SQL查询-选择前5行并仅在存在用户的情况下进一步选择行

Question

I kind of stuck on how to implement this query - this is pretty similar to the query I posted earlier but I'm not able to crack it.

I have a shopping table where everytime a user buys anything, a record is inserted.

Some of the fields are

* shopping_id (primary key)
* store_id
* user_id

Now what I need is to pull only the list of those stores where he's among the top 5 visitors:

When I break it down - this is what I want to accomplish:

* Find all stores where this UserA has visited
* For each of these stores - see who the top 5 visitors are.
* Select the store only if UserA is among the top 5 visitors.

The corresponding queries would be:

select store_id from shopping where user_id = xxx

select user_id,count(*) as 'visits' from shopping 
where store_id in (select store_id from shopping where user_id = xxx)
group by user_id
order by visits desc
limit 5

Now I need to check in this resultset if UserA is present and select that store only if he's present. For example if he has visited a store 5 times - but if there are 5 or more people who have visited that store more than 5 times - then that store should not be selected.

So I'm kind of lost here.

Thanks for your help

Answer 1

This should do it. It uses an intermediate VIEW to figure out how many times each user has shopped at each store. Also, it assumes you have a stores table somewhere with each store_id listed once. If that's not true, you can change SELECT store_id FROM stores to SELECT DISTINCT store_id FROM shopping for the same effect but slower results.

 CREATE VIEW shop_results (store_id, user_id, purchase_count) AS
     SELECT store_id, user_id, COUNT(*)
     FROM shopping GROUP BY store_id, user_id

 SELECT store_id FROM stores 
    WHERE 'UserA' IN 
      (SELECT user_id FROM shop_results 
       WHERE shop_results.store_id = stores.store_id 
       ORDER BY purchase_count DESC LIMIT 5)

You can combine these into a single query by placing the SELECT from the VIEW inside the sub-query, but I think it's easier to read this way and it may well be true that you want that aggregated information elsewhere in the system — more consistent to define it once in a view than repeat it in multiple queries.