SQL 3 table join - mysql

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.

Answer 2

select o.order_id from orders o inner join orders_items oi on o.order_id = oi.item_id inner join items i on oi.item_id = i.id where i.vendor='3';
Many ways to do the same like joins, sub query, in clause. Depends on the need like terms of time or terms of memory which will best to use also major dependance on the INDEX columns of table and amount of data join table having.

Answer 3

You don't need the GROUP BY, just make a DISTINCT if you need to remove duplicates:
SELECT DISTINCT o.order_id
FROM orders o
INNER JOIN orders_items oi ON oi.order_id = o.order_id
INNER JOIN items i ON i.id = oi.items_id
where i.vendor = '3'
And also, use INNER JOIN on all tables :)

Answer 4

This is efficient and will work too::
SELECT
DISTINCT(orders.order_id)
FROM
items
INNER JOIN orders_items on (items.id=orders_items.item_id )
inner join orders on (orders.order_id=order_items.order_id)
WHERE
items.vendor = '3'

Answer 5

SELECT
orders.order_id
FROM
orders o
INNER JOIN orders_items oi ON o.order_id = oi.order_id
INNER JOIN items i ON oi.item_id = i.item_id
WHERE
i.vendor = 3
The table1, table2 syntax isn't something that I've used, but I imagine listing the tables as joins is more efficient as that seems to be the most accepted way.
Also, you don't need to put speech marks on the vendor criteria if the field is an integer.

Answer 6

SELECT O.order_id AS Id
FROM orders O
INNER JOIN orders_items OI
ON O.order_id = OI.order_id
INNER JOIN items I
ON OI.item_id = I.id
WHERE I.vendor = '3'
GROUP BY O.order_id