SQL: Use char and nchar for queries.

Use the SQL Server northwind database's MERs table
1) Code I

1 Declare   @ Ccoustomerid   Char ( 5 )
2 Select   @ Nccoustomerid   =   Null ;
3 Select   *  
4 From MERs
5 Where Customerid Like   Isnull ( @ Nccoustomerid , ' % ' )

 
2) Code 2

1 Declare   @ Ccoustomerid  N Char ( 5 )
2 Select   @ Nccoustomerid   =   Null ;
3 Select   *  
4 From MERs
5 Where Customerid Like   Isnull ( @ Nccoustomerid , ' % ' )

3) Code 3

1 Declare   @ Ccoustomerid VaR Char ( 5 )
2 Select   @ Nccoustomerid   =   Null ;
3 Select   *  
4 From MERs
5 Where Customerid Like   Isnull ( @ Nccoustomerid , ' % ' )

4) code 4 1 Declare   @ Ccoustomerid  NVaR Char ( 5 )
2 Select   @ Nccoustomerid   =   Null ;
3 Select   *  
4 From MERs
5 Where Customerid Like   Isnull ( @ Nccoustomerid , ' % ' )

Four pieces of code are put into the SQL queryserver. Result 1) and 2) the returned message is:
(0 rows affected)

3) and 4) the returned message is:
(91 rows affected)

Cause:
1) and 2) if the variable is null, spaces will be filled after %, and 3) and 4) will not.
Damn it, why does it need to fill in spaces automatically. Really annoying, and rushed to the msdn rummaging, the original:
Char is a fixed-length data type. If the length of the inserted value is smaller than that of the char not null column, spaces are filled on the right of the value until the column length is reached. (Nchar does the same)
Varchar is a variable-length data type. A value smaller than the column length is not filled on the right of the value to reach the column length. (Likewise, nvarchar)
So when the declared variable is Char or nchar, if it is null, it will automatically help you fill in spaces to know the length that meets the definition.
Be careful later ~ Hmm

Now, I don't have to worry about it. I went to the SQL Server online series and found that,ArticleThe name is Char and varchar data. The address is MS-help: // Ms. sqlcc. V9/ms. sqlsvr. v9.zh-CHS/udb9/html/07cc67f6-5057-463b-8975-694a5b4820.6.htm.