意外的 SQL 除法结果？

Question

我使用 SQL Server。

与部门合作，我在这两个查询中收集了我的发现，您可以使用您的 SQL Server 版本重现这些结果。

我期望 Division with Integer 在两种情况下都具有相同的值。 因为无论我的表中的值如何，如果我得到意想不到的结果，我最终会遇到一个逻辑错误，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

您的第一个表全是整数。

SELECT 'Float', 1 is an INT 不是一个浮点数，因为你把这个词放在那里。它将被暗示为 INT，因为没有小数位。

然后，您知道整数除法使用整数...例如...

select 3/10 是 0，而不是使用 select 3/10.0 得到的 .3000。如果要返回小数/浮点数，则分子或分母必须是浮点数（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联合在一起时，就像您在第二个 CTE 中所做的那样，SQL 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。 See the order of precedence here

因此，如果您只是从该 cte 中选择 *... 您将看到您的 1 已转换为 1.0。一列只能有一个数据类型。在此示例中，SQL Server 必须选择 INT 或 FLOAT。如果选择了 INT，您将遇到数据完整性问题。

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

select * from s

Answer 2

如果您希望这两个数据集的结果相同，请使用 FLOOR() 函数

SQL A

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

SQL B

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

两个查询的结果

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