Question

What I did was, I wanted each user to have their own “unique” numbering system. Instead of auto incrementing the item number by 1, I did it so that Bob’s first item would start at #1 and Alice’s number would also start at #1. The same goes for rooms and categories. I achieved this by creating “mapping” tables for items, rooms and categories.

The query below works, but I know it can definitely be refactored. I have primary keys in each table (on the “ids”).

SELECT unique_item_id as item_id, item_name, category_name, item_value, room_name
                FROM
                    users_items, users_map_item, users_room, users_map_room, users_category, users_map_category
                WHERE
                    users_items.id = users_map_item.map_item_id AND 
                    item_location = users_map_room.unique_room_id AND 
                    users_map_room.map_room_id = users_room.room_id AND 
                    users_map_room.map_user_id = 1 AND 
                    item_category = users_map_category.unique_category_id AND 
                    users_map_category.map_category_id = users_category.category_id AND 
                    users_category.user_id = users_map_category.map_user_id AND 
                    users_map_category.map_user_id = 1
                ORDER BY item_name

users_items

|  id  |  item_name  |  item_location  |item_category  |
--------------------------------------------------------
|   1  |   item_a    |        1        |      1        |
|   2  |   item_b    |        2        |      1        |
|   3  |   item_c    |        1        |      1        |

users_map_item

|  map_item_id  |  map_user_id  |  unique_item_id  |
----------------------------------------------------
|       1       |       1       |          1       |
|       2       |       1       |          2       |
|       3       |       2       |          1       |

users_rooms

|  id  |  room_name  |
----------------------
|   1  |   basement  |
|   2  |   kitchen   |
|   3  |   attic     |

users_map_room

|  map_room_id  |  map_user_id  |  unique_room_id  |
----------------------------------------------------
|       1       |       1       |          1       |
|       2       |       1       |          2       |
|       3       |       2       |          1       |

users_category

|  id  |  room_name  |
----------------------
|   1  |   antiques  |
|   2  |   appliance |
|   3  |   sporting goods |

users_map_category

|  map_room_id  |  map_user_id  |  unique_category_id  |
----------------------------------------------------
|       1       |       1       |          1       |
|       2       |       1       |          2       |
|       3       |       2       |          1       |

Answer 1

Rewriting your query with explicit JOIN conditions makes it more readable (while doing the same).

SELECT mi.unique_item_id AS item_id
     , i.item_name
     , c.category_name
     , i.item_value
     , r.room_name
FROM   users_map_item     mi
JOIN   users_items        i  ON i.id = mi.map_item_id
JOIN   users_map_room     mr ON mr.unique_room_id = i.item_location
JOIN   users_room         r  ON r.room_id = mr.map_room_id
JOIN   users_map_category mc ON mc.unique_category_id = i.item_category
JOIN   users_category     c  ON (c.user_id, c.category_id)
                              = (mc.map_user_id, mc.map_category_id)
WHERE  mr.map_user_id = 1
AND    mc.map_user_id = 1
ORDER  BY i.item_name

The result is unchanged. Query plan should be the same. I see no way to improve the query further.

You should use LEFT [OUTER] JOIN instead of [INNER] JOIN if you want to keep rows in the result where no matching rows are found in the right hand table. You may want to move the additional WHERE clauses to the JOIN condition in this case, as it changes the outcome.