while using IF...THEN - when the condition evaluates to false, the it ignores the action in THEN clause and proceeds with the next command. so, nor explicit ELSE stmt with BTEQ IF...THEN but its obvious.
If you want to prove a method related with IF- ELSE but with Querys maybe this example can help you:
SELECT TOP 10 * FROM DB.TABLE; --Q1
.IF ERRORCODE !=0 THEN .GOTO DoQuery1;
.IF ERRORCODE =0 THEN .GOTO DoQuery2;
SELECT TOP 1 * FROM DB.TABLE; --Q2
SELECT TOP 2 * FROM DB.TABLE; --Q3
Depending of the result of Q1, the sequence of the bteq will be defined, if it goes to DoQuery1 or DoQuery2, but it's very important put de command .GOTO End to keep de estructure of the method IF-ELSE, if you don´t put it you can enter in a sequence where your code will process both Query´s (first Q2 and then Q3).