SQL: JOIN vs LEFT OUTER JOIN?

In SQL, understanding the difference between a JOIN (often implying INNER JOIN) and a LEFT OUTER JOIN is crucial for writing accurate queries.

An INNER JOIN (or just JOIN) returns only the rows that have matching values in both tables. If there’s no match, the row is excluded from the result.

A LEFT OUTER JOIN returns all rows from the left table, and the matching rows from the right table. If there’s no match in the right table, the result will include NULL values for columns from the right table.

Here’s a quick breakdown:

INNER JOIN (JOIN):

   Only matched rows from both tables are returned.

   No match = row excluded.

LEFT OUTER JOIN:

   All rows from the left table are included.

   Unmatched rows in the right table return with NULLs.

Example:

-- INNER JOIN

SELECT A.name, B.order_id 

FROM Customers A 

JOIN Orders B ON A.customer_id = B.customer_id;

-- LEFT OUTER JOIN

SELECT A.name, B.order_id 

FROM Customers A 

LEFT OUTER JOIN Orders B ON A.customer_id = B.customer_id;

When to use what?

• Use INNER JOIN when you only want data that exists in both tables.
  
• Use LEFT OUTER JOIN when you want all records from the left table, even if there's no match in the right one.
  

This difference is especially important when working with reports, optional relationships, or filtering incomplete data.