Can you explain what Anti JOIN in PostgreSQL is and how it is used?

Anti JOIN in PostgreSQL is a way to select rows from one table for which there are no corresponding rows in another table. In SQL, this is usually implemented using the NOT EXISTS clause or a LEFT JOIN with a NULL check.

Example of using Anti JOIN with NOT EXISTS:

SELECT *
FROM table_a a
WHERE NOT EXISTS (
    SELECT 1 FROM table_b b WHERE b.key = a.key
);

This query selects all rows from table_a for which there are no matching rows in table_b based on the key.

Alternatively, using LEFT JOIN:

SELECT a.*
FROM table_a a
LEFT JOIN table_b b ON a.key = b.key
WHERE b.key IS NULL;

Here, a left join is performed, and only the rows from table_a that have no matches in table_b are selected.

Anti JOIN is useful for finding discrepancies, for example, to find records that have not been processed or do not have related data in another table.