T-SQL IF THEN ELSE Control Flow

CREATE PROCEDURE [NBS].SelContextLabel
 @ContextInd bigint,
 @RowPrimaryKeyID nvarchar(100)
AS
BEGIN
 Declare @SQL nvarchar(1000)

 IF (@ContextInd = 1)
  SET @SQL = 'SELECT ''UnitTest'''
 ELSE IF (@ContextInd = 2)
  SET @SQL = 'SELECT Firstname + '' ''+ Lastname FROM NBS.Person WHERE PersonID=' + @RowPrimaryKeyID
 ELSE IF (@ContextInd = 3)
  SET @SQL = 'SELECT InstName FROM NBS.Inst WHERE InstID = ' + @RowPrimaryKeyID
 ELSE
  SET @SQL = 'SELECT '''''
 
 Execute(@SQL)
END