mySQL一对多查询

Question

I've got 3 tables that are something like this (simplified here ofc):

• users
  • user_id
  • user_name
• info
  • info_id
  • user_id
  • rate
• contacts
  • contact_id
  • user_id
  • contact_data

users has a one-to-one relationship with info, although info doesn't always have a related entry.

users has a one-to-many relationship with contacts, although contacts doesn't always have related entries.

I know I can grab the proper 'users' + 'info' with a left join, is there a way to get all the data I want at once?

For example, one returned record might be:

user_id: 5
user_name: tom
info_id: 1
rate: 25.00
contact_id: 7
contact_data: 555-1212
contact_id: 8
contact_data: 555-1315
contact_id: 9
contact_data: 555-5511

Is this possible with a single query? Or must I use multiple?

Answer 1

It is possible to do what you're asking in one query, but you'd either need a variable number of columns which is evil because SQL isn't designed for that, or you'd have to have a fixed number of columns, which is even more evil because there is no sensible fixed number of columns you could choose.

I'd suggest using one of two alternatives:

1. Return one row for each contact data, repeating the data in other columns:

5 tom 1 25.00 7 555-1212
5 tom 1 25.00 8 555-1315
5 tom 1 25.00 9 555-5511

The problem with this of course is that redundant data is normally a bad idea, but if you don't have too much redundant data it will be OK. Use your judgement here.

2. Use two queries. This means a slightly longer turnaround time, but less data to transfer.

In most cases I'd prefer the second solution.

You should try to avoid making a large number of queries inside a loop. This can almost always be rewritten to a single query. But if using two queries is the most natural way to solve your problem, just use two queries. Don't try to cram all the data you need into a single query just for the sake of reducing the number of queries.

Answer 2

Each row of result must have the same columns, so you can't aggregate multiple rows of contact not having the other columns as well.

Hopefully, this query would achieve what you need:

SELECT
    u.user_id as user_id,
    u.user_name as user_name,
    i.info_id as info_id,
    i.rate as rate,
    c.contact_id as contact_id,
    c.contact_data as contact_data
FROM users as u
LEFT JOIN info as i ON i.user_id = u.user_id
LEFT JOIN contacts as c ON c.user_id = u.user_id