PLpgSQL (or ANSI SQL?) Conditional calculation on a column - sql

Question

I want to write a stored procedure that performs a conditional calculation on a column. Ideally the implementation of the SP will be db agnostic - if possible. If not the underlying db is PostgreSQL (v8.4), so that takes precedence.
The underlying tables being queried looks like this:
CREATE TABLE treatment_def ( id PRIMARY SERIAL KEY,
name VARCHAR(16) NOT NULL
);
CREATE TABLE foo_group_def ( id PRIMARY SERIAL KEY,
name VARCHAR(16) NOT NULL
);
CREATE TABLE foo ( id PRIMARY SERIAL KEY,
name VARCHAR(16) NOT NULL,
trtmt_id INT REFERENCES treatment_def(id) ON DELETE RESTRICT,
foo_grp_id INT REFERENCES foo_group_def(id) ON DELETE RESTRICT,
is_male BOOLEAN NOT NULL,
cost REAL NOT NULL
);
I want to write a SP that returns the following 'table' result set:
treatment_name, foo_group_name, averaged_cost
where averaged cost is calcluated differently, depending on whether the row field *is_male* flag is set to true or false.
For the purpose of this question, lets assume that if the is_male flag is set to true, then the averaged cost is calculated as the SUM of the cost values for the grouping, and if the is_male flag is set to false, then the cost value is calculated as the AVERAGE of the cost values for the grouping.
(Obviously) the data is being grouped by trmt_id, foo_grp_id (and is_male?).
I have a rough idea about how to to write the SQL if there was no conditional test on the is_male flag. However, I could do with some help in writing the SP as defined above.
Here is my first attempt:
CREATE TYPE FOO_RESULT AS (treatment_name VARCHAR(16), foo_group_name VARCHAR(64), averaged_cost DOUBLE);
// Outline plpgsql (Pseudo code)
CREATE FUNCTION somefunc() RETURNS SETOF FOO_RESULT AS $$
BEGIN
RETURN QUERY SELECT t.name treatment_name, g.name group_name, averaged_cost FROM foo f
INNER JOIN treatment_def t ON t.id = f.trtmt_id
INNER JOIN foo_group_def g ON g.id = f.foo_grp_id
GROUP BY f.trtmt_id, f.foo_grp_id;
END;
$$ LANGUAGE plpgsql;
I would appreciate some help on how to write this SP correctly to implement the conditional calculation in the column results

Answer 1

Could look like this:
CREATE FUNCTION somefunc()
RETURNS TABLE (
treatment_name varchar(16)
, foo_group_name varchar(16)
, averaged_cost double precision)
AS
$BODY$
SELECT t.name -- AS treatment_name
, g.name -- AS group_name
, CASE WHEN f.is_male THEN sum(f.cost)
ELSE avg(f.cost) END -- AS averaged_cost
FROM foo f
JOIN treatment_def t ON t.id = f.trtmt_id
JOIN foo_group_def g ON g.id = f.foo_grp_id
GROUP BY 1, 2, f.is_male;
$BODY$ LANGUAGE sql;
Major points
I used an sql function, not plpgsql. You can use either, I just did it to shorten the code. plpgsql might be slightly faster, because the query plan is cached.
I skipped the custom composite type. You can do that simpler with RETURNS TABLE.
I would generally advise to use the data type text instead of varchar(n). Makes your life easier.
Be careful not to use names of the RETURN parameter without table-qualifying (tbl.col) in the function body, or you will create naming conflicts. That is why I commented the aliases.
I adjusted the GROUP BY clause. The original didn't work. (Neither does the one in #Ken's answer.)

Answer 2

You should be able to use a CASE statement:
SELECT t.name treatment_name, g.name group_name,
CASE is_male WHEN true then SUM(cost)
ELSE AVG(cost) END AS averaged_cost
FROM foo f
INNER JOIN treatment_def t ON t.id = f.trtmt_id
INNER JOIN foo_group_def g ON g.id = f.foo_grp_id
GROUP BY 1, 2, f.is_male;
I'm not familiar with PLpgSQL, so I'm not sure of the exact syntax for the BOOLEAN column, but the above should at least get you started in the right direction.