Get a Combined Result in MySQL - mysql

Question

I have 5 tables in Mysql and i need a combined result from all the tables
The tables are as follows
1)library_book_relation
CREATE TABLE db.library_book_relation (
id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
library_id int ,
book_item_id int ,
seller_price decimal(10,2) ,
pack_quantity decimal(10,2) ,
is_discontinue tinyint ,
total_quantity decimal(10,2) ,
) ;
2)book_item
CREATE TABLE db.`book_item` (
id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
book_id int ,
size_unit_id tinyint ,
size decimal(10,2) ,
is_active boolean ,
is_deleted boolean ,
) ;
3) book
CREATE TABLE db.book(
id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
name varchar(100) ,
author_id int ,
description varchar(256) ,
is_active boolean ,
is_deleted boolean ,
) ;
4) author
CREATE TABLE db.author(
id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
name varchar(100) ,
image varchar(256) ,
description varchar(256) ,
is_active tinyint ,
is_deleted tinyint ,
) ;
5)size_unit
CREATE TABLE db.size_unit (
id tinyint NOT NULL PRIMARY KEY,
name varchar(100) ,
is_active tinyint ,
) ;
i want a combined result where i get
author id
author name
count of books for the author in library
i got all 3 in different queries but dont know how to combine the result
to get author id and name
select id,name FROM author where id IN( select e.id from author e inner join book r on e.id = r.author_id where r.id IN( select q.id from book q inner join book_item w on q.id = w.book_id where w.id IN( Select s.id from book_item s inner join library_book_relation d on s.id=d.book_item_id where d.library_id = 1 )))
to get count of books for author in library
(Select COUNT(*) FROM book where author_id IN ( select e.id from author e inner join book r on e.id = r.author_id where r.id IN( select q.id from book q inner join book_item w on q.id = w.book_id where w.id IN( Select s.id from book_item s inner join library_book_relation d on s.id=d.book_item_id where d.library_id = 1 ))) GROUP BY author_id)

Answer 1

The easiest way is to join all tables, group by author and library and count:
select b.author_id, a.name, lbr.library_id, count(*)
from book b
join book_item bi on bi.book_id = b.id
join library_book_relation lbr on lbr.book_item_id = bi.id
join author a on a.id = b.author_id
group by b.author_id, a.name, lbr.library_id
order by b.author_id, a.name, lbr.library_id;
Per SQL standard you only need to group by b.author_id, lbr.library_id, because you want one result row per author and library. MySQL should be able to do this, so you can remove a.name from the group by clause. (There is only one name per author ID.)
You can also count before joining with the authors table. It is considered good style to aggregate before joining and can help a lot (or be even necessary) in more complicated queries.
select c.author_id, a.name, c.library_id, c.books
from
(
select b.author_id, lbr.library_id, count(*) as books
from book b
join book_item bi on bi.book_id = b.id
join library_book_relation lbr on lbr.book_item_id = bi.id
group by b.author_id, lbr.library_id
) c
join author a on a.id = c.author_id
order by c.author_id, a.name, c.library_id;
Above queries do not count zeros, though. They just show how often to find authors in the libraries. Here is how to count how many books author #1 has in library #123:
select a.id, a.name, coalesce(b.books, 0)
from author a
left join
(
select b.author_id, count(*) as books
from book b
join book_item bi on bi.book_id = b.id
join library_book_relation lbr on lbr.book_item_id = bi.id
where lbr.library_id = 123
group by b.author_id
) c on c.author_id = a.id
where a.id = 1;
Or with the subquery in the select clause:
select
a.id,
a.name,
(
select count(*)
from book b
join book_item bi on bi.book_id = b.id
join library_book_relation lbr on lbr.book_item_id = bi.id
where lbr.library_id = 123
and b.author_id = a.id
) as books
from author a
where a.id = 1;
At last a query to get all authors and all libraries including zeros:
select a.id as author_id, a.name, l.id as library_id, coalesce(c.books, 0)
from author a
cross join library l
left join
(
select b.author_id, lbr.library_id, count(*) as books
from book b
join book_item bi on bi.book_id = b.id
join library_book_relation lbr on lbr.book_item_id = bi.id
group by b.author_id, lbr.library_id
) c on c.author_id = a.id and c.library_id = l.id
order by a.id, l.id;