[sql] if condition in sql server update query

I have a SQL server table in which there are 2 columns that I want to update either of their values according to a flag sent to the stored procedure along with the new value, something like:

UPDATE
    table_Name

SET
    CASE
        WHEN @flag = '1' THEN column_A += @new_value
        WHEN @flag = '0' THEN column_B += @new_value
    END AS Total

WHERE
    ID = @ID

What is the correct SQL server code to do so??

Since you're using SQL 2008:

UPDATE
    table_Name

SET
    column_A  
     = CASE
        WHEN @flag = '1' THEN @new_value
        ELSE 0
    END + column_A,

    column_B  
     = CASE
        WHEN @flag = '0' THEN @new_value
        ELSE 0
    END + column_B 
WHERE
    ID = @ID

If you were using SQL 2012:

UPDATE
    table_Name
SET
    column_A  = column_A + IIF(@flag = '1', @new_value, 0),
    column_B  = column_B + IIF(@flag = '0', @new_value, 0)
WHERE
    ID = @ID

this worked great:

UPDATE
    table_Name
SET 
  column_A = CASE WHEN @flag = '1' THEN column_A + @new_value ELSE column_A END,
  column_B = CASE WHEN @flag = '0' THEN column_B + @new_value ELSE column_B END
WHERE
    ID = @ID

Something like this should work:

UPDATE
    table_Name
SET 
  column_A = CASE WHEN @flag = '1' THEN column_A + @new_value ELSE column_A END,
  column_B = CASE WHEN @flag = '0' THEN column_B + @new_value ELSE column_B END
WHERE
    ID = @ID

DECLARE @JCnt int=null
SEt @JCnt=(SELECT COUNT( ISNUll(EmpCode,0)) FROM tbl_Employees WHERE EmpCode=1  )

UPDATE #TempCode
SET janCA= CASE WHEN @JCnt>0 THEN (SELECT SUM (ISNUll(Amount,0)) FROM tbl_Salary WHERE Code=1 )ELSE 0 END
WHERE code=1