内容简介:在开源中国每日动弹中看到这么一道题目,蛮有意思,还学到了一个新的 MySQL 语法:CTE。尝试 MySQL CTE:结果非常诡异:
在开源中国每日动弹中看到这么一道题目,蛮有意思,还学到了一个新的 MySQL 语法:CTE。
题目
原始数据
SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score
+------+-------+ | Name | Score | +------+-------+ | A | 8 | | B | 7 | | C | 6 | | D | 5 | +------+-------+
目标数据
+------+-------+------+-------+ | name | score | name | score | +------+-------+------+-------+ | A | 8 | B | 7 | | C | 6 | D | 5 | +------+-------+------+-------+
过程
尝试 MySQL CTE:
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score ) SELECT * FROM t1;
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT * FROM t2;
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1 WHERE MOD(t2.row_, 2) = 0 UNION SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1 WHERE MOD(t2.row_, 2) != 0;
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT a.name, a.score, b.name, b.score FROM (SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1 WHERE MOD(t2.row_, 2) != 0) a, (SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1 WHERE MOD(t2.row_, 2) = 0) b WHERE a.r = b.r;
如果行数是奇数呢?
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score UNION SELECT 'E' Name, 4 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT a.name, a.score, b.name, b.score FROM (SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1 WHERE MOD(t2.row_, 2) != 0) a, (SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1 WHERE MOD(t2.row_, 2) = 0) b WHERE a.r = b.r;
+------+-------+------+-------+ | name | score | name | score | +------+-------+------+-------+ | A | 8 | A | 8 | | C | 6 | C | 6 | | E | 4 | E | 4 | +------+-------+------+-------+
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score UNION SELECT 'E' Name, 4 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT * FROM (SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1 WHERE MOD(t2.row_, 2) != 0) a LEFT JOIN (SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1 WHERE MOD(t2.row_, 2) = 0) b ON a.r = b.r;
结果非常诡异:
+------+-------+------+------+------+-------+------+------+ | Name | Score | row_ | r | Name | Score | row_ | r | +------+-------+------+------+------+-------+------+------+ | A | 8 | 1 | 1 | A | 8 | 6 | 1 | | C | 6 | 3 | 2 | C | 6 | 8 | 2 | | E | 4 | 5 | 3 | E | 4 | 10 | 3 | +------+-------+------+------+------+-------+------+------+
真实表 LEFT JOIN 实验
结论:不是我用错连接了, LEFT JOIN
没有问题。
可能是 MariaDB 的 CTE 实现方式有什么问题。
CREATE DATABASE `test` /*!40100 COLLATE 'utf8mb4_unicode_ci' */; CREATE TABLE `test`.`test20190419` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `Name` VARCHAR(50) NOT NULL, `Score` INT(10) UNSIGNED NOT NULL, PRIMARY KEY (`id`) ) COLLATE='utf8mb4_unicode_ci' ENGINE=InnoDB; INSERT INTO `test`.`test20190419` (`Name`, `Score`) VALUES ('A', 9); INSERT INTO `test`.`test20190419` (`Name`, `Score`) VALUES ('B', 8); INSERT INTO `test`.`test20190419` (`Name`, `Score`) VALUES ('C', 7); INSERT INTO `test`.`test20190419` (`Name`, `Score`) VALUES ('D', 6); INSERT INTO `test`.`test20190419` (`Name`, `Score`) VALUES ('E', 5);
SELECT a.name, a.score, b.name, b.score FROM ( SELECT `test`.`test20190419`.*, @rownum2 := @rownum2 + 1 AS r FROM `test`.`test20190419`, ( SELECT @rownum2 := 0) a1 WHERE MOD(id, 2) != 0 ) a LEFT JOIN ( SELECT `test`.`test20190419`.*, @rownum3 := @rownum3 + 1 AS r FROM `test`.`test20190419`, ( SELECT @rownum3 := 0) b1 WHERE MOD(id, 2) = 0 ) b ON a.r = b.r;
