Postgres: INSERT if does not exist already

I'm using Python to write to a postgres database:

sql_string = "INSERT INTO hundred (name,name_slug,status) VALUES ("
sql_string += hundred + ", '" + hundred_slug + "', " + status + ");"
cursor.execute(sql_string)

But because some of my rows are identical, I get the following error:

psycopg2.IntegrityError: duplicate key value  
  violates unique constraint "hundred_pkey"

How can I write an 'INSERT unless this row already exists' SQL statement?

I've seen complex statements like this recommended:

IF EXISTS (SELECT * FROM invoices WHERE invoiceid = '12345')
UPDATE invoices SET billed = 'TRUE' WHERE invoiceid = '12345'
ELSE
INSERT INTO invoices (invoiceid, billed) VALUES ('12345', 'TRUE')
END IF

But firstly, is this overkill for what I need, and secondly, how can I execute one of those as a simple string?

Postgres 9.5 (released since 2016-01-07) offers an "upsert" command, also known as an ON CONFLICT clause to INSERT:

INSERT ... ON CONFLICT DO NOTHING/UPDATE

It solves many of the subtle problems you can run into when using concurrent operation, which some other answers propose.

> How can I write an 'INSERT unless this row already exists' SQL statement?

There is a nice way of doing conditional INSERT in PostgreSQL:

INSERT INTO example_table
    (id, name)
SELECT 1, 'John'
WHERE
    NOT EXISTS (
        SELECT id FROM example_table WHERE id = 1
    );

CAVEAT This approach is not 100% reliable for concurrent write operations, though. There is a very tiny race condition between the SELECT in the NOT EXISTS anti-semi-join and the INSERT itself. It can fail under such conditions.

One approach would be to create a non-constrained (no unique indexes) table to insert all your data into and do a select distinct from that to do your insert into your hundred table.

So high level would be. I assume all three columns are distinct in my example so for step3 change the NOT EXITS join to only join on the unique columns in the hundred table.

1. Create temporary table. See docs here.

    CREATE TEMPORARY TABLE temp_data(name, name_slug, status);
   

2. INSERT Data into temp table.

    INSERT INTO temp_data(name, name_slug, status); 
   

3. Add any indexes to the temp table.

4. Do main table insert.

    INSERT INTO hundred(name, name_slug, status) 
        SELECT DISTINCT name, name_slug, status
        FROM hundred
        WHERE NOT EXISTS (
            SELECT 'X' 
            FROM temp_data
            WHERE 
                temp_data.name          = hundred.name
                AND temp_data.name_slug = hundred.name_slug
                AND temp_data.status    = status
        );
   

Unfortunately, PostgreSQL supports neither MERGE nor ON DUPLICATE KEY UPDATE, so you'll have to do it in two statements:

UPDATE  invoices
SET     billed = 'TRUE'
WHERE   invoices = '12345'

INSERT
INTO    invoices (invoiceid, billed)
SELECT  '12345', 'TRUE'
WHERE   '12345' NOT IN
        (
        SELECT  invoiceid
        FROM    invoices
        )

You can wrap it into a function:

CREATE OR REPLACE FUNCTION fn_upd_invoices(id VARCHAR(32), billed VARCHAR(32))
RETURNS VOID
AS
$$
        UPDATE  invoices
        SET     billed = $2
        WHERE   invoices = $1;

        INSERT
        INTO    invoices (invoiceid, billed)
        SELECT  $1, $2
        WHERE   $1 NOT IN
                (
                SELECT  invoiceid
                FROM    invoices
                );
$$
LANGUAGE 'sql';

and just call it:

SELECT  fn_upd_invoices('12345', 'TRUE')

This is exactly the problem I face and my version is 9.5

And I solve it with SQL query below.

INSERT INTO example_table (id, name)
SELECT 1 AS id, 'John' AS name FROM example_table
WHERE NOT EXISTS(
            SELECT id FROM example_table WHERE id = 1
    )
LIMIT 1;

Hope that will help someone who has the same issue with version >= 9.5.

Thanks for reading.

You can make use of VALUES - available in Postgres:

INSERT INTO person (name)
    SELECT name FROM person
    UNION 
    VALUES ('Bob')
    EXCEPT
    SELECT name FROM person;

I know this question is from a while ago, but thought this might help someone. I think the easiest way to do this is via a trigger. E.g.:

Create Function ignore_dups() Returns Trigger
As $$
Begin
    If Exists (
        Select
            *
        From
            hundred h
        Where
            -- Assuming all three fields are primary key
            h.name = NEW.name
            And h.hundred_slug = NEW.hundred_slug
            And h.status = NEW.status
    ) Then
        Return NULL;
    End If;
    Return NEW;
End;
$$ Language plpgsql;

Create Trigger ignore_dups
    Before Insert On hundred
    For Each Row
    Execute Procedure ignore_dups();

Execute this code from a psql prompt (or however you like to execute queries directly on the database). Then you can insert as normal from Python. E.g.:

sql = "Insert Into hundreds (name, name_slug, status) Values (%s, %s, %s)"
cursor.execute(sql, (hundred, hundred_slug, status))

Note that as @Thomas_Wouters already mentioned, the code above takes advantage of parameters rather than concatenating the string.

