LIMIT by distinct values in PostgreSQL - sql

Question

I have a table of contacts with phone numbers similar to this:
Name Phone
Alice 11
Alice 33
Bob 22
Bob 44
Charlie 12
Charlie 55
I can't figure out how to query such a table with LIMITing the rows not just by plain count but by distinct names. For example, if I had a magic LIMIT_BY clause, it would work like this:
SELECT * FROM "Contacts" ORDER BY "Phone" LIMIT_BY("Name") 1
Alice 11
Alice 33
-- ^ only the first contact
SELECT * FROM "Contacts" ORDER BY "Phone" LIMIT_BY("Name") 2
Alice 11
Charlie 12
Alice 33
Charlie 55
-- ^ now with Charlie because his phone 12 goes right after 11. Bob isn't here because he's third, beyond the limit
How could I achieve this result?
In other words, select all rows containing top N distinct Names ordered by Phone

Answer 1

I don't think that PostgreSQL provides any particularly efficient way to do this, but for 6 rows it doesn't need to be very efficient. You could do a subquery to compute which people you want to see, then join that subquery back against the full table.
select * from
"Contacts" join
(select name from "Contacts" group by name order by min(phone) limit 2) as limited
using (name)
You could put the subquery in an IN-list rather than a JOIN, but that often performs worse.

Answer 2

If you want all names that are in the first n rows, you can use in:
select t.*
from t
where t.name in (select t2.name
from t t2
order by t2.phone
limit 2
);
If you want the first n names by phone:
select t.*
from t
where t.name in (select t2.name
from t t2
group by t2.name
order by min(t2.phone)
limit 2
);

Answer 3

try this:
SELECT distinct X.name
,X.phone
FROM (
SELECT *
FROM (
SELECT name
,rn
FROM (
SELECT name
,phone
,row_number() OVER (
ORDER BY phone
) rn
FROM "Contacts"
) AA
) DD
WHERE rn <= 2 --rn is the "limit" variable
) EE
,"Contacts" X
WHERE EE.name = X.name
above seems to be working correctly on following dataset:
create table "Contacts" (name text, phone text);
insert into "Contacts" (name, phone) VALUES
('Alice', '11'),
('Alice', '33'),
('Bob', '22'),
('Bob', '44'),
('Charlie', '13'),
('Charlie', '55'),
('Dennis', '12'),
('Dennis', '66');