SQL计算输出精度

Question

有人可以解释这个查询的行为吗：

SELECT
    0.1 + 0.01,
    '',
    (CAST(0.1 AS numeric(18,1)))+(CAST(0.01 AS numeric (18,2))),
    0.1 + (CAST(0.01 AS numeric (18,2))),
    (CAST(0.1 AS numeric(18,1)))+ 0.01,
    '',
    (CAST(0.1 AS numeric(38,1)))+(CAST(0.01 AS numeric (38,2))),
    0.1 + (CAST(0.01 AS numeric (38,2))),
    (CAST(0.1 AS numeric(38,1)))+ 0.01

我不明白为什么 38 的行为与 18 不同？

我原以为 SQL 服务器总是会自动 return 以准确显示结果所需的精度计算结果？要改变它，您必须显式转换结果吗？

Answer 1

它遵循此处找到的规则：Precision, Scale, and Length (Transact-SQL) - msdn

当您使用最大精度时，无法在不损失精度的情况下调整比例。

Chart for Data Type Conversion (Database Engine) - msdn

如果您在计算中引入 float，您将得到 float return。

默认情况下，带小数的值将隐式转换为 decimal/numeric 数据类型。在根据您的问题修改的示例中，这些隐式转换被标记为 'und'：

select
      undefined=0.1 + 0.01
    ,[18,1+18,2] = (cast(0.1 as numeric(18,1)))+(cast(0.01 as numeric (18,2)))
    ,[und+18,2+18,1] = 0.1 + (cast(0.01 as numeric (18,2)))
    ,[18,2+und]=(cast(0.1 as numeric(18,1)))+ 0.01
    ,[38,1+38,2]=(cast(0.1 as numeric(38,1)))+(cast(0.01 as numeric (38,2)))
    ,[und+38,2] = 0.1 + (cast(0.01 as numeric (38,2)))
    ,[38,1+und]=(cast(0.1 as numeric(38,1)))+ 0.01
    ,[38,1+float]=(cast(0.1 as numeric(38,1)))+ convert(float,0.01)

rextester 演示：http://rextester.com/ULVRGS77309

returns:

+-----------+-----------+---------------+----------+-----------+----------+----------+------------+
| undefined | 18,1+18,2 | und+18,2+18,1 | 18,2+und | 38,1+38,2 | und+38,2 | 38,1+und | 38,1+float |
+-----------+-----------+---------------+----------+-----------+----------+----------+------------+
| 0,11      | 0,11      | 0,11          | 0,11     | 0,1       | 0,11     | 0,1      | 0,11       |
+-----------+-----------+---------------+----------+-----------+----------+----------+------------+

SQL计算输出精度

SQL calculation output precision

sql-server

precision

decimal-precision