How to perform the following query with Postgres? - sql

Question

This is the example
banzai=# select letter_id, length_id, word from words;
letter_id | length_id | word
-----------+-----------+-------
1 | 1 | run
3 | 1 | tea
2 | 1 | cat
2 | 2 | cast
2 | 3 | coast
1 | 3 | roast
1 | 2 | rest
3 | 2 | team
3 | 3 | toast
(9 rows)
banzai=# select letter from letters;
letter
--------
R
C
T
(3 rows)
banzai=# select length from lengths;
length
--------
4
5
3
(3 rows)
banzai=# select length, letter, word from words, lengths, letters where words.length_id = lengths.id and words.letter_id = letters.id;
length | letter | word
--------+--------+-------
3 | C | cat
3 | R | run
3 | T | tea
4 | R | rest
4 | C | cast
4 | T | team
5 | R | roast
5 | C | coast
5 | T | toast
(9 rows)
I want to produce the following table in HTML
R T C
3 run tea cat
4 rest team cast
5 roast toast coast
I have a method in my java (backend) code that will produce the data in json. Angularjs (frontend) will take the json and present the table in html

Answer 1

As you want JSON this will return a single object:
select json_object_agg(length, o)
from (
select length, json_object_agg(letter, word) as o
from
words w
inner join
lengths l on w.length_id = l.id
inner join
letters t on w.letter_id = t.id
group by length
) s;
json_object_agg
----------------------------------------------------------------------------------------------------------------------------------------------------------------
{ "4" : { "R" : "rest", "C" : "cast", "T" : "team" }, "5" : { "R" : "roast", "C" : "coast", "T" : "toast" }, "3" : { "C" : "cat", "R" : "run", "T" : "tea" } }
The above query is for 9.4. In 9.3 it is a bit more difficult but it can be done as well.