Why INNER JOIN not equal (!=) hang forever

Question

When I execute the following query:

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

I get the result within 2 s

Answer 1

Instead of this you can use this method:

SELECT * FROM `table1` 
     LEFT JOIN table2 ON table2.number = table1.number 
WHERE 
     table2.number is NULL OR table1.number is NULL