What is the difference between INNER JOIN and LEFT JOIN in PostgreSQL and when would you use each one?

Understanding INNER JOIN and LEFT JOIN in PostgreSQL

When working with SQL databases in PostgreSQL, it is essential to understand the differences between INNER JOIN and LEFT JOIN. Both types of joins are used to combine data from two or more tables based on a related column between them.

INNER JOIN

INNER JOIN is used to retrieve data that exists in both tables being joined. It returns only the rows where there is a match between the columns in both tables. If there is no match, the rows are not included in the result set.

Example:

SELECT t1.column1, t2.column2
FROM table1 t1
INNER JOIN table2 t2
ON t1.id = t2.id;

LEFT JOIN

LEFT JOIN, also known as LEFT OUTER JOIN, returns all the rows from the left table and the matched rows from the right table. If there is no match in the right table, NULL values are returned for the columns of the right table.

Example:

SELECT t1.column1, t2.column2
FROM table1 t1
LEFT JOIN table2 t2
ON t1.id = t2.id;

When to use each one?

Use INNER JOIN when you only want to retrieve data that exists in both tables and want to eliminate non-matching rows. This is useful when you want to retrieve data that has matching values in both tables.

Use LEFT JOIN when you want to retrieve all the rows from the left table, even if there are no matching rows in the right table. This is useful when you want to retrieve data from the primary table regardless of whether there is a match in the related table.

Understanding the differences between INNER JOIN and LEFT JOIN can help you optimize your SQL queries and retrieve the desired results efficiently in PostgreSQL.