3.20 Calculating LIMIT Values from Expressions
3.20.1 Problem
You want to use
expressions to specify the arguments for LIMIT.
3.20.2 Solution
Sadly, you cannot. You can use only literal integers—unless you
issue the query from within a program, in which case you can evaluate
the expressions yourself and stick the resulting values into the
query string.
3.20.3 Discussion
Arguments to LIMIT must be literal integers, not
expressions. Statements such as the following are illegal:
SELECT * FROM profile LIMIT 5+5;
SELECT * FROM profile LIMIT @skip_count, @show_count;
The same "no expressions allowed"
principle applies if you're using an expression to
calculate a LIMIT value in a program that
constructs a query string. You must evaluate the expression first,
then place the resulting value in the query. For example, if you
produce a query string in Perl (or PHP) as follows, an error will
result when you attempt to execute the query:
$str = "SELECT * FROM profile LIMIT $x + $y";
To avoid the problem, evaluate the expression first:
$z = $x + $y;
$str = "SELECT * FROM profile LIMIT $z";
Or do this (but don't omit the parentheses or the
expression won't evaluate properly):
$str = "SELECT * FROM profile LIMIT " . ($x + $y);
If you're constructing a two-argument
LIMIT clause, evaluate both expressions before
placing them into the query string.
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