SQL “IF”, “BEGIN”, “END”, “END IF”?

It has to do with the Normal Form for the SQL language. IF statements can, by definition, only take a single SQL statement. However, there is a special kind of SQL statement which can contain multiple SQL statements, the BEGIN-END block.

If you omit the BEGIN-END block, your SQL will run fine, but it will only execute the first statement as part of the IF.

Basically, this:

IF @Term = 3
    INSERT INTO @Classes
    SELECT              
        XXXXXX  
    FROM XXXX blah blah blah

is equivalent to the same thing with the BEGIN-END block, because you are only executing a single statement. However, for the same reason that not including the curly-braces on an IF statement in a C-like language is a bad idea, it is always preferable to use BEGIN and END.