新特性解读 | MySQL 8.0 通用表达式（WITH）深入用法

爱可生开源社区

发布于 2021-06-16 19:11:25

8820

发布于 2021-06-16 19:11:25

作者：杨涛涛

资深数据库专家，专研 MySQL 十余年。擅长 MySQL、PostgreSQL、MongoDB 等开源数据库相关的备份恢复、SQL 调优、监控运维、高可用架构设计等。目前任职于爱可生，为各大运营商及银行金融企业提供 MySQL 相关技术支持、MySQL 相关课程培训等工作。

本文来源：原创投稿

*爱可生开源社区出品，原创内容未经授权不得随意使用，转载请联系小编并注明来源。

MySQL 8.0 发布已经好几年了，之前介绍过 WITH 语句（通用表达式）的简单用途以及使用场景，类似如下的语句：

with tmp(a) as (select 1 union all select 2) select * from tmp;

正巧之前客户就咨询我，WITH 有没有可能和 UPDATE、DELETE 等语句一起来用？或者说有没有可以简化日常 SQL 的其他用法，有点迷惑，能否写几个例子简单说明下？

其实 WITH 表达式除了和 SELECT 一起用，还可以有下面的组合：

insert with 、with update、with delete、with with、with recursive(可以模拟数字、日期等序列)、WITH 可以定义多张表

我们来一个一个看看：

1. 用 WITH 表达式来造数据

用 WITH 表达式来造数据，非常简单，比如下面例子：给表 y1 添加100条记录，日期字段要随机。

localhost:ytt>create table y1 (id serial primary key, r1 int,log_date date);
Query OK, 0 rows affected (0.09 sec)

localhost:ytt>INSERT y1 (r1,log_date)
    -> WITH recursive tmp (a, b) AS
    -> (SELECT
    ->   1,
    ->   '2021-04-20'
    -> UNION
    -> ALL
    -> SELECT
    ->   ROUND(RAND() * 10),
    ->   b - INTERVAL ROUND(RAND() * 1000) DAY
    -> FROM
    ->   tmp
    -> LIMIT 100) TABLE tmp;
Query OK, 100 rows affected (0.03 sec)
Records: 100  Duplicates: 0  Warnings: 0

localhost:ytt>table y1 limit 10;
+----+------+------------+
| id | r1   | log_date   |
+----+------+------------+
|  1 |    1 | 2021-04-20 |
|  2 |    8 | 2020-04-02 |
|  3 |    5 | 2019-05-26 |
|  4 |    1 | 2018-01-21 |
|  5 |    2 | 2016-09-08 |
|  6 |    9 | 2016-06-14 |
|  7 |    7 | 2016-02-06 |
|  8 |    6 | 2014-03-18 |
|  9 |    6 | 2011-08-25 |
| 10 |    9 | 2010-02-02 |
+----+------+------------+
10 rows in set (0.00 sec)

2. 用 WITH 表达式来更新表数据

WITH 表达式可以与 UPDATE 语句一起，来执行要更新的表记录：

localhost:ytt>WITH recursive tmp (a, b, c) AS
    -> (SELECT
    ->   1,
    ->   1,
    ->   '2021-04-20'
    -> UNION ALL
    -> SELECT
    ->   a + 2,
    ->   100,
    ->   DATE_SUB(
    ->     CURRENT_DATE(),
    ->     INTERVAL ROUND(RAND() * 1000, 0) DAY
    ->   )
    -> FROM
    ->   tmp
    -> WHERE a < 100)
    -> UPDATE
    ->   tmp AS a,
    ->   y1 AS b
    -> SET
    ->   b.r1 = a.b
    -> WHERE a.a = b.id;
Query OK, 49 rows affected (0.02 sec)
Rows matched: 50  Changed: 49  Warnings: 0

localhost:ytt>table y1 limit 10;
+----+------+------------+
| id | r1   | log_date   |
+----+------+------------+
|  1 |    1 | 2021-04-20 |
|  2 |    8 | 2019-12-26 |
|  3 |  100 | 2018-06-12 |
|  4 |    8 | 2017-07-11 |
|  5 |  100 | 2016-08-10 |
|  6 |    9 | 2015-09-14 |
|  7 |  100 | 2014-12-19 |
|  8 |    2 | 2014-08-13 |
|  9 |  100 | 2014-08-05 |
| 10 |    8 | 2011-11-12 |
+----+------+------------+
10 rows in set (0.00 sec)

