如何强制列别名为特定数据类型？

Question

我在选择中有一个别名：

SELECT
    CASE ....
    WHEN 1 THEN somefield * -10
    WHEN 0 THEN somefield * 10
    END as myAlias
FROM
    ....

问题是在mysql寻找的第一条记录中，它将“myAlias”标记为UNSIGNED 但在一些下一条记录中，相同的别名产生一个SIGNED整数，然后我有错误：

BIGINT UNSIGNED value is out of range in....

那么，如何告诉mysql在执行“myAlias”是签名整数的任何操作之前？

谢谢！

编辑：

我试过

SELECT
    CAST(CASE ....
    WHEN 1 THEN somefield * -10
    WHEN 0 THEN somefield * 10
    END AS SIGNED) as myAlias
FROM
    ....

没有成功！

Answer 1

我假设您表中的字段声明为UNSIGNED INTEGER。在这种情况下，您必须在THEN表达式的CASE子句中键入强制转换字段。下面的例子说明了一个样本。

创建并插入脚本：

CREATE TABLE mytable
(
     numberfield INTEGER UNSIGNED
   , bitfield BIT
);

INSERT INTO mytable (numberfield, bitfield) VALUES
  (134, 1),
  (673, 0),
  (855, 1),
  (133, 0),
  (778, 1),
  (234, 0);

在THEN子句中输入字段：SQL Fiddle Demo

SELECT
    CASE bitfield
        WHEN 1 THEN CAST(numberfield AS SIGNED) * -10
        WHEN 0 THEN CAST(numberfield AS SIGNED) * 10
    END as myAlias
FROM mytable

输出：

MYALIAS
-------
-1340
6730
-8550
1330
-7780
2340

编写上述查询的替代方法：SQL Fiddle Demo

SELECT
    CAST(numberfield AS SIGNED) * 
    (
        CASE bitfield
            WHEN 1 THEN -10
            WHEN 0 THEN 10
        END
    ) as myAlias
FROM mytable

输出：

MYALIAS
-------
-1340
6730
-8550
1330
-7780
2340

没有类型转换字段：SQL Fiddle Demo

SELECT
    CASE bitfield
      WHEN 1 THEN numberfield * -10
      WHEN 0 THEN numberfield * 10
    END as myAlias
FROM mytable

错误：

  

数据截断：BIGINT UNSIGNED值超出范围'{db_5b3bb。mytable。numberfield * - （10））'：SELECT CASE位域WHEN 1 THEN numberfield * - 10当0，那么numberfield * 10 END为myAlias from mytable

Answer 2

尝试使用“cast（）”函数：

http://dev.mysql.com/doc/refman/5.1/en/cast-functions.html#function_cast

EX： select * from mytable where myfield=cast(somefield as UNSIGNED INTEGER)