Keeping rows from double-counting in a GROUP BY - sql

Question

Here's the basic guts of my schema and problem: http://sqlfiddle.com/#!1/72ec9/4/2
Note that the periods table can refer to a variable range of time - it could be an entire season, it could be a few games or one game. For a given team and year all period rows represent exclusive ranges of time.
I've got a query written which joins up tables and uses a GROUP BY periods.year to aggregate scores for a season (see sqlfiddle). However, if a coach had two positions in the same year the GROUP BY will count the same period row twice. How can I ditch the duplicates when a coach held two positions but still sum up periods when a year is comprised of multiple periods? If there's a better way to do the schema I'd also appreciate it if you pointed it out to me.

Answer 1

The underlying problem (joining to multiple tables with multiple matches) is explained in this related answer:
Two SQL LEFT JOINS produce incorrect result
To fix, I first simplified & formatted your query:
select pe.year
, sum(pe.wins) AS wins
, sum(pe.losses) AS losses
, sum(pe.ties) AS ties
, array_agg(po.id) AS position_id
, array_agg(po.name) AS position_names
from periods_positions_coaches_linking pp
join positions po ON po.id = pp.position
join periods pe ON pe.id = pp.period
where pp.coach = 1
group by pe.year
order by pe.year;
Yields the same, incorrect result as your original, but simpler / faster / easier to read.
No point in joining the table coach as long as you don't use columns in the SELECT list. I removed it completely and replaced the WHERE condition with where pp.coach = 1.
You don't need COALESCE. NULL values are ignored in the aggregate function sum(). No need to substitute 0.
Use table aliases to make it easier to read.
Next, I solved your problem like this:
SELECT *
FROM (
SELECT pe.year
, array_agg(DISTINCT po.id) AS position_id
, array_agg(DISTINCT po.name) AS position_names
FROM periods_positions_coaches_linking pp
JOIN positions po ON po.id = pp.position
JOIN periods pe ON pe.id = pp.period
WHERE pp.coach = 1
GROUP BY pe.year
) po
LEFT JOIN (
SELECT pe.year
, sum(pe.wins) AS wins
, sum(pe.losses) AS losses
, sum(pe.ties) AS ties
FROM (
SELECT period
FROM periods_positions_coaches_linking
WHERE coach = 1
GROUP BY period
) pp
JOIN periods pe ON pe.id = pp.period
GROUP BY pe.year
) pe USING (year)
ORDER BY year;
Aggregate positions and periods separately before joining them.
In the first sub-query po list positions only once with array_agg(DISTINCT ...).
In the second sub-query pe ...
GROUP BY period, because a coach can have multiple positions per period.
JOIN to periods-data after that, and then aggregate to get sums.
db<>fiddle here
Old sqlfiddle

Answer 2

use distinct as shown here
code:
select periods.year as year,
sum(coalesce(periods.wins, 0)) as wins,
sum(coalesce(periods.losses, 0)) as losses,
sum(coalesce(periods.ties, 0)) as ties,
array_agg( distinct positions.id) as position_id,
array_agg( distinct positions.name) as position_names
from periods_positions_coaches_linking
join coaches on coaches.id = periods_positions_coaches_linking.coach
join positions on positions.id = periods_positions_coaches_linking.position
join periods on periods.id = periods_positions_coaches_linking.period
where coaches.id = 1
group by periods.year, positions.id
order by periods.year;

Answer 3

In your case, the easiest way might be to divide out the positions:
select periods.year as year,
sum(coalesce(periods.wins, 0))/COUNT(distinct positions.id) as wins,
sum(coalesce(periods.losses, 0))/COUNT(distinct positions.id) as losses,
sum(coalesce(periods.ties, 0))/COUNT(distinct positions.id) as ties,
array_agg(distinct positions.id) as position_id,
array_agg(distinct positions.name) as position_names
from periods_positions_coaches_linking join
coaches
on coaches.id = periods_positions_coaches_linking.coach join
positions
on positions.id = periods_positions_coaches_linking.position join
periods
on periods.id = periods_positions_coaches_linking.period
where coaches.id = 1
group by periods.year
order by periods.year;
The number of positions scales the wins, losses, and ties, so dividing it out adjusts the counts.