How to get unique values in the self join and how to get LIMIT number dynamically in psql - sql

Question

Hi i am just learning databases and practicing my skills on the table shown below
id | name | wins | matches
-----+-------------------+------+---------
205 | Twilight Sparkle | 0 | 0
206 | Fluttershy | 0 | 0
207 | Applejack | 0 | 0
208 | Pinkie Pie | 0 | 0
209 | Rarity | 0 | 0
210 | Rainbow Dash | 0 | 0
211 | Princess Celestia | 0 | 0
212 | Princess Luna | 0 | 0
My Job is here is Returns a list of pairs of players for the next round of a match.
Assuming that there are an even number of players registered, each player
appears exactly once in the pairings. Each player is paired with another
player with an equal or nearly-equal win record, that is, a player adjacent to him or her in the standings.
Returns:
A list of tuples, each of which contains (id1, name1, id2, name2)
id1: the first player's unique id
name1: the first player's name
id2: the second player's unique id
name2: the second player's name
to achieve those goals i have done self joined that table and have writen code something like this
SELECT a.id, a.name, b.id, b.name
FROM results AS a, results AS b
WHERE a.id > b.id and a.wins = b.wins
LIMIT COUNT(a.id)/2;
It seems not working. Please help me to dealing with this.
Thanks.

Answer 1

You can sequence them based on their wins then join on the sequence, so they may have the same wins or next closest:
WITH seq_results AS
(
SELECT
id,
name,
ROW_NUMBER() OVER(ORDER BY wins DESC) AS seq
FROM
results
)
SELECT
r1.id,
r1.name,
r2.id,
r2.name
FROM
seq_results r1
JOIN
seq_results r2
ON (r1.seq = (r2.seq - 1))
AND (r2.seq % 2 = 0);
Per your request, here is some information on how this works. I will highly recommend that you visit the documentation for PostgreSQL - it really is some of the best documentation out there: http://www.postgresql.org/docs/current/static/
The first part is a common-table expression (CTE). It allows me to essentially create a table in-memory for use in subsequent queries. You could just as easily create a temp table, but these don't have to be dropped, etc.
See: http://www.postgresql.org/docs/current/static/queries-with.html
WITH seq_results AS
(
SELECT
id,
name,
ROW_NUMBER() OVER(ORDER BY wins DESC) AS seq
FROM
results
)
In this CTE, I am sequencing/sequentially numbering each record using a window function. I will use these numbers later in my join. See: http://www.postgresql.org/docs/current/static/functions-window.html
SELECT
r1.id,
r1.name,
r2.id,
r2.name
FROM
seq_results r1
JOIN
seq_results r2
ON (r1.seq = (r2.seq - 1))
AND (r2.seq % 2 = 0);
Above I am joining the CTE to itself using the sequence. I "offset" the sequence of the second instance of the CTE r2 by -1, essentially joining two sequential records together.
Had I only specified that condition in the join, I would return more than the 4 records expected. I needed to make sure that the ids and names on the "left" are not also on the "right", so I decided to include only the odd-numbered sequenced records on the left and the evens on the right. To do this, I used the modulus operator % to ensure that r2 only returned records where the sequence was even.
Lastly, because the join was an inner join (JOIN is the same as INNER JOIN), any even-numbered sequences in r1 are not returned.