1、CTE简介
MySQL从8.0开始支持CTE，慢慢地向Oracle学习，CTE确实是个很好用的东西，特别是针对OLAP类型的SQL,可以大大简化，优化SQL.
那么什么是CTE呢？
个人理解：CTE(common table expression)是一个临时的结果集，类似一个函数，一旦定义好，可以多次调用。
2、CTE语法
with_clause:
WITH [RECURSIVE]
cte_name [(col_name [, col_name] ...)] AS (subquery)
[, cte_name [(col_name [, col_name] ...)] AS (subquery)] ...(1)列别名可以在不同的位置定义
mysql> WITH cte (col1, col2) AS
-> (
-> SELECT 1, 2
-> UNION ALL
-> SELECT 3, 4
-> )
-> SELECT col1, col2 FROM cte;
+------+------+
| col1 | col2 |
+------+------+
| 1 | 2 |
| 3 | 4 |
+------+------+
2 rows in set (0.00 sec)
等价与：
mysql> WITH cte AS
-> (
-> SELECT 1 AS col1, 2 AS col2
-> UNION ALL
-> SELECT 3, 4
-> )
-> SELECT col1, col2 FROM cte;
+------+------+
| col1 | col2 |
+------+------+
| 1 | 2 |
| 3 | 4 |
+------+------+
2 rows in set (0.00 sec)(2) CTE用在Select操作
mysql> create table t1(a int,b int);
mysql> insert into t1 values(1,1),(2,2),(3,3);
mysql> with t as
(select a+2 c,b from t1)
select c,b from t;
+------+------+
| c | b |
+------+------+
| 3 | 1 |
| 4 | 2 |
| 5 | 3 |
+------+------+
3 rows in set (0.00 sec)(3)CTE用在DML操作
mysql> with t as (select a+2 as a,b from t1) update t1,t set t1.a=t.a+10 where t1.a=t.a;
mysql> select * from t1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 13 | 3 |
+------+------+
3 rows in set (0.00 sec)
mysql> with t as (select a+2 as a,b from t1) delete t1 from t1,t where t1.a=t.a;
mysql> select * from t1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 2 | 2 |
+------+------+
2 rows in set (0.00 sec)
mysql> insert into t1 with t as (select 10*a as a,b from t1) select * from t;
mysql> select * from t1;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 10 | 1 |
| 20 | 2 |
| 30 | 3 |
+------+------+
6 rows in set (0.00 sec)3、CTE可以优化SQL
(1)下面第一条SQL可以改写成如下两种CTE简化形式
mysql> select count(*) from employees e1
left join (select * from employees) e2 on e1.emp_no=e2.emp_no
left join (select * from employees) e3 on e2.emp_no=e3.emp_no;
mysql> with e2 as (select * from employees),
e3 as (select * from employees)
select count(*) from employees e1
left join e2 on e1.emp_no=e2.emp_no
left join e3 on e2.emp_no=e3.emp_no;
mysql> with e as(select * from employees) select count(*) from employees e1
left join e e2 on e1.emp_no=e2.emp_no
left join e e3 on e2.emp_no=e3.emp_no;(2)CTE的本质是子查询，所以子查询的一些特性都适用，如子查询合并。
mysql> desc with e as(select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from employees)
-> select count(*) from employees e1
-> left join e e2 on e1.emp_no=e2.emp_no
-> left join e e3 on e2.emp_no=e3.emp_no;
+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
| 1 | PRIMARY | e1 | NULL | index | NULL | PRIMARY | 4 | NULL | 299512 | 100.00 | Using index |
| 1 | PRIMARY | | NULL | ref | | | 4 | testdb.e1.emp_no | 10 | 100.00 | NULL |
| 1 | PRIMARY | | NULL | ref | | | 4 | e2.emp_no | 10 | 100.00 | NULL |
| 2 | DERIVED | employees | NULL | ALL | NULL | NULL | NULL | NULL | 299512 | 100.00 | NULL |
+----+-------------+------------+------------+-------+---------------+-------------+---------+------------------+--------+----------+-------------+
4 rows in set, 2 warnings (0.00 sec) (3)CTE可以起到减少插入临时表数据，优化SQL的作用
mysql> flush status;
Query OK, 0 rows affected (0.02 sec)
mysql> select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from
