MySQL Join從左表中返回所有數據以及在右表中找到的所有數據

Question

腦子不好 我知道這很簡單，這不是單擊即可。 我正在盡力避免子查詢，但這可能是不可避免的。

共有11條記錄中左表[ cards_types ]和1之間和11條記錄中右表[ users_cards 。 我需要從左表以及在右表中找到的所有內容返回所有記錄。 右表的唯一警告是，如果執行card_types則執行一些IF / ELSE語句以返回0值。 id中找不到users_cards 。 另外，在cards_types上有一個外鍵約束。 id => users_cards 。 type_id （如果有關系）。

詢問

SELECT
  t.id,
  t.slug,
  t.label AS type_label,
  t.points AS point_value,
  IF (c.mtg_id IS NULL, 0, c.mtg_id) AS mtg_id,
  IF (c.label IS NULL, 0, c.label ) AS card_label,
  IF (uc.points IS NULL, 0, uc.points ) AS card_score
FROM cards_types t
JOIN users_cards uc
  ON uc.type_id = t.id
JOIN cards c
  ON c.id = uc.card_id
WHERE uc.user_id = 1
  AND uc.season_id = 1
ORDER BY t.priority ASC

Answer 1

您當前正在使用INNER JOIN ，將其更改為LEFT JOIN 。 我也將您的WHERE子句過濾器移到了JOIN因此您將從cards_type返回所有行。 如果將過濾器保留在WHERE子句中，則其作用類似於INNER JOIN ：

SELECT
  t.id,
  t.slug,
  t.label AS type_label,
  t.points AS point_value,
  COALESCE(c.mtg_id, 0) AS mtg_id,
  COALESCE(c.label, 0) AS card_label,
  COALESCE(uc.points, 0) AS card_score
FROM cards_types t
LEFT JOIN users_cards uc
  ON uc.type_id = t.id
  AND uc.user_id = 1  -- < -- move the where filters here
  AND uc.season_id = 1
LEFT JOIN cards c
  ON c.id = uc.card_id
ORDER BY t.priority ASC

Answer 2

嘗試像這樣的左連接

 SELECT
  t.id,
  t.slug,
  t.label AS type_label,
  t.points AS point_value,
  COALESCE(c.mtg_id, 0) AS mtg_id,
  COALESCE(c.label, 0) AS card_label,
  COALESCE(uc.points, 0) AS card_score
FROM cards_types t
LEFT JOIN users_cards uc
  ON uc.type_id = t.id
  AND uc.user_id = 1
  AND uc.season_id = 1
LEFT JOIN cards c
  ON c.id = uc.card_id
ORDER BY t.priority ASC