Question

I have multiple tables that I want to join to get a specific result.

Tables are:

ad_categories, ad_image, ad_data, ad_location

I need a row output for every one of the following rows in the ad_categories table, regardless if there exists data for any of these categories for a specific location when using the WHERE conditional.

mysql> SELECT type FROM ad_categories;
+---------------+
| type          |
+---------------+
| restaurants   |
| fitness       |
| funactivities |
| shopping      |
| homes         |
| men           |
+---------------+

For example:

mysql> SELECT alias, type, sha, originalname FROM ad_categories LEFT JOIN ad_data ON ad_data.cid=ad_categories.id LEFT JOIN ad_image ON ad_image.pid=ad_data.id LEFT JOIN ad_location ON ad_location.id=ad_data.lid GROUP BY type;
+--------+---------------+------------------------------------------+---------------------------------+
| alias  | type          | sha                                      | originalname                    |
+--------+---------------+------------------------------------------+---------------------------------+
| malibu | fitness       | ad8b277202f4ded274274744b3fa28f34e9f1c21 | thai_body_works.jpg             |
| malibu | funactivities | 6a226df8ff827aa020b077e9e0d48e4701ae2fca | rosenthal-the_malibu_estate.jpg |
| NULL   | homes         | NULL                                     | NULL                            |
| NULL   | men           | NULL                                     | NULL                            |
| malibu | restaurants   | 98f357dfa5bdb2eb1d480dc0e8b7156b1eecac31 | moonshadows.jpg                 |
| malibu | shopping      | 1b2ef538691569842b9f9fb6c3816673f334205a | malibu_surf_shack.jpg           |
+--------+---------------+------------------------------------------+---------------------------------+
6 rows in set (0.00 sec)

Lists all the category types I need, but I haven’t specified the location which is what I need to do. I want only one row per type, per location specified. If no data exists for the category type for that location, fill in null values for the columns other than type.

mysql> SELECT alias, type, sha, originalname FROM ad_categories LEFT JOIN ad_data ON ad_data.cid=ad_categories.id JOIN ad_image ON ad_image.pid=ad_data.id LEFT JOIN ad_location ON ad_location.id=ad_data.lid WHERE ad_location.alias='malibu' GROUP BY type;
+--------+---------------+------------------------------------------+---------------------------------+
| alias  | type          | sha                                      | originalname                    |
+--------+---------------+------------------------------------------+---------------------------------+
| malibu | fitness       | ad8b277202f4ded274274744b3fa28f34e9f1c21 | thai_body_works.jpg             |
| malibu | funactivities | 6a226df8ff827aa020b077e9e0d48e4701ae2fca | rosenthal-the_malibu_estate.jpg |
| malibu | restaurants   | 98f357dfa5bdb2eb1d480dc0e8b7156b1eecac31 | moonshadows.jpg                 |
| malibu | shopping      | 1b2ef538691569842b9f9fb6c3816673f334205a | malibu_surf_shack.jpg           |
+--------+---------------+------------------------------------------+---------------------------------+
4 rows in set (0.00 sec)

I am not a pro SQL ninja. Can someone correct me if I am building my SQL query incorrectly or educate me with a few clues on how to properly accomplish what I want to do? Thanks.

Answer 1

Left this alone for a while, came back to it. Here’s the answer I needed, thanks to a coworker at a new job:

SELECT 
    alias, type, sha, originalname 
FROM 
    ad_categories 
LEFT JOIN 
    ad_location ON ad_location.alias = '?' 
LEFT JOIN 
    ad_data ON ad_data.cid=ad_categories.id AND ad_location.id=ad_data.lid 
LEFT JOIN 
    ad_image ON ad_image.pid=ad_data.id GROUP BY type;

Thanks coworker 😉