3. 用 WITH 表达式来删除表数据

比如删除 ID 为奇数的行，可以用 WITH DELETE 形式的删除语句：

localhost:ytt>WITH recursive tmp (a) AS
    -> (SELECT
    ->   1
    -> UNION
    -> ALL
    -> SELECT
    ->   a + 2
    -> FROM
    ->   tmp
    -> WHERE a < 100)
    -> DELETE FROM y1 WHERE id IN (TABLE tmp);
Query OK, 50 rows affected (0.02 sec)

localhost:ytt>table y1 limit 10;
+----+------+------------+
| id | r1   | log_date   |
+----+------+------------+
|  2 |    6 | 2019-05-16 |
|  4 |    8 | 2015-12-07 |
|  6 |    2 | 2014-05-14 |
|  8 |    7 | 2010-05-07 |
| 10 |    3 | 2007-03-27 |
| 12 |    6 | 2006-12-14 |
| 14 |    3 | 2004-04-22 |
| 16 |    7 | 2001-09-16 |
| 18 |    7 | 2001-01-04 |
| 20 |    7 | 2000-02-12 |
+----+------+------------+
10 rows in set (0.00 sec)

与 DELETE 一起使用，要注意一点：WITH 表达式本身数据为只读，所以多表 DELETE 中不能包含 WITH 表达式。比如把上面的语句改成多表删除形式会直接报 WITH 表达式不可更新的错误。

localhost:ytt>WITH recursive tmp (a) AS
    ->  (SELECT
    ->    1
    ->  UNION
    ->  ALL
    ->  SELECT
    ->    a + 2
    ->  FROM
    ->    tmp
    ->  WHERE a < 100)
    ->  delete a,b from y1 a join tmp b where a.id = b.a;
ERROR 1288 (HY000): The target table b of the DELETE is not updatable

4. WITH 和 WITH 一起用

前提条件：WITH 表达式不能在同一个层级，一个层级只允许一个 WITH 表达式

localhost:ytt>SELECT * FROM  
    ->   (
    ->     WITH tmp1 (a, b, c) AS 
    ->     (
    ->       VALUES
    ->         ROW (1, 2, 3),
    ->         ROW (3, 4, 5),
    ->         ROW (6, 7, 8)
    ->     ) SELECT  * FROM
    ->         (
    ->           WITH tmp2 (d, e, f) AS (
    ->             VALUES
    ->               ROW (100, 200, 300),
    ->               ROW (400, 500, 600)
    ->             ) TABLE tmp2
    ->         ) X
    ->           JOIN tmp1 Y
    ->   ) Z ORDER BY a;
+-----+-----+-----+---+---+---+
| d   | e   | f   | a | b | c |
+-----+-----+-----+---+---+---+
| 400 | 500 | 600 | 1 | 2 | 3 |
| 100 | 200 | 300 | 1 | 2 | 3 |
| 400 | 500 | 600 | 3 | 4 | 5 |
| 100 | 200 | 300 | 3 | 4 | 5 |
| 400 | 500 | 600 | 6 | 7 | 8 |
| 100 | 200 | 300 | 6 | 7 | 8 |
+-----+-----+-----+---+---+---+
6 rows in set (0.01 sec)

