Insert multiple rows where not exists PostgresQL

Question

I'd like to generate a single sql query to mass-insert a series of rows that don't exist on a table. My current setup makes a new query for each record insertion similar to the solution detailed in WHERE NOT EXISTS in PostgreSQL gives syntax error, but I'd like to move this to a single query to optimize performance since my current setup could generate several hundred queries at a time. Right now I'm trying something like the example I've added below:

INSERT INTO users (first_name, last_name, uid) 
SELECT ( 'John', 'Doe', '3sldkjfksjd'), ( 'Jane', 'Doe', 'adslkejkdsjfds')
WHERE NOT EXISTS (
  SELECT * FROM users WHERE uid IN ('3sldkjfksjd', 'adslkejkdsjfds')
)

Postgres returns the following error:

PG::Error: ERROR:  INSERT has more target columns than expressions

The problem is that PostgresQL doesn't seem to want to take a series of values when using SELECT. Conversely, I can make the insertions using VALUES, but I can't then prevent duplicates from being generated using WHERE NOT EXISTS.

http://www.techonthenet.com/postgresql/insert.php suggests in the section EXAMPLE - USING SUB-SELECT that multiple records should be insertable from another referenced table using SELECT, so I'm wondering why I can't seem to pass in a series of values to insert. The values I'm passing are coming from an external API, so I need to generate the values to insert by hand.

Answer 1

Your select is not doing what you think it does.

The most compact version in PostgreSQL would be something like this:

with data(first_name, last_name, uid)  as (
   values
      ( 'John', 'Doe', '3sldkjfksjd'),
      ( 'Jane', 'Doe', 'adslkejkdsjfds')
) 
insert into users (first_name, last_name, uid) 
select d.first_name, d.last_name, d.uid
from data d
where not exists (select 1
                  from users u2
                  where u2.uid = d.uid);

Which is pretty much equivalent to:

insert into users (first_name, last_name, uid) 
select d.first_name, d.last_name, d.uid
from (
   select 'John' as first_name, 'Doe' as last_name, '3sldkjfksjd' as uid
   union all
   select 'Jane', 'Doe', 'adslkejkdsjfds'
) as d
where not exists (select 1
                  from users u2
                  where u2.uid = d.uid);