What are the differences between the JOIN and LEFT JOIN operators in SQL, and when is each better to use?

The JOIN operator (usually INNER JOIN) returns only those rows that have matches in both tables based on the join condition. If there are no matches, the rows are not included in the result.

LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table and the matching rows from the right table. If there is no match for a row from the left table, the result will include the values from the left table, and the fields from the right will be filled with NULL.

When to use:

• JOIN (INNER JOIN) — when only related data from both tables is needed.
• LEFT JOIN — when you want to get all data from the left table, even if there are no related records in the right table.

Example:

SELECT a.id, b.value
FROM tableA a
JOIN tableB b ON a.id = b.a_id; -- only matching records

SELECT a.id, b.value
FROM tableA a
LEFT JOIN tableB b ON a.id = b.a_id; -- all from tableA, with data from tableB or NULL