Left Association in Oracle

What is the cause of the large amount of empty data that sometimes occurs when you use the left association?

This is because the field in the primary table is empty until the primary table is associated with another table, and then the field associated with the other table is used, and a null value appears.

For example:

SELECT * FROM student T left join EMP T1 on T. Math =t1.empno

This is a table with an empty data associated with another table

The results are as follows:

So keep an eye on the empty data when left-associative!

Left Association in Oracle