Test the query with EXPLAIN to check whether you can find something that is obviously wrong. See Section 12.8.2, “EXPLAIN Syntax”.

Select only those columns that are used in the WHERE clause.

Remove one table at a time from the query until it returns some rows. If the tables are large, it is a good idea to use LIMIT 10 with the query.

Issue a SELECT for the column that should have matched a row against the table that was last removed from the query.

If you are comparing FLOAT or DOUBLE columns with numbers that have decimals, you can't use equality (=) comparisons. This problem is common in most computer languages because not all floating-point values can be stored with exact precision. In some cases, changing the FLOAT to a DOUBLE fixes this. See Section B.5.5.8, “Problems with Floating-Point Values”.

Similar problems may be encountered when comparing DECIMAL values prior to MySQL 5.0.3.

If you still can't figure out what is wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file by dumping the tables with mysqldump --quick db_name tbl_name_1 ... tbl_name_n > query.sql. Open the file in an editor, remove some insert lines (if there are more than needed to demonstrate the problem), and add your SELECT statement at the end of the file.

Verify that the test file demonstrates the problem by executing these commands:

shell> mysqladmin create test2
shell> mysql test2 < query.sql

Attach the test file to a bug report, which you can file using the instructions in Section 1.7, “How to Report Bugs or Problems”.