Conditional delete and insert in postgres

Question

I have two tables Table_A and Table_B . How can I write a conditional SQL that does the following logic

If table A records match table B records on id
then
    delete records from table A and Insert records into Table B

How can I do this with SQL most likely using with

delete from Table_A where Exists (select a.id from TABLE_A
join TABLE_B as b on a.id = b.id)

The Insert is: Insert into Table_A (id) select id from TABLE_B

Answer 1

Use a CTE to catch the ids of the deleted records, and re-join these with the b records:

WITH del AS (
        DELETE FROM a
        WHERE EXISTS ( SELECT *
                FROM b
                WHERE b.id = a.id
                )
        returning *
        )
INSERT INTO a (id, x, y, z)
SELECT id, x, y, z
FROM b
WHERE EXISTS (
        SELECT *
        FROM del
        WHERE del.id = b.id
        );

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BTW: you should have very good reasons (such as wanting to activate the triggers) to prefer delete+insert to a update.