意外的SQL除法结果？

Question

我使用SQL Server.

与分部合作我在这两个您可以使用SQL Server版本重现的查询中收集了我的发现.

我希望Division with Integer在这两种情况下具有相同的值.因为无论我的表中的值如何,如果我得到意想不到的结果,我最终会得到一个逻辑错误,有时很难调查.

SQL A:

WITH s AS (
    SELECT 'Integer' AS c, 1 AS k UNION
    SELECT 'Float', 1 UNION
    SELECT 'NULL', NULL
) 
SELECT 
    c AS [Type],
    k/10 AS                          'Division',
    CAST(k as numeric(38,4))/10 AS 'Cast',
    COALESCE(k, 0)/10           AS 'Coalesce',
    COALESCE(k, .0)/10          AS 'Coalesce with float'
FROM s; 

结果A.

Type        Division    Cast        Coalesce    Coalesce with float
Float       0           0.100000    0           0.100000
Integer     0           0.100000    0           0.100000
NULL        NULL        NULL        0           0.000000

SQL B:

WITH s AS (
    SELECT 'Integer' AS c, 1 AS k UNION
    SELECT 'Float', .1 UNION
    SELECT 'NULL', NULL
) 
SELECT 
    c AS [Type],
    k/10 AS                          'Division',
    CAST(k as numeric(38,4))/10 AS 'Cast',
    COALESCE(k, 0)/10           AS 'Coalesce',
    COALESCE(k, .0)/10          AS 'Coalesce with float'
FROM s; 

结果A.

Type        Division    Cast        Coalesce    Coalesce with float
Float       0.010000    0.010000    0.010000    0.010000
Integer     0.100000    0.100000    0.100000    0.100000
NULL        NULL        NULL        0.000000    0.000000

Answer 1

你的第一个表是全部整数.

SELECT 'Float', 1 is an INT不是因为你把这个词放在那里而浮动.它将隐含为INT,因为没有小数位.

然后,你知道整数除法使用整数......例如......

select 3/10是0而不是.3000,你会得到select 3/10.0.如果要返回decimal/float,则分子或分母需要是float(或两者)

编辑

在您上次发表评论后,我更了解您的问题.当您将不同的数据类型组合在一起时,您在第二步中将它们联合起来,CTESQL Server将隐式地将它们转换为必要的数据类型以伴随所有输入.请在此处查看优先顺序

所以,如果你只是从那个cte中选择*,你会看到你的1被转换为1.0.列只能有一种数据类型.在此示例中,SQL Server必须选择INT或FLOAT.如果选择了INT,则会出现数据完整性问题.

WITH s AS (
    SELECT 'Integer' AS c, 1 AS k UNION
    SELECT 'Float', .1 UNION
    SELECT 'NULL', NULL
) 

select * from s