递归函数with recursive的用法

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AS 用法:

AS在mysql用来给列/表起别名.
有时,列的名称是一些表达式,使查询的输出很难理解。要给列一个描述性名称,可以使用列别名。
要给列添加别名,可以使用AS关键词后跟别名

例子1:

SELECT 
 [column_1 | expression] AS col_name
FROM table_name;

如果别名包含空格,则必须引用以下内容:

例子2:

SELECT 
 [column_1 | expression] AS 'col name'
FROM table_name;

with(Common Table Expressions/CTE)用法:

with在mysql中被称为公共表达式,可以作为一个临时表然后在其他结构中调用.如果是自身调用那么就是后面讲的递归.

语法:

with_clause:
    WITH [RECURSIVE]
        cte_name [(col_name [, col_name] ...)] AS (subquery)
        [, cte_name [(col_name [, col_name] ...)] AS (subquery)] ...

cte_name :公共表达式的名称,可以理解为表名,用来表示as后面跟着的子查询
col_name :公共表达式包含的列名,可以写也可以不写

例子1:

WITH
  cte1 AS (SELECT a, b FROM table1),
  cte2 AS (SELECT c, d FROM table2)
SELECT b, d FROM cte1 JOIN cte2
WHERE cte1.a = cte2.c;

例子2:

WITH cte (col1, col2) AS
(
  SELECT 1, 2
  UNION ALL
  SELECT 3, 4
)
SELECT col1, col2 FROM cte;

例子3:

这里的第一个as后面接的是子查询,第二个as表示列名,而不是子查询.

WITH cte AS
(
  SELECT 1 AS col1, 2 AS col2
  UNION ALL
  SELECT 3, 4
)
SELECT col1, col2 FROM cte;

with的合法用法:

  • 在子查询(包括派生的表子查询)的开始处
SELECT ... WHERE id IN (WITH ... SELECT ...) ...
SELECT * FROM (WITH ... SELECT ...) AS dt ...
  • 同一级别只允许一个WITH子句。同一级别的WITH后面跟着WITH是不允许的,下面是非法用法:
WITH cte1 AS (...) WITH cte2 AS (...) SELECT ...

改为合法用法:

WITH cte1 AS (SELECT 1)
SELECT * FROM (WITH cte2 AS (SELECT 2) SELECT * FROM cte2 JOIN cte1) AS dt;

在这里面as代表列名,sql不是顺序执行的,这一点了解的话就很好理解这个as了

简单递归用法:

首先我们引出一个问题: 什么叫做递归?
递归:给定函数初始条件,然后反复调用自身直到终止条件.

例子1:递归得到依次递增的序列:

WITH RECURSIVE cte (n) AS
(
  SELECT 1
  UNION ALL
  SELECT n + 1 FROM cte WHERE n < 5
)
SELECT * FROM cte;

运行结果:

+------+
| n    |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
+------+

官方文档中对于这个写法的解释:

At each iteration, that SELECT produces a row with a new value one greater than the value of n from the previous row set. The first iteration operates on the initial row set (1) and produces 1+1=2; the second iteration operates on the first iteration’s row set (2) and produces 2+1=3; and so forth. This continues until recursion ends, which occurs when n is no longer less than 5.

用python实现就是:

def cte(n):
	print(n)
	if n<5:
		cte(n+1)

也就是说,一个with recursive 由两部分组成.第一部分是非递归部分(union all上方),第二部分是递归部分(union all下方).递归部分第一次进入的时候使用非递归部分传递过来的参数,也就是第一行的数据值,进而得到第二行数据值.然后根据第二行数据值得到第三行数据值.

