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 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”.
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”.
User Comments
when using float(5,5) or something like float(m,m), storing 1 into this field will return 0.99999!!!! and not 1!!!! to work you need float(m+1,m)
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