SQL aliasing with FROM AS - sql

Question

SELECT A.barName AS BarName1, B.barName AS BarName2
FROM (
SELECT Sells.barName, COUNT(barName) AS count
FROM Sells
GROUP BY barName
) AS A, B
WHERE A.count = B.count
I'm trying to do a self join on this table that I created, but I'm not sure how to alias the table twice in this format (i.e. FROM AS). Unfortunately, this is a school assignment where I can't create any new tables. Anyone have experience with this syntax?
edit: For clarification I'm using PostgreSQL 8.4. The schema for the tables I'm dealing with are as follows:
Drinkers(name, addr, hobby, frequent)
Bars(name, addr, owner)
Beers(name, brewer, alcohol)
Drinks(drinkerName, drinkerAddr, beerName, rating)
Sells(barName, beerName, price, discount)
Favorites(drinkerName, drinkerAddr, barName, beerName, season)
Again, this is for a school assignment, so I'm given read-only access to the above tables.
What I'm trying to find is pairs of bars (Name1, Name2) that sell the same set of drinks. My thinking in doing the above was to try and find pairs of bars that sell the same number of drinks, then list the names and drinks side by side (BarName1, Drink1, BarName2, Drink2) to try and compare if they are indeed the same set.

Answer 1

You have not mentioned what RDBMS you use.
If Oracle or MS SQL, you can do something like this (I use my sample data table, but you can try it with your tables):
create table some_data (
parent_id int,
id int,
name varchar(10)
);
insert into some_data values(1, 2, 'val1');
insert into some_data values(2, 3, 'val2');
insert into some_data values(3, 4, 'val3');
with data as (
select * from some_data
)
select *
from data d1
left join data d2 on d1.parent_id = d2.id
In your case this query
SELECT Sells.barName, COUNT(barName) AS count
FROM Sells
GROUP BY barName
should be placed in WITH section and referenced from main query 2 times as A and B.

Answer 2

It is slightly unclear what you are trying to achive. Are you looking for a list bar names, with how many times they appear in the table? If so, there are a couple ways you could do this. Firstly:
SELECT SellsA.barName AS BarName1, SellsB.count AS Count
FROM
(
SELECT DISTINCT barName
FROM Sells
) SellsA
LEFT JOIN
(
SELECT Sells.barName, COUNT(barName) AS count
FROM Sells
GROUP BY barName
) AS SellsB
ON SellsA.barName = SellsB.barName
Secondly, if you are using MSSQL:
SELECT barNamr, MAX(rn) AS Count
FROM
(
SELECT barName,
ROW_NUMBER() OVER(ORDR BY barName PARTITION BY barName) as rn
FROM Sells
) CountSells
GROUP BY barName
Thirdly, you could avoid a self-join in MSSQL, by using OVER():
SELECT
barName
COUNT(*) OVER(ORDER BY barName PARTITION BY barName) AS Count
FROM Sells