How to select data in a sub query only once?

Question

I have the following query:

SELECT
  ordno
from
  orders
where
  ordno NOT IN (SELECT ordno FROM invoices)

There are about 25000 records in each table. I have indexed the ordno column.

The query hangs without showing any output.

I imagine that the problem is that the select subquery is called 25000 times for each ordno in orders table.

How can I do this?

SELECT
  ordno
from
  orders
WHERE ordno NOT IN (
--  somehow select ordno from invoice table only once
)

Answer 1

If you LEFT JOIN the tables you can then check if the left ordno does not exist. Something like this:

SELECT o.ordno 
FROM orders AS o
LEFT JOIN invoices AS i 
ON i.ordno = o.ordno
WHERE i.ordno IS NULL;

I hope this helps you.

Answer 2

Another answer:

SELECT
  ordno
from
  orders
where
  NOT EXISTS (SELECT * FROM invoices WHERE invoices.ordno = orders.ordno)

Answer 3

You asked: How to

somehow select ordno from invoice table only once

You could do that by adding DISTINCT:

SELECT
  ordno
FROM
  orders
WHERE
  ordno NOT IN (SELECT DISTINCT ordno FROM invoices) ;

... but there is absolutely no reason to do it, if there is an index on invoices(ordno). You can test of course, but the explain plan should be the same and the execution times identical.

(the situation may be different if the invoices is not a base table but a complex view that returns thousands of identical order numbers).

You should also check if the two variations proposed in other answers are more efficient with your data distribution. I wouldn't expect much difference. This blog article Explain Extended: NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: MySQL has some tests that support that but it's never bad to test with your data and table sizes, in your envirorment.

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What you should really check first of all, is whether you have an index on orders(ordno) and on invoices(ordno) and whether these two columns, are of the same datatype.

If there are no indices, add them. If the two columns are of different datatype, change them into the same datatype.