Dynamic Pivotting in PostgreSQL - sql

Question

I'd like get a count of every possible value from one table associated with each possible value from another table. So if my (combined) table basically look like:
Order ID Employee Product Category
-------------------------------------------
1 Alan Automobile
2 Barry Beauty
3 Charlie Clothing
4 Alan Beauty
I would like to be able to query and get a result of:
Employee Count Auto Count Beauty Count Clothing
------------------------------------------------------------
Alan 1 1 0
Barry 0 1 0
Charlie 0 0 1
I could manually query for each count, but then if I later add new product categories, it will no longer work. What I'm doing now is basically just:
SELECT employee, category, COUNT(*) FROM sales GROUP BY employee, category;
Which returns:
Employee Category Count
-------------------------------
Alan Automobile 1
Alan Beauty 1
Alan Clothing 0
etc. But with a large number of categories this can get a bit redundant. Is there any way to have it returned as a single row for each employee with a column for every category?

Answer 1

You can use JSON approach
SELECT employee,
json_object_agg(ProductCategory,total ORDER BY ProductCategory)
FROM (
SELECT employee, ProductCategory, count(*) AS total
FROM tbl
GROUP BY employee,ProductCategory
) s
GROUP BY employee
ORDER BY employee;
or with two step approach
CREATE FUNCTION dynamic_pivot(central_query text, headers_query text)
RETURNS refcursor AS
$$
DECLARE
left_column text;
header_column text;
value_column text;
h_value text;
headers_clause text;
query text;
j json;
r record;
curs refcursor;
i int:=1;
BEGIN
-- find the column names of the source query
EXECUTE 'select row_to_json(_r.*) from (' || central_query || ') AS _r' into j;
FOR r in SELECT * FROM json_each_text(j)
LOOP
IF (i=1) THEN left_column := r.key;
ELSEIF (i=2) THEN header_column := r.key;
ELSEIF (i=3) THEN value_column := r.key;
END IF;
i := i+1;
END LOOP;
-- build the dynamic transposition query (based on the canonical model)
FOR h_value in EXECUTE headers_query
LOOP
headers_clause := concat(headers_clause,
format(chr(10)||',min(case when %I=%L then %I::text end) as %I',
header_column,
h_value,
value_column,
h_value ));
END LOOP;
query := format('SELECT %I %s FROM (select *,row_number() over() as rn from (%s) AS _c) as _d GROUP BY %I order by min(rn)',
left_column,
headers_clause,
central_query,
left_column);
-- open the cursor so the caller can FETCH right away
OPEN curs FOR execute query;
RETURN curs;
END
$$ LANGUAGE plpgsql;
then
=> BEGIN;
-- step 1: get the cursor (we let Postgres generate the cursor's name)
=> SELECT dynamic_pivot(
'SELECT employee,ProductCategory,count(*)
FROM tbl GROUP BY employee,ProductCategory
ORDER BY 1',
'SELECT DISTINCT productCategory FROM tbl ORDER BY 1'
) AS cur
\gset
-- step 2: read the results through the cursor
=> FETCH ALL FROM :"cur";
Reference