MySQL-如何从不同的表排序行？

Question

Imagine I have a database with four tables: 'shopping_list', 'sweets', 'books', 'clothes'

A user can add any number of any item(s) to their shopping list and rearrange the order before submitting it. I'm trying to work out the best way of storing that information in the order they submitted it so I can easily/efficiently display it on screen later.

Currently the item tables (sweets, books, clothes) store the shopping_list ID and an order position which is incremented as I loop through each shopping_list item and insert it into the relevant table.

The item tables have completely different structures with item specific fields.

So I might end up with something like:

shopping_list

ID: 1  UserId:123

sweets

ID: 1  ListId: 1  Order: 1  Flavour: 'Strawberry'  Colour: 'Red'
ID: 2  ListId: 1  Order: 4  Flavour: 'Cola'        Colour: 'Brown'

books

ID: 1  ListId: 1  Order: 3  Name: 'MySQL for Dummies' Author: 'Joe Bloggs'

clothes

ID: 1  ListId: 1  Order: 2  Type: 'Hat'  Size: 'Large'

The only way I have found to SELECT this data in the correct order so far is to UNION three SELECTS from the item tables and ORDER BY the common 'Order' field, but with the unique fields in each table (15-20 per table), I would have to SELECT a lot of NULL fields each time to match the number of columns.

ie

SELECT Id, Order, Flavour, Colour, NULL, NULL, NULL, NULL FROM sweets WHERE ListId=1
UNION
SELECT Id, Order, NULL, NULL, Name, Author, NULL, NULL FROM books WHERE ListId=1
UNION
SELECT Id, Order, NULL, NULL, NULL, NULL, Type, Size FROM clothes WHERE ListId=1
ORDER BY Order

This is obviously a smaller example.... To make this work on my actual tables, I have to write NULL about 20 times on each select. Am I going about this completely the wrong way?

Thanks in advance for any help.

Answer 1

You're probably going about it wrong, yes. You should have an extra table which contains an item's place on the shopping list and a field that represents where the extra information for that field is stored. That will let you query things in order and then select the other information (if you need it) from the other tables.

Right now you've scattered information that belongs together in 3 different tables, which is why you have problems sorting it properly.