Question

I’m having a bit of trouble getting the right results from a query.

At the moment I have two tables, main_cats and products.

The result I am after is 6 records, in date order, with only one unique main_cat_id.

The basic table structures are

• Main_cats: main_cat_id, main_cat_title
• Products: product_id, main_cat_id, product_name, date_added.

I am hitting problems when I join the main_cat table to the products table. It seems to totally ignore the ORDER BY clause.

SELECT date_added, product_name,main_cat_title FROM ic_products p 
JOIN ic_main_cats icm on icm.main_cat_id=p.main_cat_id 
WHERE p.main_cat_id IN (1,2,12,22,6,8)
GROUP BY p.main_cat_id
ORDER BY date_added   ASC
LIMIT 6

If I leave the join out the query works but shows more than one main_cat_id and I cannot display the main_cat_title as needed.

Answer 1

Your question is (at heart) a “select min/max date per group and associated fields” question.

SELECT p.date_added, p.product_name, icm.main_cat_title
FROM ic_products p 
LEFT JOIN ic_products p2
 ON p.main_cat_id=p2.main_cat_id
 AND p.date_added > p2.date_added
LEFT JOIN ic_main_cats icm ON icm.main_cat_id=p.main_cat_id 
WHERE p2.date_added IS NULL
AND p.main_cat_id IN (1,2,12,22,6,8)

Let me explain: look at this table, being the first LEFT JOIN of the query above:

SELECT p.date_added, p.product_name
FROM ic_products p 
LEFT JOIN ic_products p2
 ON p.main_cat_id=p2.main_cat_id
 AND p.date_added > p2.date_added
WHERE p2.date_added IS NULL

This joins products to itself: it produces a table with every combination of date_added pairs within each category, where the date in the first column is always greater than the date in the second.

Since this is a left join, when the date in the first column is the smallest for that category, the date in the second will be NULL.

So this basically selects the minimum date for each category (I assume you want the minimum date ie earliest occurence, based off your ORDER BY date_added ASC in your question — if you wanted the newest date_added you’d change the > to a < in the above join).

The second LEFT JOIN to icm is just the one in your original question, so that we can retrieve main_cat_title.

There is no need to LIMIT 6 here because firstly, only one row is retrieved per main_cat_id thanks to the first LEFT JOIN, and secondly, your AND p.main_cat_id IN (1,2,12,22,6,8) only selects 6 categories. So 6 categories at one row per category retrieves 6 rows. (Or at most 6; if you have no items in a particular category of course no rows will be retrieved).