+------+-------+------+-------+ | name | score | name | score | +------+-------+------+-------+ | A | 9 | B | 8 | | C | 7 | D | 6 | | E | 5 | NULL | NULL | +------+-------+------+-------+
找到原因
去掉奇偶判断逻辑,之后:
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score UNION SELECT 'E' Name, 4 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT * FROM (SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1) a LEFT JOIN (SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1) b ON a.r = b.r;
+------+-------+------+------+------+-------+------+------+ | Name | Score | row_ | r | Name | Score | row_ | r | +------+-------+------+------+------+-------+------+------+ | A | 8 | 1 | 1 | A | 8 | 6 | 1 | | B | 7 | 2 | 2 | B | 7 | 7 | 2 | | C | 6 | 3 | 3 | C | 6 | 8 | 3 | | D | 5 | 4 | 4 | D | 5 | 9 | 4 | | E | 4 | 5 | 5 | E | 4 | 10 | 5 | +------+-------+------+------+------+-------+------+------+
大胆猜测:
LEFT JOIN 连表操作导致 WITH 重新执行,最后 rownum 变量...悲剧了。
WITH t0 AS ( SELECT @rownum := 0 ), t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score UNION SELECT 'E' Name, 4 Score ), t2 AS ( SELECT t1.*, @rownum := @rownum + 1 AS row_ FROM t1 ) SELECT * FROM t2;
这么写,变量不起作用:
+------+-------+------+ | Name | Score | row_ | +------+-------+------+ | A | 8 | NULL | | B | 7 | NULL | | C | 6 | NULL | | D | 5 | NULL | | E | 4 | NULL | +------+-------+------+
感慨:我对 SQL 的执行方式还真是一无所知。
下面是开源中国上贴的图(看这 SQL 写法,应该是 Oracle 吧,反正不是 MySQL,MongoDB, SQLite,PostgreSQL)
(+) rownum FROM DUAL
解决方法
MariaDB 支持 row_number() over()
算行号。
我不知道是从哪个版本开始支持的,反正在我本地的 10.3 版本中运行良好:
PS:MySQL 最新版本(8.0.15)也支持,测试通过。
WITH t1 AS ( SELECT 'A' Name, 8 Score UNION SELECT 'B' Name, 7 Score UNION SELECT 'C' Name, 6 Score UNION SELECT 'D' Name, 5 Score UNION SELECT 'E' Name, 4 Score ), t2 AS ( SELECT t1.*, row_number() over() AS row_ FROM t1, (SELECT @rownum := 0) t21 ) SELECT a.name, a.score, b.name, b.score FROM (SELECT t2.*, @rownum2 := @rownum2 + 1 AS r FROM t2, (SELECT @rownum2 := 0) a1 WHERE MOD(t2.row_, 2) != 0) a LEFT JOIN (SELECT t2.*, @rownum3 := @rownum3 + 1 AS r FROM t2, (SELECT @rownum3 := 0) b1 WHERE MOD(t2.row_, 2) = 0) b ON a.r = b.r;
知识点
CTE (Common Table Expression)
之前的方案
- 派生表语句,写在 FROM 后面
由 DBMS 自动管理的临时表 - 数据库视图
CTE 语法
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; # Recursive Common Table Expression WITH RECURSIVE cte (n) AS ( SELECT 1 UNION ALL SELECT n + 1 FROM cte WHERE n < 5 ) SELECT * FROM cte;
- column_list 可以省略
MySQL/MariaDB 支持情况
MySQL
8.0 之后支持,参考:
- http://mysqlserverteam.com/mysql-8-0-labs-recursive-common-table-expressions-in-mysql-ctes/
- http://mysqlserverteam.com/mysql-8-0-labs-recursive-common-table-expressions-in-mysql-ctes-part-two-how-to-generate-series/
- http://mysqlserverteam.com/mysql-8-0-labs-recursive-common-table-expressions-in-mysql-ctes-part-three-hierarchies/
- https://dev.mysql.com/doc/refman/8.0/en/with.html
MariaDB
- https://mariadb.com/kb/en/library/with/
Common Table Expression WITH was introduced in MariaDB 10.2.1. Recursive WITH has been supported since MariaDB 10.2.2.
参考资料与拓展阅读
- 开源中国, OSChina 周五乱弹 —— 企鹅尼克号
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