Insert if not exists, else return id in postgresql

Question

I have a simple table in PostgreSQL that has three columns:

• id serial primary key
• key varchar
• value varchar

I have already see

Answer 1

And you can store value returned to variables in form of ... RETURNING field1, field2,... INTO var1, var2,...

RETURNING will normally return a query which would return Error 'query has no destination for result data' if you call it in plpgsql without using its returned result set.

Answer 2

Yes there is returning

INSERT INTO tag ("key", "value")
SELECT 'key1', 'value1'
WHERE NOT EXISTS (
    SELECT id, "key", "value"
    FROM node_tag
    WHERE key = 'key1' AND value = 'value1'
    )
returning id, "key", "value"

To return the row if it already exists

with s as (
    select id, "key", "value"
    from tag
    where key = 'key1' and value = 'value1'
), i as (
    insert into tag ("key", "value")
    select 'key1', 'value1'
    where not exists (select 1 from s)
    returning id, "key", "value"
)
select id, "key", "value"
from i
union all
select id, "key", "value"
from s

If the row does not exist it will return the inserted one else the existing one.

BTW, if the pair "key"/"value" makes it unique then it is the primary key, and there is no need for an id column. Unless one or both of the "key"/"value" pair can be null.

Answer 3

with vals as (
  select 'key5' as key, 'value2' as value
)
insert into Test1 (key, value)
select v.key, v.value
from vals as v
where not exists (select * from Test1 as t where t.key = v.key and t.value = v.value)
returning id

sql fiddle demo