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：

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

WITH cta1 AS (SELECT sum(k) from sbtest1 where id < 100) ,  
           cta2 AS (SELECT SUM(k) from sbtest2 WHERE id < 100) 
SELECT * FROM cta1 JOIN cta2 ;
+----------+----------+
| sum(k)   | SUM(k)   |
+----------+----------+
| 49529621 | 49840812 |
+----------+----------+
1 row in set (0.00 sec)

递归CTE示例：

root@sb1 09:41:34>WITH RECURSIVE cte (n) AS
    -> (
    ->   SELECT 1
    ->   UNION ALL
    ->   SELECT n + 1 FROM cte WHERE n < 5
    -> )
    -> SELECT * FROM cte;
+------+
| n    |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
+------+
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

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
UNION ALL
SELECT ....返回更多的值，并定义退出循环条件，这里引用了cte自身

其实现类似于：

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

递归的部分不可以包含:

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

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

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;

+------+-------+------------+
| 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 |
+------+-------+------------+
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:

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

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;
ERROR 3636 (HY000): Recursive query aborted after 1001 iterations. Try increasing @@cte_max_recursion_depth to a larger value.

如何实现

前文已经说过，笔者对Server层代码了解不多，这里只做简单的记录

主要提交的代码

想看实现思路可以阅读如下两个worklog：
WL#883: Non-recursive WITH clause (Common Table Expression)
WL#3634: Recursive WITH (Common Table Expression)

参考文档

官方文档
A Definitive Guide To MySQL Recursive CTE
An Introduction to MySQL CTE
MySQL | Recursive CTE (Common Table Expressions)
之前的月报文章