SqlServer_查询带有null的记录

字段的三种不同状态

select distinct status  from t_table
(NULL)
XXX
YYY

不同状态的记录数

select count(1) from t_table  35709
select count(1) from t_table where status  = 'XXX'      17579 XXX
select count(1) from t_table where status  = 'YYY'      4242  YYY
select count(1) from t_table where status  is null      13888 (NULL)
select count(1) from t_table where status  = null       0
select count(1) from t_table where status  is not null  21821 XXX+ YYY 
select count(1) from t_table where status  <> null      0
select count(1) from t_table where status  != null      0

select count(1)  from t_table where status   != 'XXX'     4242   不含null值记录
select count(1)  from t_table where status   != 'YYY'     17579  不含null值记录
select count(1)  from t_table where status   != 'XXX' and  status   != 'YYY'   0    !=  不包含 null值的记录

select count(1) from t_table where status  in (select status  from t_table)                                   21821
select count(1) from t_table where status  in (select status  from t_table where status  = 'XXX' )             17579
select count(1) from t_table where status  in (select status  from t_table where status  = 'YYY' )             4242
select count(1) from t_table where status  in (select status  from t_table where status  in ('XXX','YYY') )    21821

select count(1) from t_table where status   in ('XXX' )  17579
select status  from t_table where status  not in ('XXX' )     4242      not in  是不包含 (默认null值)的记录  
select count(1) from t_table where status  not in ('')       21821        
select count(1) from t_table where status  not in (null)      0   not in (null)   返回空   ANSISQL（SQL-92）规定
select count(1) from t_table where status  in (null)          0   in (NULL)       返回空   ANSISQL（SQL-92）规定
select count(1) from t_table where status  is null          13888            查询 带有null值得记录   最好不要使用 in 
select count(1) from t_table where status  is not null      21821            使用了 in  是 不查询 null 的记录的

select status  from t_table where status  not in ('XXX','YYY')  (默认null值)       返回空结果集
select region,isNull(region,'11') from t_table where status  not in ('XXX','YYY')  (默认null值)  加了where 后面的条件查询不到记录  ,返回空结果集,region都没有,也不会进行null判断
select COALESCE(region,'11') from t_table where status  not in ('XXX','YYY')  (默认null值)    返回空结果集

select count(1) from t_table where status  not in (select status  from t_table )                     0
select isNull(region,'22') from t_table where status  not in ('XXX','YYY')                          显示(Null),其实是 返回的 空结果集
select count(1) from t_table where status  not in ('XXX','YYY')                                     0
select count(1) from t_table where status  not in (select status  from t_table where status  not in ('XXX','YYY') )   35709  not in (空结果集)  所以,能查出所有数据
select count(1) from t_table where status  in ( null)                                                               0   in (NULL)    ANSISQL（SQL-92）规定的Null值的比较取值结果都为False，既Null=Null取值也是False    
select count(1) from t_table where status  not in ( null)                                                           0   not in (NULL)  ANSISQL（SQL-92）规定的Null值的比较取值结果都为False，既Null=Null取值也是False
select count(1) from t_table where status  in (select status  from t_table where status  not in ('XXX','YYY') )       0      in  (默认null值)
select count(1) from t_table where status  <> (select status  from t_table where status  not in ('XXX','YYY') )       0      <>  (默认null值)
select count(1) from t_table where status  != (select status  from t_table where status  not in ('XXX','YYY') )       0      !=  (默认null值)
select count(1) from t_table where status  != null                        0    ANSISQL（SQL-92）规定
select count(1) from t_table where status  <> null                        0    ANSISQL（SQL-92）规定

默认情况下，推荐使用 IS NOT NULL去判断，因为SQL默认情况下对！= Null的判断会永远返回0行，但没有语法错误。

如果你一定想要使用！= Null来判断，需要加上这个语句：

set ANSI_NULLS off

所以我们要牢记：默认情况下做比较条件时使用关键字“is null”和“is not null”。

如果你一定要使用!= null来进行条件判断，需要加上这个命令语句：SET ANSI_NULLS OFF，这时数据库进入ANSI SQL非标准模式，你会发现IS NOT NULL 和 != null 是等效的了。

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这里使用的是模式切换命令SET ANSI_NULLS[ON/OFF]。ON值采用ANSI SQL严格标准，OFF值采用非标准兼容模式。另外SET ANSI_DEFAULTS [ON/OFF]命令也可以实现标准的切换，只是这个命令控制的是一组符合SQL-92标准的设置，其中就包括Null值的标准。

默认情况下，数据库管理程序（DB-Library）是SET ANSI_NULLS为OFF的。但是我们的大多数应用程序，都是通过ODBC或者OLEDB来访问数据库的，作为一种开放兼容的数据库访问程序，或许是兼容性的考虑，SETANSI_NULLS值设置为ON。这样一来带来的一些问题是需要注意的。像存储过程或者自定义函数这样的应用程序都是基于DB-Library的，默认情况下，SETANSI_NULLS为OFF，并且在这样的程序中，不能使用SETANSI_NULLS在一个环境中修改规则，只能修改数据库配置参数。

执行 set ANSI_NULLS off 后

select count(1) from t_table where status  in ( null)                13888
select count(1) from t_table where status  not in ( null)            21821                                               
select count(1) from t_table where status  in (select status  from t_table where status  not in ('XXX','YYY') )   13888  
select count(1) from t_table where status  <> null                 21821
select count(1) from t_table where status  <> (select status  from t_table where status  not in ('XXX','YYY') )    
select status  from t_table where status  not in ('XXX','YYY')    返回  13888 条 null值记录  导致下面的报错 

[Err] 21000 - [SQL Server]Subquery returned more than 1 value. 
This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.


set ANSI_NULLS on   时 ,下面sql  返回的是 null   ,不会出现语法报错的情况
select status  from t_table where status  not in ('XXX','YYY')

select count(1) from t_table where status  != (select status  from t_table where status  not in ('XXX','YYY') )  语法报错

slq_不使用or查询 0 和null_ISNULL(a.status ,0)=0 和 case when else end

SQL中IS NOT NULL与!=NULL的区别

SqlServer_查询带有null的记录

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