-> (select * from t_group) t1
-> join (select * from t_group) t2
-> on t1.emp_no=t2.emp_no;
+--------+---------+------------+------------+--------+---------+------------+------------+
| emp_no | dept_no | from_date | to_date | emp_no | dept_no | from_date | to_date |
+--------+---------+------------+------------+--------+---------+------------+------------+
| 22744 | d006 | 1986-12-01 | 9999-01-01 | 22744 | d006 | 1986-12-01 | 9999-01-01 |
| 24007 | d005 | 1986-12-01 | 9999-01-01 | 24007 | d005 | 1986-12-01 | 9999-01-01 |
| 30970 | d005 | 1986-12-01 | 2017-03-29 | 30970 | d005 | 1986-12-01 | 2017-03-29 |
| 31112 | d002 | 1986-12-01 | 1993-12-10 | 31112 | d002 | 1986-12-01 | 1993-12-10 |
| 40983 | d005 | 1986-12-01 | 9999-01-01 | 40983 | d005 | 1986-12-01 | 9999-01-01 |
| 46554 | d008 | 1986-12-01 | 1992-05-27 | 46554 | d008 | 1986-12-01 | 1992-05-27 |
| 48317 | d008 | 1986-12-01 | 1989-01-11 | 48317 | d008 | 1986-12-01 | 1989-01-11 |
| 49667 | d007 | 1986-12-01 | 9999-01-01 | 49667 | d007 | 1986-12-01 | 9999-01-01 |
| 50449 | d005 | 1986-12-01 | 9999-01-01 | 50449 | d005 | 1986-12-01 | 9999-01-01 |
| 10004 | d004 | 1986-12-01 | 9999-01-01 | 10004 | d004 | 1986-12-01 | 9999-01-01 |
+--------+---------+------------+------------+--------+---------+------------+------------+
10 rows in set (0.00 sec)
mysql> show status like '%handler_write%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Handler_write | 20 |
+---------------+-------+
1 row in set (0.00 sec)
mysql> flush status;
Query OK, 0 rows affected (0.02 sec)
mysql> with t as(select /*+ set_var(optimizer_switch='derived_merge=off')*/ * from t_group)
-> select * from t t1
-> join t t2 on t1.emp_no=t2.emp_no;
+--------+---------+------------+------------+--------+---------+------------+------------+
| emp_no | dept_no | from_date | to_date | emp_no | dept_no | from_date | to_date |
+--------+---------+------------+------------+--------+---------+------------+------------+
| 22744 | d006 | 1986-12-01 | 9999-01-01 | 22744 | d006 | 1986-12-01 | 9999-01-01 |
| 24007 | d005 | 1986-12-01 | 9999-01-01 | 24007 | d005 | 1986-12-01 | 9999-01-01 |
| 30970 | d005 | 1986-12-01 | 2017-03-29 | 30970 | d005 | 1986-12-01 | 2017-03-29 |
| 31112 | d002 | 1986-12-01 | 1993-12-10 | 31112 | d002 | 1986-12-01 | 1993-12-10 |
| 40983 | d005 | 1986-12-01 | 9999-01-01 | 40983 | d005 | 1986-12-01 | 9999-01-01 |
| 46554 | d008 | 1986-12-01 | 1992-05-27 | 46554 | d008 | 1986-12-01 | 1992-05-27 |
| 48317 | d008 | 1986-12-01 | 1989-01-11 | 48317 | d008 | 1986-12-01 | 1989-01-11 |
| 49667 | d007 | 1986-12-01 | 9999-01-01 | 49667 | d007 | 1986-12-01 | 9999-01-01 |
| 50449 | d005 | 1986-12-01 | 9999-01-01 | 50449 | d005 | 1986-12-01 | 9999-01-01 |
| 10004 | d004 | 1986-12-01 | 9999-01-01 | 10004 | d004 | 1986-12-01 | 9999-01-01 |
+--------+---------+------------+------------+--------+---------+------------+------------+
10 rows in set, 1 warning (0.00 sec)
mysql> show status like '%handler_write%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Handler_write | 10 |
+---------------+-------+
1 row in set (0.00 sec)CTE除了一般功能外,还可以实现递归一些复杂SQL需求，参考MySQL 8.0新特性--CTE(二)
参考链接
13.2.13 WITH Syntax (Common Table Expressions)