Getting 2 columns from sets of 2 rows in Postgresql

Question

So I have my data organized as:

id(bigint) | report_item (varchar) | report_value (numeric) | stock_id (uuid) | date

1 | netIncome | 100 | stockid1 | 2019-05-01
2 | totalRevenue |1000 | stockid1 | 2019-05-01
3 | salesExpense |20 | stockid1 | 2019-05-01
4 | totalAssets | 10000 | stockid1 | 2019-05-01
5 | netIncome | 30 | stockid2 | 2019-04-23
6 | totalRevenue |600 | stockid2 | 2019-04-23
7 | salesExpense |15 | stockid2 | 2019-04-23
8 | totalAssets | 900 | stockid3 | 2019-04-23

So for each stockid, date pair, there is only one of each different report_item (there's not 2 different totalAssets for X date, Y stock id). There is a totalAssets for each stockID for every quarter (which aren't always on the same date).

What I want to do is return stock_id, date, totalAssets, totalRevenue as columns, effectively merging the 2 rows (one for totalAssets, totalRevenue) for each date/stock_id pair into 1 while discarding any rows where report_item is neither totalAssets or totalRevenue.

So this is what I did to turn turn the totalAssets and totalRevenue rows into columns, which gets rid of rows with other report_items, although for each stock_id, date pair there are still 2 rows. What I want is to have them in one row.

select date,
       stock_id,
       (case when report_item = 'netIncome' then id end) as netIncome,
       (case when report_item = 'totalRevenue' then id end) as totalRevenue
from report_items where report_item IN('netIncome', 'totalRevenue') order by date desc limit 100

Returns:

date | stock_id | netIncome | totalRevenue
2019-05-01 | stockid1 | 100 | null
2019-05-01 | stockid1 | null | 1000
2019-04-23 | stockid2 | 30 | null
2019-04-23 | stockid2 | null | 600

Answer 1

You can use a self-join for that:

select ni.date,
       ni.stock_id,
       ni.report_value as net_income, 
       nr.report_value as total_revenue
from report_items ni
  join report_items nr 
    on ni.stock_id = nr.stock_id 
   and ni."date" = nr."date" 
   and nr.report_item = 'totalRevenue'
where ni.report_item = 'netIncome'
order by date desc limit 100