Какое отличие между оператором JOIN и LEFT JOIN при объединении таблиц в SQL?

The JOIN operator (usually INNER JOIN) returns only those rows that have matches in both joined tables based on the join condition.

LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table and the matching rows from the right table. If there are no matches in the right table, the result set will contain NULL values for the right table's columns.

Example:

-- INNER JOIN
SELECT * FROM A
JOIN B ON A.id = B.a_id;

-- LEFT JOIN
SELECT * FROM A
LEFT JOIN B ON A.id = B.a_id;

In the first case, only rows where A.id matches B.a_id will be returned. In the second case, all rows from A will be returned, and if there is no match in B for a row from A, the fields from B will be NULL.