例子2:递归得到不断复制的字符串

这里的as表示列名,表示说这个CTE有两个列,也可以写为with cte(n,str) as (subquery)

WITH RECURSIVE cte AS
(
  SELECT 1 AS n, 'abc' AS str
  UNION ALL
  SELECT n + 1, CONCAT(str, str) FROM cte WHERE n < 3
)
SELECT * FROM cte;

结果:

+------+------+
| n    | str  |
+------+------+
|    1 | abc  |
|    2 | abc  |
|    3 | abc  |
+------+------+

这里的话concat是每一次都连接一个str,这个str来自上一行的结果,但是最终输出却是每一行都没有变化的值,这是为什么?
这是因为我们在声明str的时候限制了它的字符长度,使用 类型转换CAST(‘abc’ AS CHAR(30)) 就可以得到复制的字符串了.
**注意:**这里也可能会报错,看mysql模式.在严格模式下这里会显示Error Code: 1406. Data too long for column 'str' at row 1
关于strict SQL mode和nonstrict SQL mode:mysql 严格模式 Strict Mode说明

WITH RECURSIVE cte AS
(
  SELECT 1 AS n, CAST('abc' AS CHAR(20)) AS str
  UNION ALL
  SELECT n + 1, CONCAT(str, str) FROM cte WHERE n < 3
)
SELECT * FROM cte;
+------+--------------+
| n    | str          |
+------+--------------+
|    1 | abc          |
|    2 | abcabc       |
|    3 | abcabcabcabc |
+------+--------------+

当然,如果上一行的值有多个,我们还可以对多个值进行重新组合得到我们想要的结果,比如下面这个例子.

例子3:生成斐波那契数列

WITH RECURSIVE fibonacci (n, fib_n, next_fib_n) AS
(
  SELECT 1, 0, 1
  UNION ALL
  SELECT n + 1, next_fib_n, fib_n + next_fib_n
    FROM fibonacci WHERE n < 10
)
SELECT * FROM fibonacci;

结果:

+------+-------+------------+
| n    | fib_n | next_fib_n |
+------+-------+------------+
|    1 |     0 |          1 |
|    2 |     1 |          1 |
|    3 |     1 |          2 |
|    4 |     2 |          3 |
|    5 |     3 |          5 |
|    6 |     5 |          8 |
|    7 |     8 |         13 |
|    8 |    13 |         21 |
|    9 |    21 |         34 |
|   10 |    34 |         55 |
+------+-------+------------+

语法说明:

UNION ALL与UNION DISTINCT

  • UNION ALL:
    非递归部分和递归部分用UNION ALL分隔,那么所有的行都会被加入到最后的表中
  • UNION DISTINCT:
    非递归部分和递归部分用UNION DISTINCT分隔,重复的行被消除。这对于执行传递闭包的查询非常有用,以避免无限循环。

limit控制递归次数

recursive(第二个select)不能使用的结构:

官网的描述:

  • The recursive SELECT part must not contain these constructs:

    Aggregate functions such as SUM()
    
    Window functions
    
    GROUP BY
    
    ORDER BY
    
    DISTINCT
    

限制递归次数/时间:

当出现不符合设置情况的会报错,分为以下几种设置方法:

  • cte_max_recursion_depth :default 设置为1000,表达递归的层数.可以使用如下语句修改这个值:
SET SESSION cte_max_recursion_depth = 10;      -- permit only shallow recursion
SET SESSION cte_max_recursion_depth = 1000000; -- permit deeper recursion

当然也可以设置为global,也就是set global cte_max_recursion_depth = 1000000;这样子就对全局的递归都有限制

  • max_execution_time :设置最近的递归时间
SET max_execution_time = 1000; -- impose one second timeout
  • MAX_EXECUTION_TIME:设置全局的递归时间

官网文档说明如下:

  • The cte_max_recursion_depth system variable enforces a limit on the
    number of recursion levels for CTEs. The server terminates execution
    of any CTE that recurses more levels than the value of this variable.

  • The max_execution_time system variable enforces an execution timeout
    for SELECT statements executed within the current session.

  • The MAX_EXECUTION_TIME optimizer hint enforces a per-query execution
    timeout for the SELECT statement in which it appears.

