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MySQL Cookbook
book

MySQL Cookbook

by Paul DuBois
October 2002
Intermediate to advanced content levelIntermediate to advanced
1024 pages
27h 26m
English
O'Reilly Media, Inc.
Content preview from MySQL Cookbook

Calculating LIMIT Values from Expressions

Problem

You want to use expressions to specify the arguments for LIMIT.

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.

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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Publisher Resources

ISBN: 0596001452Catalog PageErrata