Question

I have three tables

   table name: tags
          id (int)
          tag_name (vharchar)    

   table name: messages
          id (int)
          message_title (vharchar)
          message_content(vharchar)

    table name: message_tag
          message_id (int)
          tag_id (int)

I have tried this code but it didn’t work(returns empty)

SELECT messages.message_title, messages.message_content, tags.tag_name
            FROM messages
            INNER JOIN message_tag ON messages.id = message_tag.message_id
            INNER JOIN tags ON tags.id = message_tag.tag_id
            WHERE message_tag.tag_id =191
            AND message_tag.tag_id =19
            AND message_tag.tag_id =31
            ORDER BY RAND( )
            LIMIT 20

I want to select random 20 rows which are connected to tag 191, 19 and 31

Answer 1

SELECT messages.message_title, messages.message_content, tags.tag_name
        FROM messages
        INNER JOIN message_tag ON messages.id = message_tag.message_id
        INNER JOIN tags ON tags.id = message_tag.tag_id
        WHERE message_tag.tag_id IN (191, 19, 31)
        ORDER BY RAND( )
        LIMIT 20

Use IN instead of AND. This will bring you messages which are linked to one at least of those particular tags.

Update: IF you wanted to get messages where all 3 tags are present, you would do this instead (assuming your rows in message_tag are unique):

    SELECT messages.message_title, messages.message_content, tags.tag_name
        FROM messages
        INNER JOIN message_tag ON messages.id = message_tag.message_id
        INNER JOIN tags ON tags.id = message_tag.tag_id
        WHERE message_tag.tag_id IN (191, 19, 31)
        GROUP BY messages.id
        HAVING COUNT(messages.id) > 2
        ORDER BY RAND( )
        LIMIT 20