Constraint on row value based on values in other rows
-
01-10-2020 - |
Domanda
Is it possible to have a table with four columns:
CREATE TABLE accrual (
add_date date NOT NULL,
user_id integer NOT NULL,
rate integer NOT NULL,
amount numeric(7,3)
);
and then constrain it so that if I already have one entry like
('2016-04-01', 3, 120, 25.6)
an attempt to insert another entry with the same date and user_id but a different rate would fail? i.e.
('2016-04-01', 3, 140, 15)
would fail, but
('2016-04-02', 3, 140, 15)
or
('2016-04-01', 4, 140, 15)
or
('2016-04-01', 3, 120, 15)
would be fine?
To be clear, the date and user_id are basically like an index, except duplicate indices are totally fine as long as the rates are the same. The amount is not constrained.
I am working with a postrgesql 9.5 database, and I am using PHP. My guess is that the easiest thing to do is to write some PHP code to perform the check. The closest question I could find to this was this one:
trigger or check constraint insert values based upon existence of others
but it is about an Oracle database and is slightly different.
Thanks for any suggestions!!!
Soluzione
As @NeilMcGuigan suggested, an Exclusion Constraint is the key here. The part I was unfamiliar with was the ability to use the <> operator. As available post Postgres 9.0:
CREATE TABLE accrual (
add_date date NOT NULL,
user_id integer NOT NULL,
rate integer NOT NULL,
amount numeric(7,3),
EXCLUDE USING gist
(date WITH =,
user_id WITH =,
rate WITH <>)
);
Two helpful links are:
- http://www.tutorialspoint.com/postgresql/postgresql_constraints.htm -- a short tutorial on constraints in Postgres, with an example for the Exclusion constraint that uses the <> operator
and
- http://www.slideshare.net/pgconf/not-just-unique-exclusion-constraints (specifically slide 23) -- a presentation on the need for and use of Exclusion constraints. Slide 23 introduces the <> operator and its usefulness.