SQL Server 2014中的ISNULL（）函数表现异常

Question

Declare @Var1 Varchar(5) = Null
Declare @Var2 Varchar(10) = 'ABCDEFGHIJ'

SELECT ISNULL(@Var1, @Var2) AS Result

I excepted the result to be ' ABCDEFGHIJ' , but the result is 'ABCDE' .

Is this the normal behavior of the function or I am missing anything?

Answer 1

You have used varchar(5) that's why this is happening

use below

 Declare @Var1 Varchar(50) = Null

this will give your expected answer

Answer 2

You should use coalesce function because the Coalesce() function returns the first non-null value among its arguments.

SELECT COALESCE(@Var1, @Var2) AS Result

Or you can change size of your declared variable. Both will give you expected output.

Answer 3

To answer your question, yes this is the normal behavior of ISNULL . it takes the type of the first expression.

reference here https://msdn.microsoft.com/en-us/library/ms184325.aspx

The value of check_expression is returned if it is not NULL; otherwise, replacement_value is returned after it is implicitly converted to the type of check_expression, if the types are different. replacement_value can be truncated if replacement_value is longer than check_expression.

As an alternative you should rather use COALESCE

Answer 4

使用COALESCE函数：不确定为什么ISNULL返回这样的输出，但是假设在检查ISNULL(p1,p2) ，它将仅考虑第一个参数的大小。

SELECT COALESCE(@Var1, @Var2) AS Result

Answer 5

That is because your source variable is of length 5 so it has truncated the other part

Declare @Var1 Varchar(10) = Null
Declare @Var2 Varchar(10) = 'ABCDEFGHIJ'

SELECT ISNULL(@Var1, @Var2) AS Result

Answer 6

ISNULL(@Var1, @Var2)语句将拷贝的价值@Var2到@var1如果@var1是空的，长度@var1是5 ，这就是为什么它是只显示前五个字符。