Mysql – Select rows with multiple AND conditions that override eachother

You need to use GROUP_CONCAT to aggregate the citizen names first. Then use CONCAT

SELECT CONCAT(id,' # ',name,' # ',citizens) listing FROM
(
    SELECT A.id,A.name,GROUP_CONCAT(B.name,' ',B.surname) citizens
    FROM city A INNER JOIN citizen B
    ON A.id = B.city_id GROUP BY A.id,A.name
) AA;

Here is the SQL Fiddle to prove it : http://sqlfiddle.com/#!2/9f4b3/1

If the names become too long due to truncation, you will have to extend GROUP_CONCAT's maximum length. To set it to 16K, run the following:

SET group_concat_max_len = 1024 * 16;
SET GLOBAL group_concat_max_len = 1024 * 16;

First line is for the session, the second is for all new DB Connections.

Because you desire a categorization hierarchy of indeterminate depth, you will have to use a single category table structured along these lines:

CREATE TABLE Category (
    Category_ID           INT NOT NULL AUTO_INCREMENT primary key,
    Parent_Category_ID    INT NOT NULL REFERENCES Category(Category_ID),
    Description           varchar(255) not null
)

This type of structure is not as well supported by MySQL as some other RDBMS's, but these issues can all be worked through.

You will also need a revised BookCategory mapping table such as this:

CREATE TABLE BookCategory (
    Book_ID        INT NOT NULL references Book(Book_ID),
    Category_ID    INT NOT NULL references Category(Category_ID)
)

My apologies if the Foreign Key syntax above is not quite correct for MySQL; I usually work with SQL Server.

Update With this INSERT:

insert Category(Parent_Category_ID,Description)
values (1,'Mystery')
      ,(2,'SciFi')
      ,(2,'Aliens')
      ,(3,'Humanoid')
      ,(3,'Reptilian')
      ,(3,'Other')
;

the following SELECT:

select
    root.Category_ID,
    cast(
      coalesce(grandparent.Description + '-',' ') +
      coalesce(parent.Description + '-',' ') +
            root.Description 
    as varchar(255)) as Description
from Category                           root
left join Category                      parent
   on parent.Category_ID            = root.Parent_Category_ID
  and root.Parent_Category_ID      <> root.Category_ID
left join Category                      grandparent
   on grandparent.Category_ID       = parent.Parent_Category_ID
  and parent.Parent_Category_ID    <> parent.Category_ID

yields this:

Category_ID Description
----------- 
1             Mystery
2             SciFi
3            SciFi-Aliens
4           SciFi-Aliens-Humanoid
5           SciFi-Aliens-Reptilian
6           SciFi-Aliens-Other

When Common Table Expressions are supported (not yet in MySQL I understand) the following will provide automatic nesting of the hierarchy levels:

with data as (
    select
        Category_ID,
        cast(Description as varchar(255)) as Description
    from Category
    where Parent_Category_ID = Category_ID

    union all

    select
        child.Category_ID,
        cast(parent.Description + '-' + child.Description as varchar(255)) as Description
    from data       parent
    join Category   child
       on child.Parent_Category_ID  = parent.Category_ID
    and child.Parent_Category_ID <> child.Category_ID
)
select 
     Category_ID,
     Description
from data