Question

I have 3 tables which need to be linked in an SQL statement (I’m using PHP – MySQL if it helps). I need to extract all orders where the vendor field from the third table equals ‘3’, as below:

orders   -  orders_items - items

order_id -> order_id
            item_id     -> id
                           vendor = '3'

There are many ways to do this I believe with various WHERE and JOINS but I’m asking for the most efficient methods in comparison to my method below:

SELECT
    orders.order_id
FROM
    items, orders
INNER JOIN
    orders_items
ON
    orders.order_id = orders_items.order_id
WHERE
    orders_items.item_id = items.id
AND
    items.vendor = '3'
GROUP BY
    orders.order_id

Answer 1

Using , notation is not universally considered bad practice, but I think it’s quite a minority now that agree with it. Even Oracle (whose users seems to be the most vocal supporters of that syntax) recommend to not use it.

But I don’t know anyone who would support mixing , and ANSI-92’s JOIN syntax. It’s just asking for trouble.

SELECT
  orders.order_id
FROM 
  orders
INNER JOIN
  orders_items
    ON orders.order_id = orders_items.order_id
INNER JOIN
  items
    ON orders_items.item_id = items.id
WHERE
  items.vendor = '3'
GROUP BY
  orders.order_id

The SQL Optimiser doesn’t execute that exactly as you specified it. SQL is just a expression from which the SQL Optimiser derives a plan to give a result that fits. By writing it as above the optimiser will find what it sees as the best order to filter, join, sort, etc, and which are the best indexes, etc to use to do those things.

EDIT

I’ve noticed people supporting DISTINCT over GROUP BY.

While DISTINCT is slightly shorter, it is not any quicker, and does place restrictions on you. You can’t later add COUNT(*) for example, but with GROUP BY you can.

In short, GROUP BY can do anything DISTINCT can, but that’s not true the other way around. I only use DISTINCT in very trivial pieces of code so I can get a shole query on one line. Even then I often later regret it a little as the code develops and I need to rever to GROUP BY.