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

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