Skipping WHERE statement if the variable for that statement is null

Question

I will have a table

TABLE1
ID  Name   Country
1   Anna   Singapore
2   Brad   UK
3   Cassie US

declare @place varchar(20);
set @place=\'US\';

select * from Tab         


        

Answer 1

For this simple case in your question just use

IF ( @place IS NULL )
  SELECT *
  FROM   table1
ELSE
  SELECT *
  FROM   table1
  WHERE  country = @place  

If your actual situation is more complex you could use

select * 
from Table1 
where @place is null or country=@place 
option (recompile)

The reason for needing the recompile hint is to avoid having a single plan catering for both cases and doing an unnecessary scan in the event that you do supply an explicit value.

These, and other alternatives such as generating the query dynamically, are discussed in detail in the article Dynamic Search Conditions in T-SQL

Answer 2

like and = are equivalent in your situation because you are dealing with text (char, nchar, varchar, nvarchar) values.

Instead of

WHERE  country = @place  

try

WHERE  country like @place  

For the variable set try Instead of

set @place='US'

Try

set @country = isnull('''US''','''%''')

To test this:

declare @country nvarchar(50)
set @country = isnull('''US''','''%''')
print @Country
set @country = isnull(NULL,'''%''')
print @Country

The extra quotes are necessary because in your example you are using explicit values - ' is an escape character and you need a single quote around each value to signify it is NOT a table column. If you were using the COLUMN_NAME you would simply use a single quote around the % character.

Depending on your SQL version you may have to use a different wildcard character. This is fairly a fairly direct and linear solution, although there are risks associated with open variables.