IF EXISTS 
   (
       SELECT *
       FROM   Table1
       WHERE  Monthcol = @Month
   )
    SELECT col1,
           col2,
           col3
    FROM   Table1
    WHERE  Monthcol = @Month
ELSE
    SELECT col1,
           col2,
           col3
    FROM   Table2
    WHERE  Monthcol = @Month;

if EXISTS (select * from [@temp] WHERE Monthcol = @Month)
select * from [@temp] WHERE Monthcol = @Month
else
select * from [@m_temp1] WHERE Monthcol = @Month

-- This query returns 0 rows
SELECT 1 WHERE 0 = 1

-- So this query prints "No"
IF EXISTS (SELECT 1 WHERE 0 = 1) 
	PRINT 'Yes';
ELSE 
	PRINT 'No';
-------------------------------
-- This query returns one row
SELECT 1;

-- So this query prints "yes"
IF EXISTS (SELECT 1) 
	PRINT 'Yes';
ELSE 
	PRINT 'No';