PostgreSQL - Conditional aggregation - Avg() in Select statement

Question

I have this table

| user         | Mark       | Points   |
|--------------|------------|----------|
| John         |    0       |    2     |
| Paul         |    5       |    3     |
| John         |    4       |    4     |
| Paul         |    7       |    5     |

I would like to build a query with one select statement that returns the rows shown below. Avg(Mark) - should be average only if Mark>0 Sum(Points) - should be sum of all records.

| user         | Avg(Mark)  | Sum(Points) |
|--------------|------------|-------------|
| John         |    4       |    6        |
| Paul         |    6       |    8        |

Can anyone point to a proper syntax?

I believe it should like :

select user, avg(Mark>0), sum(Points) from Table group by user;

Inego · Accepted Answer · 2018-03-23 03:00:24Z

27

Starting with version 9.4, PostgreSQL directly supports filtering aggregates.

https://www.postgresql.org/docs/9.4/static/sql-expressions.html

If FILTER is specified, then only the input rows for which the filter_clause evaluates to true are fed to the aggregate function; other rows are discarded.

By using it, your example can be rewritten as:

SELECT
    "user",
    AVG(mark) FILTER (WHERE mark > 0),
    SUM(points)
FROM
    "table"
GROUP BY
    "user"

edited Mar 23, 2018 at 3:00

answered Feb 20, 2018 at 7:18

Inego

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Comments

David Aldridge · Accepted Answer · 2013-06-20 13:26:11Z

13

How about:

select user,
       avg(case when mark > 0 then mark end),
       sum(mark)
from   ...

answered Jun 20, 2013 at 13:26

David Aldridge

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1 Comment

geoyws Over a year ago

Caution to readers: your avg might be wrong if you intend to exclude the row entirely from your list of numbers. @Inego's answer with FILTER WHERE would be better for that scenario.

user80168 · Accepted Answer · 2013-06-20 12:55:52Z

1

select
    user, -- very bad choice for column name, but i assume it's just SO example, not real column
    sum( mark ) / count (nullif(mark, 0))
from
    table
group by
    user

should so the trick.

answered Jun 20, 2013 at 12:55

user80168

2 Comments

Fridolin Over a year ago

is there also possibility how to avoid division by zero in case 'John' has all 'Marks' zero values?

Fridolin Over a year ago

I think I have worked it out sum( mark ) / nullif( count (nullif(mark, 0)),0) Thanks again for your help.

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