That example is actually very good to prove the point but may be very confusing because of the "shortcut" syntax.

if @k = 0 begin declare @j-2 int = 2 end

Since @k = 1 the block "begin-end" is not executed.

Right?

Wrong. Otherwise variable @j-2 would not be declared and we would get "must declare variable @j-2" error further in the code.

So, it is executed. Right?

Wrong. Because @j-2 does not get initialised with value 2.

What's the... ?

Explanation is simple.

There are actually 2 statements combined together:

declare @j-2 int set @j-2 = 2

Now it all becomes clear:

- DECLARE is executed during compilation, outside of IF..THEN control;

- SET is executed during run time, under IF..THEN control.

Actually, this syntax is one of the worst and most dangerous things introduced by MS in SQL2008.

I saw extremely powerful servers brought down by improper use of this syntax.

Not just tables, not databases. Whole servers, with losing ability to connect to master database.

I probably need to write an article about that experience, if nobody did it before.