Question

I am wanting to select all images that a user does not already have.

I have three tables: user, image, and user_image:

• _user (id, user, pass)
• _image (id, data)
• _user_image (id, user_id, image_id)

Answer 1

Using LEFT JOIN/IS NULL

---

   SELECT DISTINCT
          i.data
     FROM IMAGE i
     JOIN USER_IMAGE ui ON ui.image_id = i.id
LEFT JOIN USER u ON u.id = ui.user_id
                AND u.user = ?
    WHERE u.id IS NULL

Using NOT IN

---

SELECT DISTINCT
       i.data
  FROM IMAGE i
  JOIN USER_IMAGE ui ON ui.image_id = i.id
 WHERE ui.user_id NOT IN (SELECT u.id
                            FROM USER u
                           WHERE u.user = ?)

Using NOT EXISTS

---

SELECT DISTINCT
       i.data
  FROM IMAGE i
  JOIN USER_IMAGE ui ON ui.image_id = i.id
 WHERE NOT EXISTS(SELECT NULL
                    FROM USER u
                   WHERE u.id = ui.user_id
                     AND u.user = ?)

Performance:

---

The LEFT JOIN/IS NULL and NOT IN provide equivalent performance – the NOT EXISTS is ~30% less efficient. See this for more details.