If you have a complicated query that uses many tables but that doesn't return any rows, you should use the following procedure to find out what is wrong:
Test the query with EXPLAIN to check
whether you can find something that is obviously wrong.
See 項6.2.1. 「EXPLAINを使用して、クエリを最適化する」.
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's 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 項B.1.5.8. 「Problems with Floating-Point Comparisons」.
If you still can't figure out what's 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 test2shell>mysql test2 < query.sql
Attach the test file to a bug report, which you can file using the instructions in 項1.7. 「質問またはバグの報告」.