  • limit:限之最大行的数量

WITH RECURSIVE cte (n) AS
(
  SELECT 1
  UNION ALL
  SELECT n + 1 FROM cte LIMIT 10000
)
SELECT * FROM cte;

mysql with recursive 递归用法

with recursive 是一个递归的查询子句,他会把查询出来的结果再次代入到查询子句中继续查询。

语法:

WITH RECURSIVE  cte_name  AS ( 
	初始语句(非递归部分) 
	UNION ALL 
	递归部分语句
)
[ SELECT| INSERT | UPDATE | DELETE]

例子1:

WITH RECURSIVE cte (n) AS
(
  SELECT 1
  UNION ALL
  SELECT n + 1 FROM cte WHERE n < 5
)
SELECT * FROM cte;


该语句书写包括如下几步

  1. 设定递归语法,首先初始执行第一句SELECT 1,也可以写成select xxx from xxx where xxx
  2. 其结果给到n,当n值发生改变,就会执行:SELECT n + 1 FROM cte WHERE n < 5
  3. 最终结果给到n输出

注意:WITH AS () 后面必须跟着 [ SELECT| INSERT | UPDATE | DELETE] 语句,否则报错。

例子2:

有如下行政区划表

create table administrative_division2
(
    administrative_division_sn        bigint            not null comment '行政区划代码'
        primary key,
    parent_administrative_division_sn bigint            null comment '上级行政区划代码',
    status                            tinyint default 1 not null comment '状态',
    name                              varchar(28)       not null comment '名称*',
    full_name                         varchar(200)      null comment '行政区划全路径名称,省市县全路径名称'
)
    comment '行政区划';

INSERT INTO administrative_division2 (administrative_division_sn, parent_administrative_division_sn, status, name, full_name) VALUES (110000, null, 1, '北京市', '');
INSERT INTO administrative_division2 (administrative_division_sn, parent_administrative_division_sn, status, name, full_name) VALUES (110100, 110000, 1, '北京市市辖区', '');
INSERT INTO administrative_division2 (administrative_division_sn, parent_administrative_division_sn, status, name, full_name) VALUES (110101, 110100, 1, '东城区', '');
INSERT INTO administrative_division2 (administrative_division_sn, parent_administrative_division_sn, status, name, full_name) VALUES (110102, 110100, 1, '西城区', '');
INSERT INTO administrative_division2 (administrative_division_sn, parent_administrative_division_sn, status, name, full_name) VALUES (110105, 110100, 1, '朝阳区', '');


我们想补充full_name的值变成如下

那么可以用如下语句

update administrative_division2 ad,(
    with recursive res as (select administrative_division_sn sn, parent_administrative_division_sn pSn, name
                                       from administrative_division2 ad
                                       where parent_administrative_division_sn is null 
                                       union
                                       select ad2.administrative_division_sn as     sn,
                                              ad2.parent_administrative_division_sn pSn,
                                              concat(res.name, ad2.name)     as     name
                                       from res
                                                inner join administrative_division2 ad2
                                                           on res.sn = ad2.parent_administrative_division_sn)
    select * from res
    ) as t1 set ad.full_name=t1.name where ad.administrative_division_sn=t1.sn;

该语句执行解析如下

  1. 先设定开始的条件为父ID为空,即所有的父节点
select administrative_division_sn sn, parent_administrative_division_sn pSn, name from administrative_division2 ad where parent_administrative_division_sn is null
  1. 设置循环的条件为等于这个父节点的子节点
select ad2.administrative_division_sn as sn, ad2.parent_administrative_division_sn pSn, concat(res.name, ad2.name) as name from res inner join administrative_division2 ad2 on res.sn = ad2.parent_administrative_division_sn
  1. 执行完2此时的res.sn就等于子sn了,然后递归执行2
  2. 最终执行set ad.full_name=t1.name where ad.administrative_division_sn=t1.sn把名称赋值为2中的拼接了的名称concat(res.name, ad2.name)

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