PostgreSQL: a valid variable assignation sample? - mysql

Question

After reading this question, I'm trying to convert some SQL from MySQL to PostgreSQL. Thus I need variable assignation:
INSERT INTO main_categorie (description) VALUES ('Verbe normal');
SET #PRONOMINAL := SELECT LAST_INSERT_ID();
INSERT INTO main_mot (txt,im,date_c,date_v_d,date_l)
VALUES ('je m''abaisse',1,NOW(),NOW(),NOW());
SET #verbe_149 = SELECT LAST_INSERT_ID();
INSERT INTO main_motcategorie (mot_id,categorie_id) VALUES (#verbe_149,#PRONOMINAL);
How would you do this with PostgreSQL? No useful sample in the documentation of v9 and v8 (almost the same).
NB: I dont want to use a stored procedure like here, I just want "raw sql" so I can inject it through CLI interface.

Answer 1

There are no variables in Postgres SQL (you can use variables only in procedural languages).
Use RETURNING in WITH query:
WITH insert_cat AS (
INSERT INTO main_categorie (description)
VALUES ('Verbe normal')
RETURNING id
),
insert_mot AS (
INSERT INTO main_mot (txt,im,date_c,date_v_d,date_l)
VALUES ('je m''abaisse',1,NOW(),NOW(),NOW())
RETURNING id
)
INSERT INTO main_motcategorie (mot_id,categorie_id)
SELECT m.id, c.id
FROM insert_mot m, insert_cat c;
As an alternative, you can use custom configuration parameters in the way described in this post.
Create two functions:
create or replace function set_var (name text, value text)
returns void language plpgsql as $$
begin
execute format('set mysql.%s to %s', name, value);
end $$;
create or replace function get_var (name text)
returns text language plpgsql as $$
declare
rslt text;
begin
execute format('select current_setting(''mysql.%s'')', name) into rslt;
return rslt;
end $$;
With the functions you can simulate variables, like in the example:
INSERT INTO main_categorie (description)
VALUES ('Verbe normal');
SELECT set_var('PRONOMINAL', (SELECT currval('main_categorie_id_seq')::text));
INSERT INTO main_mot (txt,im,date_c,date_v_d,date_l)
VALUES ('je m''abaisse',1,NOW(),NOW(),NOW());
SELECT set_var('verbe_149', (SELECT currval('main_mot_id_seq')::text));
INSERT INTO main_motcategorie (mot_id,categorie_id)
SELECT get_var('verbe_149')::int, get_var('PRONOMINAL')::int;
This is certainly not an example of good code.
Particularly the necessity of casting is troublesome.
However, the conversion can be done semi-automatically.

Answer 2

You can run PostgreSQL scripts outside of a function using the do construct. Here's an example with Donald Ducks' nephews. First the nephew will be added to the nephew table, and then we'll add a baseball cap using the newly inserted nephew's id.
First, create two tables for nephews and baseball caps:
drop table if exists nephew;
drop table if exists cap;
create table nephew (id serial primary key, name text);
create table cap (id serial, nephewid bigint, color text);
Now add the first nephew:
do $$declare
newid bigint;
begin
insert into nephew (name) values ('Huey') returning id into newid;
insert into cap (nephewid, color) values (newid, 'Red');
end$$;
The returning ... into ... does in Postgres what currval does in MySQL. Huey's new id is assigned to the newid variable, and then used to insert a new row into the cap table. You can run this script just like any other SQL statement. Continue with Dewey and Louie:
do $$declare
newid bigint;
begin
insert into nephew (name) values ('Dewey') returning id into newid;
insert into nephew (name) values ('Louie') returning id into newid;
insert into cap (nephewid, color) values (newid, 'Green');
end$$;
And you end up with:
# select * from nephew;
id | name
----+-------
1 | Huey
2 | Dewey
3 | Louie
(3 rows)
# select * from cap;
id | nephewid | color
----+----------+-------
1 | 1 | Red
2 | 3 | Green
(2 rows)
See it working at SQL Fiddle.