SQL CTE Syntax to DELETE / INSERT rows - sql

Question

What's the CTE syntax to delete from a table, then insert to the same table and return the values of the insert?
Operating on 2 hours of sleep and something doesn't look right (besides the fact that this won't execute):
WITH delete_rows AS (
DELETE FROM <some_table> WHERE id = <id_value>
RETURNING *
)
SELECT * FROM delete_rows
UNION
(
INSERT INTO <some_table> ( id, text_field )
VALUES ( <id_value>, '<text_field_value>' )
RETURNING *
)
The expected behavior is to first clear all the records for an ID, then insert records for the same ID (intentionally not an upsert) and return those inserted records (not the deletions).

Answer 1

Your question update made clear that you cannot do this in a single statement.
Packed into CTEs of the same statement, both operations (INSERT and DELETE) would see the same snapshot of the table and execute virtually at the same time. I.e., the INSERT would still see all rows that you thought to be deleted already. The manual:
All the statements are executed with the same snapshot (see Chapter 13), so they cannot "see" one another's effects on the target tables.
You can wrap them as two independent statements into the same transaction - which doesn't seem strictly necessary either, but it would allow the whole operation to succeed / fail atomically:
BEGIN;
DELETE FROM <some_table> WHERE id = <id_value>;
INSERT INTO <some_table> (id, text_field)
VALUES ( <id_value>, '<text_field_value>')
RETURNING *;
COMMIT;
Now, the INSERT can see the results of the DELETE.

Answer 2

CREATE TABLE test_table (value TEXT UNIQUE);
INSERT INTO test_table SELECT 'value 1';
INSERT INTO test_table SELECT 'value 2';
WITH delete_row AS (DELETE FROM test_table WHERE value='value 2' RETURNING 0)
INSERT INTO test_table
SELECT DISTINCT 'value 2'
FROM (SELECT 'dummy') dummy
LEFT OUTER JOIN delete_row ON TRUE
RETURNING *;
The query above handles the situations when DELETE deletes 0/1/some rows.

Answer 3

Elaborating on skif1979's "DelSert" CTE method, the "Logged DelSert:"
-- setups
DROP TABLE IF EXISTS _zx_t1 ;
CREATE TEMP TABLE
IF NOT EXISTS
_zx_t1
( id bigint
, fld2 bigint
, UNIQUE (id)
);
-- unique records
INSERT INTO _zx_t1 SELECT 1, 99;
INSERT INTO _zx_t1 SELECT 2, 98;
WITH
_cte_del_row AS
( DELETE
FROM _zx_t1
WHERE id = 2
RETURNING id as _b4_id, fld2 as _b4_fld2 -- returns complete deleted row
)
, _cte_delsert AS
( INSERT
INTO _zx_t1
SELECT DISTINCT
_cte_del_row._b4_id
, _cte_del_row._b4_fld2 + 1
from (SELECT null::integer AS _zunk) _zunk -- skif1979's trick here
LEFT OUTER JOIN _cte_del_row -- clever LOJ magic
ON TRUE -- LOJ cartesian product
RETURNING id as _aft_id , fld2 as _aft_fld2 -- return newly "delserted" rows
)
SELECT * -- returns before & after snapshots from CTE's
FROM
_cte_del_row
, _cte_delsert ;
RESULT:
_b4_id | _b4_fld2 | _aft_id | _aft_fld2
--------+----------+---------+-----------
2 | 209 | 2 | 210
AFAICT these all occur linearly w/in a unit of work, akin to a journaled or logged update.
Workable for
Child records
OR Schema w/ no FK
OR FK w/ cascading deletes
Not workable for
Parent records w/ FK & no cascading deletes

Answer 4

A related (& IMO better) answer, akin to the "Logged DelSert" is this, a logged "SelUp" :
-- setups
DROP TABLE IF EXISTS _zx_t1 ;
CREATE TEMP TABLE
IF NOT EXISTS
_zx_t1
( id bigint
, fld2 bigint
, UNIQUE (id)
);
-- unique records
INSERT INTO _zx_t1 SELECT 1, 99;
INSERT INTO _zx_t1 SELECT 2, 98;
WITH
_cte_sel_row AS
( SELECT -- start unit of work with read
id as _b4_id -- fields need to be aliased
,fld2 as _b4_fld2 -- to prevent ambiguous column errors
FROM _zx_t1
WHERE id = 2
FOR UPDATE
)
, _cte_sel_up_ret AS -- we're in the same UOW
( UPDATE _zx_t1 -- actual table
SET fld2 = _b4_fld2 + 1 -- some actual work
FROM _cte_sel_row
WHERE id = _b4_id
AND fld2 < _b4_fld2 + 1 -- gratuitous but illustrates the point
RETURNING id as _aft_id, fld2 as _aft_fld2
)
SELECT
_cte_sel_row._b4_id
,_cte_sel_row._b4_fld2 -- before
,_cte_sel_up_ret._aft_id
,_cte_sel_up_ret._aft_fld2 -- after
FROM _cte_sel_up_ret
INNER JOIN _cte_sel_row
ON TRUE AND _cte_sel_row._b4_id = _cte_sel_up_ret._aft_id
;
RESULT:
_b4_id | _b4_fld2 | _aft_id | _aft_fld2
--------+----------+---------+-----------
2 | 209 | 2 | 210
See also:
https://rob.conery.io/2018/08/13/transactional-data-operations-in-postgresql-using-common-table-expressions/