MySQL–在5.6之前的选择中断中同时进行浮点增量和赋值

为什么像SELECT@sum:=@var:=@sum+some_table.val这样的查询。。。在MySQL中自动将@var:=@sum转换为整数≤5.5对于十进制类型，一些表val和双精度/浮点数的下限值？更改了什么功能以允许5.6中的预期行为

背景与精化： 考虑下表：

CREATE TABLE t ( 
    id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, 
    sum_component FLOAT 
);
INSERT INTO t (sum_component) VALUES (0.5), (0.6), (0.4), (0.5);

我正在设计一个累积和查询，它获取累积和大于某个值的条目的id。通常，此查询符合以下条件：

SELECT t.id, 
    @cumulative_sum
    FROM t 
    CROSS JOIN (SELECT @cumulative_sum:=0) a 
    WHERE (@cumulative_sum:=@cumulative_sum+t.sum_component) > 1.3 
    ORDER BY id ASC LIMIT 1;

…但我碰巧还需要在所选条目之前存储累积和，以便以后进行计算，并且此查询的累积和不会返回预期结果，重复计算最后一个条目。在本例中，我希望此查询设置一个存储值1.10.5+0.6的变量，而无需进行额外的数学运算

如果我在递增步骤中将@cumulative_sum的旧值赋给@another_变量，我应该能够做到这一点

SELECT t.id, 
    @cumulative_sum
    FROM t 
    CROSS JOIN (SELECT @cumulative_sum:=0) a 
    WHERE (@cumulative_sum:=(@another_variable:=@cumulative_sum)+t.sum_component) > 1.3
    ORDER BY id ASC LIMIT 1;

在我的两台机器上（一台运行MySQL 5.6，另一台运行MariaDB 10.0.7），上面的查询按预期执行：

MariaDB [a51]>     SELECT  t.id, @cumulative_sum
    FROM  t
    CROSS JOIN  
      ( SELECT  @cumulative_sum:=0) a
    WHERE  (@cumulative_sum:=(@another_variable:=@cumulative_sum)
                             +t.sum_component) > 1.3
    ORDER BY  id ASC
    LIMIT  1;

+----+--------------------+
| id | @cumulative_sum    |
+----+--------------------+
|  3 | 1.5000000298023224 |
+----+--------------------+
1 row in set (0.00 sec)

MariaDB [a51]> SELECT @another_variable;
+-------------------+
| @another_variable |
+-------------------+
| 1.100000023841858 |
+-------------------+
1 row in set (0.01 sec)

但在MySQL 5.5上，它没有：

mysql>     SELECT  t.id, @cumulative_sum
    FROM  t
    CROSS JOIN  
      ( SELECT  @cumulative_sum:=0) a
    WHERE  (@cumulative_sum:=(@another_variable:=@cumulative_sum)+t.sum_component) > 1.3
    ORDER BY  id ASC
    LIMIT  1;

Empty set (0.18 sec)

mysql> SELECT @another_variable;
+-------------------+
| @another_variable |
+-------------------+
|                 0 |
+-------------------+
1 row in set (0.01 sec)

观察查询是如何递增的，我们可以看到潜在的问题。以下是浮点和_分量的结果：

下面是十进制和的组成部分：

mysql> ALTER TABLE t MODIFY sum_component DECIMAL(4,2);
Query OK, 4 rows affected, 2 warnings (0.16 sec)
Records: 4  Duplicates: 0  Warnings: 2

mysql>     SELECT  t.id, (@cumulative_sum := (@another_variable:=@cumulative_sum) 
                           + t.sum_component) AS cumulative_sum,
        sum_component
    FROM  t
    CROSS JOIN  
      ( SELECT  @cumulative_sum:=0) a
    ORDER BY  id ASC;

+----+----------------+---------------+
| id | cumulative_sum | sum_component |
+----+----------------+---------------+
|  1 |           0.50 |          0.50 |
|  2 |           1.60 |          0.60 |
|  3 |           2.40 |          0.40 |
|  4 |           2.50 |          0.50 |
+----+----------------+---------------+
4 rows in set (0.18 sec)

---

这是因为MySQL在旧版本中将0视为整数；您的初始赋值@cumulative_sum:=0将变量设置为整数。将赋值更改为@cumulative_sum:=0.0将导致5.5上的预期行为：

SELECT t.id,
    @cumulative_sum
    FROM t
    CROSS JOIN (SELECT @cumulative_sum:=0.0) a
    WHERE (@cumulative_sum:=(@another_variable:=@cumulative_sum)+t.sum_component) > 1.3
    ORDER BY id ASC LIMIT 1;

解决了这个问题，但我没有发现5.5和5.6之间的行为变化：

为变量赋值并读取同一non SET语句中的值的另一个问题是，变量的默认结果类型基于其在语句开头的类型。以下示例说明了这一点：

mysql>SET@a='test'

mysql>从tbl_名称中选择@a，@a:=20

对于这个SELECT语句，MySQL向客户端报告第1列是字符串，并将@a的所有访问转换为字符串，即使第二行的@a设置为数字。SELECT语句执行后，@a被视为下一个语句的数字

为避免此行为出现问题，请不要在单个语句中为同一变量赋值并读取该变量的值，或者将该变量设置为0、0.0，或者在使用前定义其类型

---

你有一个84的代表，你做的背景，精化，和小提琴！这个蒙面人是谁：>@Drew一个不知道自己是否鄙视MySQL怪癖的人，或者在内心深处对这些怪癖有罪恶感的人：P@Drew在Stackoverflow上获得信誉点与用户提出好问题的能力无关。纪律事项：我希望传染病传播@VohumanI也有类似的问题。我在dba.stackexchange上看到了这一点，看看它是否有助于您理解。

SELECT t.id,
    @cumulative_sum
    FROM t
    CROSS JOIN (SELECT @cumulative_sum:=0.0) a
    WHERE (@cumulative_sum:=(@another_variable:=@cumulative_sum)+t.sum_component) > 1.3
    ORDER BY id ASC LIMIT 1;

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mysql> create database test;
Query OK, 1 row affected (0.00 sec)

mysql> use test;
Database changed
mysql> CREATE TABLE t ( 
    ->     id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, 
    ->     sum_component FLOAT 
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> INSERT INTO t (sum_component) VALUES (0.5), (0.6), (0.4), (0.5);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT t.id,
    ->     @cumulative_sum
    ->     FROM t
    ->     CROSS JOIN (SELECT @cumulative_sum:=0.0) a
    ->     WHERE (@cumulative_sum:=(@another_variable:=@cumulative_sum)+t.sum_component) > 1.3
    ->     ORDER BY id ASC LIMIT 1;
+----+----------------------------------+
| id | @cumulative_sum                  |
+----+----------------------------------+
|  3 | 1.500000029802322400000000000000 |
+----+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT @another_variable;
+-------------------+
| @another_variable |
+-------------------+
| 1.100000023841858 |
+-------------------+
1 row in set (0.00 sec)