5. WITH 多个表达式来 JOIN

用上面的例子，改写多个 WITH 为一个 WITH：

localhost:ytt>WITH 
    -> tmp1 (a, b, c) AS 
    -> (
    -> VALUES
    -> ROW (1, 2, 3),
    -> ROW (3, 4, 5),
    -> ROW (6, 7, 8)
    -> ),
    -> tmp2 (d, e, f) AS (
    ->     VALUES
    ->       ROW (100, 200, 300),
    ->       ROW (400, 500, 600)
    -> )
    -> SELECT * FROM  tmp2,tmp1 ORDER BY a;
+-----+-----+-----+---+---+---+
| d   | e   | f   | a | b | c |
+-----+-----+-----+---+---+---+
| 400 | 500 | 600 | 1 | 2 | 3 |
| 100 | 200 | 300 | 1 | 2 | 3 |
| 400 | 500 | 600 | 3 | 4 | 5 |
| 100 | 200 | 300 | 3 | 4 | 5 |
| 400 | 500 | 600 | 6 | 7 | 8 |
| 100 | 200 | 300 | 6 | 7 | 8 |
+-----+-----+-----+---+---+---+
6 rows in set (0.00 sec)

6. with 生成日期序列

用 WITH 表达式生成日期序列，类似于 POSTGRESQL 的 generate_series 表函数，比如，从 ‘2020-01-01’ 开始，生成一个月的日期序列：

localhost:ytt>WITH recursive seq_date (log_date) AS
    ->      (SELECT
    ->        '2020-01-01'
    ->      UNION
    ->      ALL
    ->      SELECT
    ->        log_date + INTERVAL 1 DAY
    ->      FROM
    ->        seq_date
    ->      WHERE log_date + INTERVAL 1 DAY < '2020-02-01')
    ->      SELECT
    ->        log_date
    ->      FROM
    ->        seq_date;
+------------+
| log_date   |
+------------+
| 2020-01-01 |
| 2020-01-02 |
| 2020-01-03 |
| 2020-01-04 |
| 2020-01-05 |
| 2020-01-06 |
| 2020-01-07 |
| 2020-01-08 |
| 2020-01-09 |
| 2020-01-10 |
| 2020-01-11 |
| 2020-01-12 |
| 2020-01-13 |
| 2020-01-14 |
| 2020-01-15 |
| 2020-01-16 |
| 2020-01-17 |
| 2020-01-18 |
| 2020-01-19 |
| 2020-01-20 |
| 2020-01-21 |
| 2020-01-22 |
| 2020-01-23 |
| 2020-01-24 |
| 2020-01-25 |
| 2020-01-26 |
| 2020-01-27 |
| 2020-01-28 |
| 2020-01-29 |
| 2020-01-30 |
| 2020-01-31 |
+------------+
31 rows in set (0.00 sec)

7. with 表达式做派生表

使用刚才那个日期列表，

localhost:ytt>SELECT
    ->        *
    ->      FROM
    ->        (
    ->          WITH recursive seq_date (log_date) AS
    ->          (SELECT
    ->            '2020-01-01'
    ->          UNION
    ->          ALL
    ->          SELECT
    ->            log_date + INTERVAL 1 DAY
    ->          FROM
    ->            seq_date
    ->          WHERE log_date+ interval 1 day  < '2020-02-01')
    ->  select * 
    ->          FROM
    ->            seq_date
    ->          ) X
    ->          LIMIT 10;
+------------+
| log_date   |
+------------+
| 2020-01-01 |
| 2020-01-02 |
| 2020-01-03 |
| 2020-01-04 |
| 2020-01-05 |
| 2020-01-06 |
| 2020-01-07 |
| 2020-01-08 |
| 2020-01-09 |
| 2020-01-10 |
+------------+
10 rows in set (0.00 sec)

WITH 表达式使用非常灵活，不同的场景可以有不同的写法，的确可以简化日常 SQL 的编写。

本文参与腾讯云自媒体同步曝光计划，分享自微信公众号。

原始发表：2021-05-19，如有侵权请联系 cloudcommunity@tencent.com 删除

postgresql