What is the difference between Left, Right, Outer and Inner Joins?

Question

I am wondering how to differentiate all these different joins ...

Answer 1

At first you have to understand what does join do? We connect multiple table and get specific result from the joined tables. The simplest way to do this is cross join.

Lets say tableA has two column A and B. And tableB has three column C and D. If we apply cross join it will produce lot of meaningless row. Then we have to match using primary key to get actual data.

Left: it will return all records from left table and matched record from right table.

Right: it will return opposite to Left join. It will return all records from right table and matched records from left table.

Inner: This is like intersection. It will return only matched records from both table.

Outer: And this is like union. It will return all available record from both table.

Some times we don't need all of the data, and also we should need only common data or records. we can easily get it using these join methods. Remember left and right join also are outer join.

You can get all records just using cross join. But it could be expensive when it comes to millions of records. So make it simple by using left, right, inner or outer join.

thanks