Null 值会被聚合函数忽略，默认情况下，Sql Server会给出Warning：
Warning: Null value is eliminated by an aggregate or other SET operation.
这个Warning说明Null value 被聚合函数忽略，此时的 SET ANSI_WARNINGS 选项是ON状态，例如，count()函数不会计算null值，min/max函数不会计算null值。如果设置 SET ANSI_WARNINGS OFF ，SQL Server不会抛出 warning 消息。
1，忽略NULL 值
例如，结果是1，正确，但是出现一个Warning。
use tempdb go set ANSI_WARNINGS on if object_id('#dt_test','U') is not null drop table #dt_test create table #dt_test ( id int ) insert into #dt_test values(1),(null) select min(id) from #dt_test
强烈推荐：将ANSI_Warnings选项设置为ON
2，查看当前DB的 ANSI_Warnings 选项的设置
select name, database_id, is_ansi_warnings_on from sys.databases where database_id=db_id()
引用《SET ANSI_WARNINGS》：
As the Warning says, NULLs are being ignored because we are using aggregate function (SUM, AVG). To avoid the warning we can use “set ansi_warnings off” before the script. Here is the modified script.
SET ANSI_WARNINGS OFF
When set to ON, if null values appear in aggregate functions, such as SUM, AVG, MAX, MIN, STDEV, STDEVP, VAR, VARP, or COUNT, a warning message is generated. When set to OFF, no warning is issued.
SET ANSI_WARNINGS must be ON when you are creating or manipulating indexes on computed columns or indexed views. If SET ANSI_WARNINGS is OFF, CREATE, UPDATE, INSERT, and DELETE statements on tables with indexes on computed columns or indexed views will fail
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