PostgreSQL `num_nulls` and `num_nonnulls`

July 10, 2020 ( last updated : July 10, 2020 )
postgres rails ruby active-record

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PostgreSQL offers a few comparison functions that are very useful if your table has columns that require exactly one entry. I ran into a situation recently that utilized the num_nonnulls function combined with a DB constraint to ensure that only one of a group of tables had data.

Problem

The table I was designing looked something like this:

Column name	Data type	Notes
`bool_response`	boolean	Must be null if there is a value in any other `*_response` columns
`int_response`	integer	Must be null if there is a value in any other `*_response` columns
`text_response`	text	Must be null if there is a value in any other `*_response` columns

We want each row in this table to only have ONE non-null value in any *_response column, and I wanted to implement a DB constraint on top of the application-level validation to catch any race conditions (such as multiple entries being saved to the DB at the same time).

Solution

Luckily I am using Postgres as our database which provides a couple comparison functions that count the number of nulls in a set of columns: num_nonnulls and num_nulls.

This table from the documentation linked above explains the two functions:

Function	Description	Example	Example Result
num_nonnulls(VARIADIC “any”)	returns the number of `non-null` arguments	num_nonnulls(1, NULL, 2)	2
num_nulls(VARIADIC “any”)	returns the number of `null` arguments	num_nulls(1, NULL, 2)	1

I decided to use the first function (num_nonnulls), adding a constraint that checks those columns and ensure there is only a single non-null value:

ALTER TABLE my_example_table
    ADD CONSTRAINT only_one_non_null_response
    CHECK (num_nonnulls(bool_response, int_response, text_response) = 1);

Finally I paired this with an app-level validation (this project is using Rails aActiveRecord):

class MyResponseClass < ActiveRecord::Base
  validate :only_one_response
  # ...
  def only_one_response
    if non_null_responses.count != 1
      non_null_responses = [bool_response, int_response, text_response].compact 
    end

    errors.add(:base, "Must only have a single response" )
  end
end

PostgreSQL num_nulls and num_nonnulls

Problem

Solution

PostgreSQL `num_nulls` and `num_nonnulls`