MySql 加入一对多

Question

I want to join a couple of tables that have a one to many relationship, but I'm having trouble working out how to do it.

I have one table with a list of items, and a second table which has a list of their ids against user id and an action. Each user can only have one action against each item, and I want to join these in such a way that I get an output with the item along with the action performed by each user.

items table

item_id name
1       Desk
2       Phone
3       Fax

actions table

item_id user_id action
1       1       sell
1       2       buy
1       3       sell
2       1       sell
2       2       buy
2       3       sell
3       1       sell
3       3       sell

desired output

item_id name  user_id1 user_id2 user_id3
1       Desk  sell     buy      sell
2       Phone sell     buy      sell
3       Fax   sell     NULL     sell

Many thanks

Answer 1

If you know the users in advance, you can use conditional aggregation:

select i.item_id, i.name,
  max(case when a.user_id = 1 then a.action end) as user_1,
  max(case when a.user_id = 2 then a.action end) as user_2,
  max(case when a.user_id = 3 then a.action end) as user_3
from items i left join 
  actions_table a 
  on a.item_id = i.item_id
group by i.item_id, i.name;

If you want to handle a variable number of users, then you cannot use a simple select because the columns in a select have to be explicitly defined. One method is dynamic SQL, where you construct the SQL with all the users (using logic similar to the above). Another method would store the results using JSON.

Answer 2

It will be better if we are using modern sql wrap functin for this problem, but unfortunatelly there is still no factory function available until now. So it seems we should create any user defined extension to be embedded into the system (engine) or manipulate the resultset (query result) from application layer as an alternative.

Otherwise we can also use ANSI/ISO Join to solve this problem easily.

Select items.*, a1.action user_id1, a2.action user_id2, a3.action user_id3
From items
Left Join actions a1
 On items.id = a1.item_id and a1.user_id = 1
Left Join actions a2
 On items.id = a2.item_id and a2.user_id = 2
Left Join actions a3
 On items.id = a3.item_id and a3.user_id = 3

Hope my answer could help.

Cheers.