Calculate Multiple Aggregate Functions (with diff combinations of fixed WHERE conditions) in a Single Query

multipleAggregateFunctions_SingleQuery_diffsWHEREs.sql

-- https://dzone.com/articles/how-to-calculate-multiple-aggregate-functions-in-a?edition=297992
-- Calculate Multiple Aggregate Functions (diff WHERE conditions) in a Single Query

-- ***************** PROBLEM to solve:
-- Number of films with a given length / language_id
SELECT count(*)
FROM film
WHERE length BETWEEN 120 AND 150
AND language_id = 1;
-- Number of films with a given length / rating
SELECT count(*)
FROM film
WHERE length BETWEEN 120 AND 150
AND rating = 'PG';
-- Number of films with a given language_id / rating
SELECT count(*)
FROM film
WHERE language_id = 1
AND rating = 'PG';
-- Number of films with a given length / language_id / rating
SELECT count(*)
FROM film
WHERE length BETWEEN 120 AND 150
AND language_id = 1
AND rating = 'PG';

-- we observe always combinations of 3 basic conditions:
length BETWEEN 120 AND 150
language_id = 1
rating = 'PG';


-- ************** SOLUTION 1
-- With DERIVED TABLE with nulls for false
/*
Create a derived table with the different predicates to combine, which should return null when false and 1 when true. 
Then we can count each case as nulls won't sum. 
The trick to combine cases is the fact that 
  1+null=null
so we can also do count(length + language_id) and it will only count when both conditions are true.
*/

SELECT 
  count(*),
  count(length),
  count(language_id),
  count(rating),
  -- 1+null=null trick
  count(length + language_id),
  count(length + rating),
  count(language_id + rating),
  count(length + language_id + rating)
FROM (
  -- Heart of the trick (notice no use of ELSE so we get those usefull NULLs):
  SELECT
    CASE WHEN length BETWEEN 120 AND 150 THEN 1 END length,
    CASE WHEN language_id = 1            THEN 1 END language_id,
    CASE WHEN rating = 'PG'              THEN 1 END rating
  FROM film
) film
;

/* result:
col1  col2  col3  col4  col5  col6  col7  col8
1000  224   1000  194   224   43    194   43
*/


-- ************** SOLUTION 2
-- With FILTER clause syntax (PostgreSQL and HSQLDB and in the SQL standard)
SELECT 
  count(*),
  count(*) FILTER (WHERE length BETWEEN 120 AND 150),
  count(*) FILTER (WHERE language_id = 1),
  count(*) FILTER (WHERE rating = 'PG'),
  count(*) FILTER (
    WHERE length BETWEEN 120 AND 150 AND language_id = 1),
  count(*) FILTER (
    WHERE length BETWEEN 120 AND 150 AND rating = 'PG'),
  count(*) FILTER (
    WHERE language_id = 1 AND rating = 'PG'),
  count(*) FILTER (
    WHERE length BETWEEN 120 AND 150 
    AND language_id = 1 AND rating = 'PG')
FROM film
;





-- *************** How the derived table variant works:
-- the derived table:
  SELECT
    CASE WHEN length BETWEEN 120 AND 150 THEN 1 END length,
    CASE WHEN language_id = 1            THEN 1 END language_id,
    CASE WHEN rating = 'PG'              THEN 1 END rating
  FROM film
  ;
/*
length  language_id  rating
---------------------------
NULL    1            1
NULL    1            NULL
NULL    1            NULL
NULL    1            NULL
1       1            NULL
NULL    1            1
NULL    1            NULL
...
*/