Question

When I execute the following query:

SELECT * FROM `table1` 
 INNER JOIN table2 ON table2.number = table1.number

I get the result within 2 seconds. There are about 6 millions records in table2 and 1 million records in table1

table2.number and table1.number are indexed.

Now I want to get a list of numbers that not exist. Like this:

SELECT * FROM `table1` 
 INNER JOIN table2 ON table2.number != table1.number

It take forever and still hanging.. How to fix?

Answer 1

Let’s say your first INNER JOIN returns 75% of the 1,000,000 rows in table1. The second query does not return the 250,000 other rows as you think. Instead, it attempts to create a Cartesian product and remove the 750,000 matching rows. Thus it’s trying to return 6,000,000×1,000,000-750,000 rows. That’s a bulging 6×10¹² row result set.

You probably want this:

SELECT * FROM table1
LEFT JOIN table2 ON table2.number = table1.number
WHERE table2.number IS NULL

This returns rows in table1 not present in table2.

You might also be interested in FULL OUTER JOIN:

SELECT * FROM table1
FULL OUTER JOIN table2 ON table2.number = table1.number
WHERE table1.number IS NULL AND table2.number IS NULL

This returns rows in both tables that don’t have a match on the other table.