Question

I have two MySQL tables structed as follows;

advert

+-------------------+--------------+
| Field             | Type         |
+-------------------+--------------+
| id                | int(11)      |
| title             | varchar(150) |
+-------------------+--------------+

advert_interest_record

+--------------+------------+
| Field        | Type       |
+--------------+------------+
| id           | int(11)    | 
| advert_id    | int(11)    |
| message      | longtext   |
+--------------+------------+

I want to select a list of all records from the “advert” table – and get the number of rows from the “advert_interest_record” table where advert.id=response.advert_id.

The SQL I’m using to join the tables is as follows;

SELECT advert.id, advert.title 
FROM advert 
LEFT JOIN advert_interest_record ON advert.id = advert_interest_record.advert_id  
GROUP BY advert_interest_record.advert_id 

The first problem I’ve got is that when there are no records in the “advert_interest_record” table, the record from the “advert” table isn’t included in the result – and I need it to be.

My second question is, how would I go about counting the rows in the “advert_interest_record” table?

Any advice on this would be appreciated.

Thanks.

Answer 1

First of all; you’re selecting two fields from the advert table; while gouping on one field from the advert_interest_record table.

Assuming that for all advert.id values, the advert.title value is the same, it might give correct results.

Anyways; since you want the data in advert to be leading, it’s best to group on the fields of that table that you want uniquely back (which also helps in getting the count of records associated with that in the other table).

For example, you could query as following:

SELECT a.id, a.title, COUNT(air.id)
FROM advert a
LEFT JOIN advert_interest_records air ON a.id = air.advert_id
GROUP BY a.id, a.title

This would give you a result of all unique combinations of advert.id and advert.title, together with the number of records in advert_interest_records that are associated with the advert.id value.

I am assuming that advert.id is unique, and that every id only occurs once. Otherwise the grouping on both id and title (which is not per se needed for MySQL, I believe, in contrast to other DBMS’es), might give you incorrect results.