Postgresql insert if does not exist - sql

Question

I have the following query
INSERT INTO address (house_number, street, city_id)
values(11, 'test st', (select id from city where LOWER(city) = LOWER('somecity')))
Is there anyway to insert "somecity" in the city table if "somecity" does not exist in city then after inserting, it would return the ID for the inserted row?
I did find this answer that says upsert can be used to achieve this
https://stackoverflow.com/a/31742830/492015
but I can't find an example that inserts if select does not return the row.

Answer 1

Instead of nesting the INSERTs, you could use a CTE
to perform the INSERTs one after the other but as a single statement:
WITH tmp AS (
INSERT INTO test_city (city) VALUES ('somecity')
ON CONFLICT (lower(city)) DO UPDATE SET city = excluded.city
RETURNING id, city
)
INSERT INTO test_address (house_number, street, city_id)
SELECT house_number, street, id
FROM (VALUES (11, 'test st', 'somecity')) val (house_number, street, city)
LEFT JOIN tmp USING (city)
RETURNING *
Using this setup:
DROP TABLE IF EXISTS test_address;
DROP TABLE IF EXISTS test_city;
CREATE TABLE test_address (
house_number int
, street text
, city_id int
);
CREATE TABLE test_city (
id int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY
, city text
);
CREATE UNIQUE INDEX test_city_uniq_idx ON test_city USING btree (lower(city));
INSERT INTO test_city (city) VALUES ('Somecity');
and with the INSERT above, the query
SELECT * FROM test_address;
yields
| house_number | street | city_id |
|--------------+---------+---------|
| 11 | test st | 1 |
and
SELECT * FROM test_city;
yields
| id | city |
|----+----------|
| 1 | somecity |
Note that the CTE replaces
(select id from city where LOWER(city) = LOWER('somecity'))
with an INSERT .. ON CONFLICT .. DO UPDATE statement:
INSERT INTO test_city (city) VALUES ('somecity')
ON CONFLICT (lower(city)) DO UPDATE SET city = excluded.city
RETURNING id, city
I used DO UPDATE instead of DO NOTHING so that RETURNING id, city will always return something. If you use DO NOTHING, then nothing is returned when there is a conflict.
Note however that a consequence of using city = excluded.city is that the original 'Somecity'
gets replaced by 'somecity'. I'm not sure you'll find that behavior acceptable, but unfortunately I haven't figured out how to do nothing when there is a conflict and yet return id and city at the same time.
Another issue you may have with the above solution is that I used a unique index on lower(city):
CREATE UNIQUE INDEX test_city_uniq_idx ON test_city USING btree (lower(city));
This allows you to use the identical condition in the INSERT statement:
INSERT ... ON CONFLICT (lower(city))
as a substitute for the condition LOWER(city) = LOWER('somecity') which appeared in your SELECT statement. It produces the desired effect, but the trade-off is that now you have a unique index
on (lower(city)).
Regarding the followup question
of how to insert into more than 2 tables:
You can chain together more than one CTE, and the subsequent CTEs can even reference the prior CTEs. For example,
CREATE UNIQUE INDEX city_uniq_idx ON city USING btree (lower(city));
CREATE UNIQUE INDEX state_uniq_idx ON state USING btree (lower(state_code));
WITH tmpcity AS
(
INSERT INTO
city (city)
VALUES
(
'Miami'
)
ON CONFLICT (lower(city)) DO
UPDATE
SET
city = excluded.city RETURNING id, city
)
, tmpstate as
(
INSERT INTO
state (state_code)
VALUES
(
'FL'
)
ON CONFLICT (lower(state_code)) DO
UPDATE
SET
state_code = excluded.state_code RETURNING id, state_code
)
INSERT INTO
address (house_number, street, city_id, state_id)
SELECT
house_number,
street,
tmpcity.id,
tmpstate.id
FROM
(
VALUES
(
12,
'fake st.',
'Miami',
'FL'
)
)
val (house_number, street, city, state_code)
LEFT JOIN
tmpcity USING (city)
LEFT JOIN
tmpstate USING (state_code)
ON CONFLICT (street) DO NOTHING