How to limit values when using distinct

Question

PHP

  SELECT DISTINCT bk.title AS Title, bk.year AS Year, aut.authorname AS Author, cat.category AS Category
         FROM book bk 

         JOIN book_category          


        

Answer 1

  SELECT DISTINCT bk.title AS Title, bk.year AS Year, aut.autName AS Author, cat.category AS Category
     FROM book bk 

     JOIN book_category bk_cat 
     ON bk_cat.book_id = bk.bookid

     JOIN categories cat 
     ON cat.id = bk_cat.category_id


     JOIN (SELECT GROUP_CONCAT(aut.authorname) as autName FROM authors JOIN books_authors ON books_authors.author_id=authors.author_id WHERE books_authors.book_id=bk.bookid) as aut


     ORDER BY bk.title ASC

Answer 2

The easiest way to achieve different data only in the columns you want is using GROUP BY clause. By default, it'll group the rows, depending on the value of the column, showing only distinct values so, if you want to group and show only different titles and categories, you should write your query as:

  SELECT bk.title AS Title, bk.year AS Year, aut.authorname AS Author, cat.category AS Category
  FROM book bk 

  JOIN book_category bk_cat 
  ON bk_cat.book_id = bk.bookid

  JOIN categories cat 
  ON cat.id = bk_cat.category_id

  JOIN books_authors bk_aut 
  ON bk_aut.book_id = bk.bookid

  JOIN authors aut
  ON aut.id = bk_aut.author_id

  GROUP BY  bk.title, cat.category
  ORDER BY bk.title ASC

As you may see, no DISTINCT is used, but you'll get all books with distincts title and categories. The more fields you added into the GROUP BY clause, the more distinct data you'd get.

Same way, if you only wanted list books by title, you should only leave bk.title in the GROUP BY clause

Answer 3

DISTINCT works on the entire row, not a single column. The only way to ensure not repeating rows is to exclude category column from the select.

If your objective is to list all categories, you will have to either look up categories as you're looping over the query result set during output (not very efficient though) OR process your result set in the code before you output it by collapsing repeating rows and merging/concatenating category values into a string.

You could try some fancy SQL with subqueries to convert categories into a single delimited string but that will sure make your code hard to read and query hard to debug.

Answer 4

I think you might want to look into subquery-ing with the group_concat() so please give this a try:

group_concat() will merge the results of the sub-query and provide you with one string of authors separated by a comma-space like this author1, author2, author3

SELECT
    DISTINCT bk.title AS Title,
    bk.year AS 'Year',
    (
        SELECT
            group_concat(aut.authorname, ', ')
        FROM
            authors aut
            LEFT OUTER JOIN
                books_authors bk_aut
                ON
                    aut.id=bk_aut.author_id
        WHERE
            bk_aut.book_id = bk.book_id
    ) AS Author
FROM
    book bk
ORDER BY
    bk.title ASC