数据库内核月报－ 2019/07 - MySQL · 引擎特性 · CTE(Common Table Expressions) - 《数据库内核月报》

前言
示例
如何实现
参考文档

前言

CTE也就是common table expressions是sql标准里的语法，很多数据库都能够支持，MySQL也在8.0版本里加入了CTE功能，本文主要简单的介绍下该语法的用法，由于笔者对server层了解不深，本文不探讨代码层

CTE与derived table最大的不同之处是

可以自引用，递归使用（recursive cte
在语句级别生成独立的临时表. 多次调用只会执行一次
一个cte可以引用另外一个cte
一个CTE语句其实和CREATE [TEMPORARY] TABLE类似，但不需要显式的创建或删除，也不需要创建表的权限。更准确的说，CTE更像是一个临时的VIEW

示例

语法：


1. with_clause:
2. WITH [RECURSIVE]
3. cte_name [(col_name [, col_name] ...)] AS (subquery)
4. [, cte_name [(col_name [, col_name] ...)] AS (subquery)] ...

一条语句里可以创建多个cte，用逗号隔开:


1. WITH cta1 AS (SELECT sum(k) from sbtest1 where id < 100) ,
2. cta2 AS (SELECT SUM(k) from sbtest2 WHERE id < 100)
3. SELECT * FROM cta1 JOIN cta2 ;
4. +----------+----------+
5. | sum(k)   | SUM(k)   |
6. +----------+----------+
7. | 49529621 | 49840812 |
8. +----------+----------+
9. 1 row in set (0.00 sec)

递归CTE示例：


1. root@sb1 09:41:34>WITH RECURSIVE cte (n) AS
2. -> (
3. ->   SELECT 1
4. ->   UNION ALL
5. ->   SELECT n + 1 FROM cte WHERE n < 5
6. -> )
7. -> SELECT * FROM cte;
8. +------+
9. | n    |
10. +------+
11. |    1 |
12. |    2 |
13. |    3 |
14. |    4 |
15. |    5 |
16. +------+
17. 5 rows in set (0.00 sec)

递归CTE需要加RECURSIVE关键字，使用Union all来产生结果


1. SELECT ...定义初始化值，不引用自身, 同时初始化值的列也定义了cte上的列的个数和类型，可以用cast重定义
2. UNION ALL
3. SELECT ....返回更多的值，并定义退出循环条件，这里引用了cte自身

5. 其实现类似于：

7. - non-recursive query block is evaluated, result goes into an internal tmp table
8. - if no rows, exit
9. - (A): recursive query block is evaluated over the tmp table's lastly inserted
10. rows, and it produces new rows which are appended to the tmp table (if UNION
11. ALL; only distinct not-already-there rows if UNION DISTINCT)
12. - if the last step didn't produce new rows, exit
13. - goto (A)

递归的部分不可以包含:


1. Aggregate functions such as SUM()
2. Window functions
3. GROUP BY
4. ORDER BY
5. LIMIT
6. DISTINCT

再举个典型的斐波拉契数(Fibonacci Series Generation)


1. WITH RECURSIVE fibonacci (n, fib_n, next_fib_n) AS
2. (
3. SELECT 1, 0, 1
4. UNION ALL
5. SELECT n + 1, next_fib_n, fib_n + next_fib_n
6. FROM fibonacci WHERE n < 10
7. )
8. SELECT * FROM fibonacci;

10. +------+-------+------------+
11. | n    | fib_n | next_fib_n |
12. +------+-------+------------+
13. |    1 |     0 |          1 |
14. |    2 |     1 |          1 |
15. |    3 |     1 |          2 |
16. |    4 |     2 |          3 |
17. |    5 |     3 |          5 |
18. |    6 |     5 |          8 |
19. |    7 |     8 |         13 |
20. |    8 |    13 |         21 |
21. |    9 |    21 |         34 |
22. |   10 |    34 |         55 |
23. +------+-------+------------+
24. 10 rows in set (0.00 sec)

关于递归的深度，除了自定义推出条件外，为了避免无限递归，也定义了一个系统参数cte_max_recursion_depth来限制深度，默认值为1000:


1. root@sb1 09:53:31>SELECT @@SESSION.cte_max_recursion_depth;
2. +-----------------------------------+
3. | @@SESSION.cte_max_recursion_depth |
4. +-----------------------------------+
5. |                              1000 |
6. +-----------------------------------+
7. 1 row in set (0.01 sec)

9. root@sb1 09:53:42>WITH RECURSIVE cte (n) AS (   SELECT 1   UNION ALL   SELECT n + 1 FROM cte WHERE n  < 1001) SELECT * FROM cte;
10. ERROR 3636 (HY000): Recursive query aborted after 1001 iterations. Try increasing @@cte_max_recursion_depth to a larger value.