how to get data based on a correlated table? - mysql

Question

Listings table
+------------+---------+
| name | id |
+------------+---------+
| Example 1 | 1 |
| Example 2 | 2 |
| Example 3 | 3 |
| Example 4 | 4 |
| Example 5 | 5 |
| Example 6 | 6 |
+------------+---------+
Categories table
+------------+---------+
| name | id |
+------------+---------+
| Catname 1 | 1 |
| Catname 2 | 2 |
| Catname 3 | 3 |
+------------+---------+
ListingCats table
+--------+---------+
| cat_id | list_id |
+--------+---------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 3 | 1 |
| 3 | 3 |
| 2 | 2 |
| 1 | 5 |
| 2 | 6 |
+--------+---------+
I am trying to build 2 queries which should be simple.
The first thing needed is to get a count of how many listings in the listings table corelate to a given category ID in the listingcats table.
The second part is getting all of the data (*) in the rows from the listings table that corelate to the given category id in the listingcats table.
I have tried a number of joins and for some reason none want to work properly. Can anyone help based on the example tables given above please. The 'given' category ID in this case would be '1'.

Answer 1

For the first query, you can use a simple join, and return a count
SELECT COUNT(Name)
FROM Listings l
JOIN ListingCats lc ON l.id = lc.cat_id
WHERE lc.cat_id = 1
This will return all rows from the listings table such that the listings id has a corresponding cat_id in the listingcats table, but exclusive to those that have a cat_id of 1. Then, the count aggregate function returns the number of rows.
For the second one, you can just use the same subquery above, but without the aggregate function, and select all values.
SELECT * FROM Listings l
JOIN ListingCats lc ON l.id = lc.cat_id
WHERE lc.cat_id = 1
Try those, please let me know if they work or not and I will try to work through them more with you.
EDIT
After looking back at the question, if you are given a specific cat_id you don't even need to use a join, you can simply query the listings table for one that has that id. If the given id is one:
SELECT COUNT(Name)
FROM Listings l
WHERE l.id = 1
And then again, even more broad for the second one:
SELECT * FROM Listings l WHERE l.id = 1

Answer 2

CREATE TABLE `listings` (
`id` int(10) unsigned NOT NULL auto_increment,
`name` varchar(10) NOT NULL default '0',
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
CREATE TABLE `categories` (
`id` int(10) unsigned NOT NULL auto_increment,
`name` varchar(10) NOT NULL default '0',
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
CREATE TABLE `listings_cats` (
`cat_id` int(10) unsigned NOT NULL,
`list_id` int(10) unsigned NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
SELECT c.id, c.name, COUNT(lc.list_id) as the_count
FROM categories c
JOIN listings_cats lc ON (lc.cat_id = c.id)
GROUP BY c.id;
SELECT l.id, l.name, c.name AS category_name
FROM listings l JOIN listings_cats lc ON (lc.list_id = l.id)
JOIN categories c ON (lc.cat_id = c.id);