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;