Are left outer joins and left joins the same?

sql

join

best-practices

performance

byNikita Barsukov·Dec 29, 2024

Both a LEFT JOIN and a LEFT OUTER JOIN are identical in SQL. Their purpose is to fetch all rows from the left table and matching rows from the right table, with NULLs for no matches.

Example:

SELECT a.*, b.*
FROM a
LEFT JOIN b ON a.key = b.key;
-- Man, I've looked left and right, this looks like a LEFT JOIN!

This yields the same results as:

SELECT a.*, b.*
FROM a
LEFT OUTER JOIN b ON a.key = b.key;
-- Alright Sherlock, it's elementary: this ain't nothing but a LEFT OUTER JOIN!

Both queries pull in every row from a matched with corresponding rows from b, throwing in NULL values for b columns when no match exists.

Distinguish between different joins

Although a LEFT OUTER JOIN and a LEFT JOIN are synonymous, deciphering various types of joins is a cornerstone of SQL expertise:

INNER JOIN: Commands a match on both tables. If it were a club, you'd need to be on both guest lists to get in!
RIGHT JOIN / RIGHT OUTER JOIN: All rows from the right table and matched ones from the left. If it were a buffet, it'd serve everything from the right table and whatever matches from the left table.
FULL JOIN: An amalgamation of LEFT and RIGHT JOIN, keeping rows from both tables irrespective of a match.
SELF JOIN: It's the SQL version of talking to oneself - a table joining with itself. Useful for hierarchy-based questions.
CARTESIAN JOIN / CROSS JOIN: Doesn't discriminate, combines all rows from two or more tables, match or no match.

Mind you, different SQL dialects may have subtle syntax differences, so ensure if OUTER is mandatory or optional.

Joining the dots: Key points regarding joins

SQL is about managing data relationships. Let's discuss some critical aspects of joins:

Maximizing utility of joins in queries

LEFT JOIN: For a complete roster of records from the primary (left) table.
INNER JOIN: Pulls the records where there's corresponding data in both tables.

Avoiding common join blunders

Steer clear of joining tables without a matching condition, unless aiming for a CARTESIAN product.
Don't mix-up FULL JOIN with LEFT or RIGHT JOIN - they serve distinct purposes.

Boosting join performance

To speed up joins, index the columns used in ON clauses. It's like putting your joins on a speedboat!
Evaluate execution plans regularly to understand how your database processes joins, then optimize fittingly.

explain-codes / Sql / Are left outer joins and left joins the same?

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