Question

I’m stuck with retrieving data from a MySQL-database in an efficient way.

I have a table with a lot of items. Each item can have a status 1, 2 or 3. I want to select item with certain statusses, depending on links to these records in other tables.

Example:

• A table items with fields: id, name, status, …
• A table bought with fields: itemId, userId
• A table rented with fields: itemId, userId

Say I want this for userId 123:

1. All the records that are bought, having status 1
2. All the records that are rented, having status 1 or 2
3. All the records that aren’t bought or rented, having status 3

My select query now looks something like this:

SELECT
  i.id,
  i.name,
  i.status,
  // bought by this user?
  SUM(IF(b.userId = 123, 1, 0)) AS bought,
  // rented by this user?
  SUM(IF(r.userId = 123, 1, 0)) AS rented,

FROM items i

LEFT JOIN bought b
  ON b.itemId = i.id

LEFT JOIN rented r
  ON r.itemId = r.id

GROUP BY i.id

HAVING
  // bought and status 1
  (bought > 0 AND status = 1)
  // rented and status 1 or 2
  OR (rented > 0 AND status IN (1, 2)
  // not bought or rented and status 3
  OR (bougth = 0 AND rented = 0 AND status = 3)

ORDER BY i.name ASC

Question 1
Is the SUM part in the SELECT clause a good way to determine whether there are entries in another table linked to an item? Assuming there’s only one entry per user, the sum will be 1 or 0, giving me the information I need. But it just seems.. weird somehow.

Question 2
Even though this works, there’s one big problem: it basically retrieves all the items and then filters them using the HAVING clause. Since there are quite a few entries, the queries is way too slow this way. I’m trying to figure out how to fix this.

I first tried a WHERE clause.. but how?

...
WHERE
  // only items with status 1 if bought or rented
  (t.status = 1 AND (bought > 0 OR rented > 0))
  // only items with status 2 if rented
  OR (t.status = 2 AND rented > 0)
  // only items with status 3 if not bought or rented
  OR (t.status = 3 AND bought = 0 AND rented = 0)
...

But you can’t use variables from the SELECT clause. And since there is no column rented or bought in the items table, this won’t work.

I also tried using user-definable variables, but this didn’t work either:

SELECT
  ...
  @bought := SUM(IF(b.userId = 123, 1, 0)) AS bought,
...
WHERE @bought = ... // does not work

Then I tried a sub-query, but I can’t get it to use the main queries item id:

...
WHERE
  ...
  // only items with status 2 if rented
  OR (
    t.status = 2
    AND (
      SELECT COUNT(r2.userId)
      FROM rented r2
      WHERE r2.userId = 123
        AND r2.itemId = i.itemId // it doesn't recognize i.itemId
    ) > 0
  )
  ...

Any ideas? I’d also like to keep everything in a single query. This is just a stripped down example, but actual one is rather large. I’m sure I can split everything up and use various queries to collect everything separately, but that’ll just add a lot more code and doesn’t make the maintainability any easier.

Answer 1

Use two subqueries (one for the bought and one for the rented) and left join them to the user table in your main query.

EDIT: forgive my MySQL, it’s been a while, but I had in mind something like:

select i.id, i.name, i.status, ifnull(b.TotalBought,0) AS ItemsBought, ifnull(r.TotalRented,0) AS ItemsRented
FROM items i 
LEFT JOIN (select itemid, COUNT(*) AS TotalBought FROM bought WHERE userid=123 GROUP BY itemid) AS b ON b.itemid=i.itemid
LEFT JOIN (select itemid, COUNT(*) AS TotalRented FROM rented WHERE userid=123  GROUP BY itemid) AS r ON r.itemid=i.itemid
WHERE (i.status=1 AND ifnull(b.TotalBought,0)>0)
OR (ifnull(r.TotalRented,0) >0 AND i.status in(1,2)
OR (ifnull(b.TotalBought,0)=0 AND ifnull(r.TotalRented,0) =0 AND i.status=3)
ORDER BY i.name ASC