The following questions are of little importance, yet I find them entertaining. I stumbled upon these while developing QueryScript.
Can you guess the results of the following statements?
Pop quiz 1
SET @x := 7; SELECT ++@x;
What is the computation result? What will be the value of @x?
Pop quiz 2
SET @ := 4; SELECT @ + @'' + @``
What is the computation result?
Pop quiz 3
SET @a := 2; SELECT @a = @'a' = @`a`
Do we get TRUE or FALSE? When?
Consider the following queries as hints to the above questions:
SELECT +++++-@x; SELECT @ = @'', @ = @`` SELECT (@a = @'a') = @`a`
- Pop quiz 1
++@x is interpreted as +(+(@x)), which is in turn evaluated as 0+(0+(@x)). No relation to C's ++ operator. @x is unchanged.
- Pop quiz 2
@x is the same as @'x' and as @`x`. What's funny is that one is allowed to create the empty-named user defined variable @''. Makes for a weird looking syntax, but nothing special about it. Computation result is 12.
- Pop quiz 3
We've already established that @a, @'a' and @`a` are the same, I just used this notation for adding pepper to the quiz. The real question is whether 2 = 2 = 2 holds true. It does not. There is no 3-way comparison. All comparisons are in pairs, which is why the expression evaluates as (2 = 2) = 2, leading to 1 = 2, "1" being the TRUE value of (2 = 2). The only value of @a for which the expression holds true is 1.