The example above can be done on a SQL Server. It is a function that performs the calculation on another table while getting the current table field Id to list data from other table, return a single value.
Question: how to do the exact thing with PostgreSQL
SELECT TOP(5) * FROM Artists;
+------------+------------------+--------------+-------------+
| ArtistId | ArtistName | ActiveFrom | CountryId |
|------------+------------------+--------------+-------------|
| 1 | Iron Maiden | 1975-12-25 | 3 |
| 2 | AC/DC | 1973-01-11 | 2 |
| 3 | Allan Holdsworth | 1969-01-01 | 3 |
| 4 | Buddy Rich | 1919-01-01 | 6 |
| 5 | Devin Townsend | 1993-01-01 | 8 |
+------------+------------------+--------------+-------------+
SELECT TOP(5) * FROM Albums;
+-----------+------------------------+---------------+------------+-----------+
| AlbumId | AlbumName | ReleaseDate | ArtistId | GenreId |
|-----------+------------------------+---------------+------------+-----------|
| 1 | Powerslave | 1984-09-03 | 1 | 1 |
| 2 | Powerage | 1978-05-05 | 2 | 1 |
| 3 | Singing Down the Lane | 1956-01-01 | 6 | 3 |
| 4 | Ziltoid the Omniscient | 2007-05-21 | 5 | 1 |
| 5 | Casualties of Cool | 2014-05-14 | 5 | 1 |
+-----------+------------------------+---------------+------------+-----------+
The function
CREATE FUNCTION [dbo].[ufn_AlbumCount] (@ArtistId int)
RETURNS smallint
AS
BEGIN
DECLARE @AlbumCount int;
SELECT @AlbumCount = COUNT(AlbumId)
FROM Albums
WHERE ArtistId = @ArtistId;
RETURN @AlbumCount;
END;
GO
Now, (at SQL Server), after update the first table fields with ALTER TABLE Artists ADD AlbumCount AS dbo.ufn_AlbumCount(ArtistId); whe can list and get the following result.
+------------+------------------+--------------+-------------+--------------+
| ArtistId | ArtistName | ActiveFrom | CountryId | AlbumCount |
|------------+------------------+--------------+-------------+--------------|
| 1 | Iron Maiden | 1975-12-25 | 3 | 5 |
| 2 | AC/DC | 1973-01-11 | 2 | 3 |
| 3 | Allan Holdsworth | 1969-01-01 | 3 | 2 |
| 4 | Buddy Rich | 1919-01-01 | 6 | 1 |
| 5 | Devin Townsend | 1993-01-01 | 8 | 3 |
| 6 | Jim Reeves | 1948-01-01 | 6 | 1 |
| 7 | Tom Jones | 1963-01-01 | 4 | 3 |
| 8 | Maroon 5 | 1994-01-01 | 6 | 0 |
| 9 | The Script | 2001-01-01 | 5 | 1 |
| 10 | Lit | 1988-06-26 | 6 | 0 |
+------------+------------------+--------------+-------------+--------------+
but how to achieve this on postgresql?
Postgres doesn't support "virtual" computed column (i.e. computed columns that are generated at runtime), so there is no exact equivalent. The most efficient solution is a view that counts this:
create view artists_with_counts
as
select a.*,
coalesce(t.album_count, 0) as album_count
from artists a
left join (
select artist_id, count(*) as album_count
from albums
group by artist_id
) t on a.artist_id = t.artist_id;
Another option is to create a function that can be used as a "virtual column" in a select - but as this is done row-by-row, this will be substantially slower than the view.
create function album_count(p_artist artists)
returns bigint
as
$$
select count(*)
from albums a
where a.artist_id = p_artist.artist_id;
$$
language sql
stable;
Then you can include this as a column:
select a.*, a.album_count
from artists a;
Using the function like that, requires to prefix the function reference with the table alias (alternatively, you can use album_count(a))