Question

For example, in my database I have 3 tables,

• the Books table for storing data about books
• the Tags table for storing data about tags
• and a link table (the Book_Tags table) for storing information about a book having multiple tags.

Its illustrated in the image below… (“M” above the arrow means the many side of the relationship)

    BOOKS                    BOOK_TAGS                      TAGS
    +----+---------+         +---------+----------+         +----+---------------+
    |ID  |  Title  |         |Book_Id  |  Tags_Id |         |ID  |  Tag_Name     |
    +----+---------+         +---------+----------+         +----+---------------+ 
    |1   |  Book_1 | 1    M  |1        | 1        | M     1 |1   |  Tag_Name_1   |
    |1   |  Book_1 |<--------|1        | 2        |-------->|2   |  Tag_Name_2   |
    +----+---------+         |1        | 3        |         |3   |  Tag_Name_3   |
                             |2        | 1        |         +----+---------------+ 
                             |2        | 3        |         
                             +---------+----------+         

My question is, how do i query my database so that i would get a result something like

    +---------------------------------------------------------+
    |ID  |  Book_Title  |   Tags                              |
    +---------------------------------------------------------+
    |1   |  Book_1      |   Tag_Name_1, Tag_Name_2, Tag_Name_3|
    |2   |  Book_2      |   Tag_Name_1, Tag_Name_3            |
    +---------------------------------------------------------+

And not something like

    +----------------------------------+
    |ID  |  Book_Title  |   Tags       |
    +----------------------------------+
    |1   |  Book_1      |   Tag_Name_1 |
    |2   |  Book_1      |   Tag_Name_2 |
    |3   |  Book_1      |   Tag_Name_3 |
    |4   |  Book_2      |   Tag_Name_1 |
    |5   |  Book_2      |   Tag_Name_3 |
    +----------------------------------+

Answer 1

You can use GROUP BY Clause to aggregate results and do string append. So something like:

SELECT bk.Book_Title, GROUP_CONCAT(bt.Book_Id SEPARATOR ', ') FROM BOOK_TAGS bt
JOIN BOOKS bk ON bk.ID = bt.Book_Id
JOIN TAGS t ON t.ID = bt.Tags_Id
GROUP BY bt.Book_Id