Get values from first and last row per group

Question

I'm new to Postgres, coming from MySQL and hoping that one of y'all would be able to help me out.

I have a table with three columns: name, week, and value. This table has a record of the names, the week at which they recorded the height, and the value of their height. Something like this:

Name  |  Week  | Value
------+--------+-------
John  |  1     | 9
Cassie|  2     | 5
Luke  |  6     | 3
John  |  8     | 14
Cassie|  5     | 7
Luke  |  9     | 5
John  |  2     | 10
Cassie|  4     | 4
Luke  |  7     | 4

What I want is a list per user of the value at the minimum week and the max week. Something like this:

Name  |minWeek | Value |maxWeek | value
------+--------+-------+--------+-------
John  |  1     | 9     | 8      | 14
Cassie|  2     | 5     | 5      | 7
Luke  |  6     | 3     | 9      | 5

In Postgres, I use this query:

select name, week, value
from table t
inner join(
select name, min(week) as minweek
from table
group by name)
ss on t.name = ss.name and t.week = ss.minweek
group by t.name
;

However, I receive an error:

column "w.week" must appear in the GROUP BY clause or be used in an aggregate function
Position: 20

This worked fine for me in MySQL so I'm wondering what I'm doing wrong here?

Answer 1

This is a bit of a pain, because Postgres has the nice window functions first_value() and last_value(), but these are not aggregation functions. So, here is one way:

select t.name, min(t.week) as minWeek, max(firstvalue) as firstvalue,
       max(t.week) as maxWeek, max(lastvalue) as lastValue
from (select t.*, first_value(value) over (partition by name order by week) as firstvalue,
             last_value(value) over (partition by name order by week) as lastvalue
      from table t
     ) t
group by t.name;

Answer 2

There are various simpler and faster ways.

2x DISTINCT ON

SELECT *
FROM  (
   SELECT DISTINCT ON (name)
          name, week AS first_week, value AS first_val
   FROM   tbl
   ORDER  BY name, week
   ) f
JOIN (
   SELECT DISTINCT ON (name)
          name, week AS last_week, value AS last_val
   FROM   tbl
   ORDER  BY name, week DESC
   ) l USING (name);

Or shorter:

SELECT *
FROM  (SELECT DISTINCT ON (1) name, week AS first_week, value AS first_val
       FROM   tbl ORDER BY 1,2) f
JOIN  (SELECT DISTINCT ON (1) name, week AS last_week, value AS last_val
       FROM   tbl ORDER BY 1,2 DESC) l USING (name);

Simple and easy to understand. Also fastest in my tests. Detailed explanation for DISTINCT ON:

• Select first row in each GROUP BY group?

first_value() of composite type

The aggregate functions min() or max() do not accept composite types as input. You would have to create custom aggregate functions (which is not that hard).
But the window functions first_value() and last_value() do. Building on that we can devise an very simple solutions:

Simple query

SELECT DISTINCT ON (name)
       name, week AS first_week, value AS first_value
     ,(first_value((week, value)) OVER (PARTITION BY name
                                        ORDER BY week DESC))::text AS l
FROM   tbl t
ORDER  BY name, week;

The output has all data, but the values for the last week are stuffed into an anonymous record. You may need decomposed values.

Decomposed result with opportunistic use of table type

For that we need a well-known type that registers the types of contained elements with the system. An adapted table definition would allow for the opportunistic use of the table type itself directly:

CREATE TABLE tbl (week int, value int, name text) -- note optimized column order

weekand value come first.

SELECT (l).name, first_week, first_val
     , (l).week AS last_week, (l).value AS last_val
FROM (
   SELECT DISTINCT ON (name)
          week AS first_week, value AS first_val
         ,first_value(t) OVER (PARTITION BY name ORDER BY week DESC) AS l
   FROM   tbl t
   ORDER  BY name, week
   ) sub;

Decomposed result from user-defined row type

However, that's probably not possible in most cases. Just use a user-defined type from CREATE TYPE (permanent) or from CREATE TEMP TABLE (for ad-hoc use):

CREATE TEMP TABLE nv(last_week int, last_val int);  -- register composite type

SELECT name, first_week, first_val, (l).last_week, (l).last_val
FROM (
   SELECT DISTINCT ON (name)
          name, week AS first_week, value AS first_val
         ,first_value((week, value)::nv) OVER (PARTITION BY name
                                               ORDER BY week DESC) AS l
   FROM   tbl t
   ORDER  BY name, week
   ) sub;

In a local test on Postgres 9.3 with a similar table of 50k rows, each of these queries was substantially faster than the currently accepted answer. Test with EXPLAIN ANALYZE.

SQL Fiddle displaying all.