isnull函数和 字段名 is null的区别

select case when sc_amount is null then 0 else sc_amount end  from salechance;

select isnull(sc_amount,0)  from salechance;

运行结果：

1000
0
700
6000
3000
7000
6700
25000
0
0

和

1000

700
6000
3000
7000
6700
25000


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前一个如果sc_amount 是空字符串和null，一样默认为null,
后一种如果sc_amount 是空字符串，不会默认为null

有待再去验证


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说明：
1，查询某字段为null语句 select * from rdrecords where iprice is null
2,查询某字段为null和为0的语句 select * from rdrecords where isnull(iprice,N'')=N'' 查询某字段为不为null且不为0的语句 select * from rdrecords where isnull(iprice,N'')<>N''
3,查询某字段，如果为null，就用0更新后的语句 select * from rdrecords where iprcie=isnull(iprice,0)



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select sc_amount from salechance;
查询结果是
sc_amount
1000

700
6000
3000
7000
6700
25000

null




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select count(*) from salechance where isnull(sc_amount,'0')='0';
select count(*) from salechance where isnull(sc_amount,0)=0;
返回结果是：
1和7

说明什么呢？待查......

结果发现
select * from salechance where sc_amount=0;
执行的是空字符串=0为true的，空字符串<>'0'

select * from salechance where isnull(sc_amount,'0')='0';

下面2句效果一样
select * from salechance where isnull(sc_amount,0)=0;
select * from salechance where isnull(sc_amount,'0')=0;