Sometimes it is useful to obtain data from modified rows while they are being manipulated. The INSERT, UPDATE, and DELETE commands all have an optional RETURNING clause that supports this. Use of RETURNING avoids performing an extra database query to collect the data, and is especially valuable when it would otherwise be difficult to identify the modified rows reliably.
The allowed contents of a RETURNING clause are the same as a SELECT command's output list (see Section 7.3). It can contain column names of the command's target table, or value expressions using those columns. A common shorthand is RETURNING *, which selects all columns of the target table in order.
In an INSERT, the data available to RETURNING is the row as it was inserted. This is not so useful in trivial inserts, since it would just repeat the data provided by the client. But it can be very handy when relying on computed default values. For example, when using a serial column to provide unique identifiers, RETURNING can return the ID assigned to a new row:
CREATE TABLE users (firstname text, lastname text, id serial primary key); INSERT INTO users (firstname, lastname) VALUES ('Joe', 'Cool') RETURNING id;
The RETURNING clause is also very useful with INSERT ... SELECT.
In an UPDATE, the data available to RETURNING is the new content of the modified row. For example:
UPDATE products SET price = price * 1.10 WHERE price <= 99.99 RETURNING name, price AS new_price;
In a DELETE, the data available to RETURNING is the content of the deleted row. For example:
DELETE FROM products WHERE obsoletion_date = 'today' RETURNING *;
If there are triggers (Chapter 37) on the target table, the data available to RETURNING is the row as modified by the triggers. Thus, inspecting columns computed by triggers is another common use-case for RETURNING.
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