How to use if else in SQL Server stored procedure

Stored procedure:

CREATE PROCEDURE validate
    @blockID VARCHAR(255)
AS
BEGIN
    IF @blockID = (SELECT blockID FROM block WHERE blockID = @blockID)
        PRINT 'exists'

    IF @blockID != (SELECT blockID FROM block WHERE blockID = @blockID)
        PRINT 'does not exist'

END

When input the wrong one blockID, the 2nd if conditions does not work.

You probably want to use EXISTS.

CREATE PROCEDURE validate
   @blockID VARCHAR(255)
AS
BEGIN
   IF EXISTS (SELECT blockID FROM block WHERE blockID = @blockID)
      PRINT'exists'
   ELSE
      PRINT'not exists'
END