There is a nice way of doing conditional INSERT in PostgreSQL using WITH query: Like:

WITH a as(
select 
 id 
from 
 schema.table_name 
where 
 column_name = your_identical_column_value
)
INSERT into 
 schema.table_name
(col_name1, col_name2)
SELECT
    (col_name1, col_name2)
WHERE NOT EXISTS (
     SELECT
         id
     FROM
         a
        )
  RETURNING id 

we can simplify the query using upsert

insert into invoices (invoiceid, billed) 
  values ('12345', 'TRUE') 
  on conflict (invoiceid) do 
    update set billed=EXCLUDED.billed;

INSERT .. WHERE NOT EXISTS is good approach. And race conditions can be avoided by transaction "envelope":

BEGIN;
LOCK TABLE hundred IN SHARE ROW EXCLUSIVE MODE;
INSERT ... ;
COMMIT;

It's easy with rules:

CREATE RULE file_insert_defer AS ON INSERT TO file
WHERE (EXISTS ( SELECT * FROM file WHERE file.id = new.id)) DO INSTEAD NOTHING

But it fails with concurrent writes ...

psycopgs cursor class has the attribute rowcount.

> This read-only attribute specifies the number of rows that the last > execute*() produced (for DQL statements like SELECT) or affected (for > DML statements like UPDATE or INSERT).

So you could try UPDATE first and INSERT only if rowcount is 0.

But depending on activity levels in your database you may hit a race condition between UPDATE and INSERT where another process may create that record in the interim.

The approach with the most upvotes (from John Doe) does somehow work for me but in my case from expected 422 rows i get only 180. I couldn't find anything wrong and there are no errors at all, so i looked for a different simple approach.

Using IF NOT FOUND THEN after a SELECT just works perfectly for me.

(described in PostgreSQL Documentation)

Example from documentation:

SELECT * INTO myrec FROM emp WHERE empname = myname;
IF NOT FOUND THEN
  RAISE EXCEPTION 'employee % not found', myname;
END IF;

Your column "hundred" seems to be defined as primary key and therefore must be unique which is not the case. The problem isn't with, it is with your data.

I suggest you insert an id as serial type to handly the primary key

If you say that many of your rows are identical you will end checking many times. You can send them and the database will determine if insert it or not with the ON CONFLICT clause as follows

  INSERT INTO Hundred (name,name_slug,status) VALUES ("sql_string += hundred  
  +",'" + hundred_slug + "', " + status + ") ON CONFLICT ON CONSTRAINT
  hundred_pkey DO NOTHING;" cursor.execute(sql_string);

I was looking for a similar solution, trying to find SQL that work work in PostgreSQL as well as HSQLDB. (HSQLDB was what made this difficult.) Using your example as a basis, this is the format that I found elsewhere.

sql = "INSERT INTO hundred (name,name_slug,status)"
sql += " ( SELECT " + hundred + ", '" + hundred_slug + "', " + status
sql += " FROM hundred"
sql += " WHERE name = " + hundred + " AND name_slug = '" + hundred_slug + "' AND status = " + status
sql += " HAVING COUNT(*) = 0 );"

Here is a generic python function that given a tablename, columns and values, generates the upsert equivalent for postgresql.

import json

def upsert(table_name, id_column, other_columns, values_hash):

    template = """
    WITH new_values ($$ALL_COLUMNS$$) as (
      values
         ($$VALUES_LIST$$)
    ),
    upsert as
    (
        update $$TABLE_NAME$$ m
            set
                $$SET_MAPPINGS$$
        FROM new_values nv
        WHERE m.$$ID_COLUMN$$ = nv.$$ID_COLUMN$$
        RETURNING m.*
    )
    INSERT INTO $$TABLE_NAME$$ ($$ALL_COLUMNS$$)
    SELECT $$ALL_COLUMNS$$
    FROM new_values
    WHERE NOT EXISTS (SELECT 1
                      FROM upsert up
                      WHERE up.$$ID_COLUMN$$ = new_values.$$ID_COLUMN$$)
    """

    all_columns = [id_column] + other_columns
    all_columns_csv = ",".join(all_columns)
    all_values_csv = ','.join([query_value(values_hash[column_name]) for column_name in all_columns])
    set_mappings = ",".join([ c+ " = nv." +c for c in other_columns])

    q = template
    q = q.replace("$$TABLE_NAME$$", table_name)
    q = q.replace("$$ID_COLUMN$$", id_column)
    q = q.replace("$$ALL_COLUMNS$$", all_columns_csv)
    q = q.replace("$$VALUES_LIST$$", all_values_csv)
    q = q.replace("$$SET_MAPPINGS$$", set_mappings)

    return q


def query_value(value):
    if value is None:
        return "NULL"
    if type(value) in [str, unicode]:
        return "'%s'" % value.replace("'", "''")
    if type(value) == dict:
        return "'%s'" % json.dumps(value).replace("'", "''")
    if type(value) == bool:
        return "%s" % value
    if type(value) == int:
        return "%s" % value
    return value


if __name__ == "__main__":

    my_table_name = 'mytable'
    my_id_column = 'id'
    my_other_columns = ['field1', 'field2']
    my_values_hash = {
        'id': 123,
        'field1': "john",
        'field2': "doe"
    }
    print upsert(my_table_name, my_id_column, my_other_columns, my_values_hash)

The solution in simple, but not immediatly.
If you want use this instruction, you must make one change to the db:

ALTER USER user SET search_path to 'name_of_schema';

after these changes "INSERT" will work correctly.