SQL check if exists and insert [duplicate] - sql

Question

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Is SELECT or INSERT in a function prone to race conditions?
(3 answers)
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I'm using a table 'Customer' with the following schema
id INTEGER NOT NULL UNIQUE,
name TEXT NOT NULL,
auth BOOLEAN DEFAULT FALSE
Now, I want to add a record if does not exist, I can do the following
IF NOT EXISTS (SELECT name from Customer where id=220)
BEGIN
INSERT into Customer (name,id) values ('Jon', 220)
END;
But at the same time, I also want to know if the id really did not exist along with the insertion i.e. True/False result of the select query. I can split it into two queries, from the first I can know if it exists and if id did not then I can insert it. But how can I do this in a single query?

Answer 1

You need to use INSERT with the RETURNING clause (PostgreSQL INSERT).

Answer 2

'on conflict' clause used with INSERT can be customized to serve your purpose.
INSERT INTO <table_name>(<column_name_list)) values(<column_values>) ON CONFLICT(<constraint_column>) DO NOTHING;
ref: https://www.postgresqltutorial.com/postgresql-upsert/
Set up
Step 1: Create the table:
create table test
(
id INTEGER NOT NULL UNIQUE,
name TEXT NOT NULL,
auth BOOLEAN DEFAULT FALSE
);
Step 2: Load the table with some sample rows:
insert into test(id,name) values(1,'vincent'),(2,'gabriel'),(3,'sebastian');
Step 3: Test with an INSERT of a row with existing id i.e 1 , the insert does not go through as the ID already exists:
INSERT INTO test(id,name) values(1,'xavier') ON CONFLICT(id) DO NOTHING;
Step 4: Now test with a row with ID that does not exist.i.e 4. It gets through.
INSERT INTO test(id,name) values(4,'xavier') ON CONFLICT(id) DO NOTHING;
Demo:
postgres=# select * from test;
id | name | auth
----+-----------+------
1 | vincent | f
2 | gabriel | f
3 | sebastian | f
(3 rows)
postgres=# INSERT INTO test(id,name) values(1,'xavier') ON CONFLICT(id) DO NOTHING;
INSERT 0 0
postgres=#
postgres=# select * from test;
id | name | auth
----+-----------+------
1 | vincent | f
2 | gabriel | f
3 | sebastian | f
(3 rows)
--- NOTE: no row inserted as ID 1 already exists.
postgres=# INSERT INTO test(id,name) values(4,'xavier') ON CONFLICT(id) DO NOTHING;
INSERT 0 1
postgres=# select * from test;
id | name | auth
----+-----------+------
1 | vincent | f
2 | gabriel | f
3 | sebastian | f
4 | xavier | f -------> new row inserted.
(4 rows)

Answer 3

you can use the following :
INSERT into Customer SELECT 'Jon', 220
Where Not EXISTS (SELECT 1
from Customer
where id=220);
Select Cast(##ROWCOUNT as bit);