Finding value with most common condition - sql

Question

I am having a lot of trouble with PostgreSQL trying to figure out how to find the most common value that fits a specific criteria. The ID is the ID number of the book, meaning repeating numbers means there are multiple copies of the book.
I have 2 tables here:
Table A:
=====+===================
ID | Condition
-------------------------
1 | Taken
1 |
1 | Taken
1 |
2 | Taken
3 | Taken
3 |
3 | Taken
3 | Taken
4 |
4 | Taken
etc.
Table B:
=====+===================
ID | Name
-------------------------
1 | BookA
2 | BookB
3 | BookC
4 | BookD
etc.
What I need is to simply find which book has the most copies taken and simply print out the name of the book. In this case all I need is:
BookC
The problem is that I can't figure out how to find how much each individual ID has books taken. I tried using a temp table something like so:
CREATE TEMP TABLE MostCommon AS
(SELECT ID
FROM TableA
WHERE SUM(CASE WHEN Condition>0 then 1 else 0 END)
)
SELECT NAME FROM TableB, MostCommon WHERE
MostCommon.ID = TableB.ID;
But it either throws an error or simply doesn't give me what I need. Any help would be greatly appreciated.

Answer 1

Ok, so firstly I assumed that your columns and tables names are case sensitive which means you must use duble quote marks. To print most "taken" book name with number of "taken" copies, you can use simple aggragete count(), then order the output descending and at the end limit the output to 1 row, like:
SELECT
b."ID",
b."Name",
count(*) as takenCount
FROM "TableA" a
JOIN "TableB" b ON a."ID" = b."ID"
WHERE a."Condition" = 'Taken'
GROUP BY b."ID", b."Name"
ORDER BY 3 DESC
LIMIT 1;

Answer 2

CREATE TEMP TABLE MostCommon AS
(SELECT id, (sum(ID)/id) book_taken FROM tableA where condition = 'Taken' group by id);
select name from tableB t2 join MostCommon mc on mc.id = t2.id where mc.id in (select max(book_taken) from MostCommon)

Answer 3

To make the data sensible (i.e. no duplicate records), I have to change the schema a little.
CREATE TABLE book_condition (
created TIMESTAMP,
book_id INTEGER,
condition VARCHAR,
PRIMARY KEY (created, book_id));
INSERT INTO book_condition (created, book_id, condition)
VALUES
('2016-01-01 08:30', 1, 'Taken'),
('2016-01-01 08:35', 1, ''),
('2016-01-01 08:40', 1, 'Taken'),
('2016-01-01 08:45', 1, ''),
('2016-01-01 08:50', 2, 'Taken'),
('2016-01-01 08:55', 3, 'Taken'),
('2016-01-01 09:00', 3, ''),
('2016-01-01 09:05', 3, 'Taken'),
('2016-01-01 09:10', 3, 'Taken'),
('2016-01-01 09:15', 4, ''),
('2016-01-01 09:20', 4, 'Taken');
CREATE TABLE book (
book_id INTEGER,
name VARCHAR,
PRIMARY KEY (book_id));
INSERT INTO book (book_id, name)
VALUES
(1, 'BookA'),
(2, 'BookB'),
(3, 'BookC'),
(4, 'BookD');
Then, the question breaks down into:
How many copies of each book were ever taken?
SELECT
book_id,
COUNT(book_id) AS total_taken
FROM book_condition
WHERE
condition = 'Taken'
GROUP BY book_id
;
book_id | total_taken
---------+-------------
1 | 2
2 | 1
3 | 3
4 | 1
(4 rows)
How to rank records by total_taken value?
SELECT
book_id,
total_taken,
RANK() OVER (
ORDER BY total_taken DESC
) AS total_taken_rank
FROM (
SELECT
book_id,
COUNT(book_id) AS total_taken
FROM book_condition
WHERE
condition = 'Taken'
GROUP BY book_id
) AS bt
ORDER BY total_taken_rank ASC
;
book_id | total_taken | total_taken_rank
---------+-------------+------------------
3 | 3 | 1
1 | 2 | 2
2 | 1 | 3
4 | 1 | 3
(4 rows)
How to get the name of the book in a query result containing its key (id) value?
SELECT
b.book_id,
b.name,
bt.total_taken,
RANK() OVER (
ORDER BY bt.total_taken DESC
) AS total_taken_rank
FROM
book AS b
LEFT JOIN (
SELECT
book_id,
COUNT(book_id) AS total_taken
FROM book_condition
WHERE
condition = 'Taken'
GROUP BY book_id
) AS bt
USING (book_id)
ORDER BY
total_taken_rank ASC,
book_id ASC
;
book_id | name | total_taken | total_taken_rank
---------+-------+-------------+------------------
3 | BookC | 3 | 1
1 | BookA | 2 | 2
2 | BookB | 1 | 3
4 | BookD | 1 | 3
(4 rows)
How to get only the highest-ranking records in the result?
SELECT
br.book_id,
br.name,
br.total_taken
FROM (
SELECT
b.book_id,
b.name,
bt.total_taken,
RANK() OVER (
ORDER BY bt.total_taken DESC
) AS total_taken_rank
FROM
book AS b
LEFT JOIN (
SELECT
book_id,
COUNT(book_id) AS total_taken
FROM book_condition
WHERE
condition = 'Taken'
GROUP BY book_id
) AS bt
USING (book_id)
) AS br
WHERE
total_taken_rank = 1
;
book_id | name | total_taken
---------+-------+-------------
3 | BookC | 3
(1 row)