Mysql insert only if limit has not been reached - php

Question

I hope I have got the title right as I don't really know how to word this!!
Here's the background...
I have a app inserting data to a db. The db holds a date field and data field (there are others but for simplicity these two are the only ones needed). There can only be 8 entries on the same date, no more. In normal operation this is not really an issue but twice a month the database gets hit hard towards the end of the day.
The way I currently do it, is to query how many records there are for a date. If that's less than 9 I insert the data.
It has never happened yet but my concern is that request A comes in and checks the DB and finds there are 7 records. OK to insert. But before A can insert, request B comes in and finds only 7 records. OK to insert. But that would then enter 9 records on one date. This can't happen.
I think there are two solutions to this but would like to know A, if I'm right at all! or B is there a better solution.
I think...
A) I could do it in a transaction. However, would I still no encounter the same problem? As far as I am aware as long as no queries in a transaction fail then it runs anyway.
or
B) Use a stored procedure to check first then insert. I have had very little experience with stored procedures so I must admit I have no idea what I'm talking about here!
Alternatively, is there a way to get a single query to check first if there is less than 9 entries??
Many Thanks in advance,
Gavin

Answer 1

You are afraid of that someone would insert a new record before other one insert it even in the million second?
I have this question too, so I just searched something for you, you can do this with two ways:
For example if you have a table which named date_table and it looks like:
date_field
----------
1998-07-13
1992-03-23
1998-09-17
1994-02-30
A) Count the rows, then insert it when it's under 8 rows. (Using PHP and MySQL)
First, get how many rows are there by this:
SELECT COUNT(*) AS Num FROM date_table WHERE date_field = '1998-07-13'
so you'll get a Num field which told you how many rows were 1998-07-13,
then using PHP to prevent user insert the same date when it's equal 8 by:
<?php if($DB['Num'] >= 8) exit('Sorry, no more same value'); ?>
otherwise insert the row.
Or you don't trust PHP, or you think someone would insert the row more earlier than 1 million second
B) Insert it when it's only under 8 rows were same with only one query:
INSERT INTO date_table (date_field)
SELECT '1998-07-13'
FROM dual
WHERE (SELECT COUNT(*) FROM date_table WHERE date_field = '1998-07-13') < 9;
BEWARE: dual is a mysql system table, so you don't need to create it
If you don't understand how the above query works:
insert into TABLE_NAME (TABLE_FIELD)
select VALUE_YOU_WANT_TO_INSERT
from dual
where (select count(*)
from TABLE_NAME
where THE_FIELD_YOU_WANT_TO_CHECK = '1998-07-13'
) < LESS_THEN_HOW_MUCH;
EDIT: add more fields change the following lines:
INSERT INTO date_table (field1, field2, field3)
SELECT value1, value2, value3