Mysql 左連接，限制返回一行的連接記錄

Question

如何使用限制加入表？ 我有以下查詢，但它沒有按預期工作。 我正在使用left join聯接到 select 表中只有一行，但它只有 select 一條記錄，如預期的第一行，而它返回 null 其他人即使他們保存在TABLE_USER_FILES文件中

TABLE_USERS

uid | u_name  
----|---------
p1  | Peter 
j1  | John
f1  | Foo
b1  | Bar

TABLE_USER_POST

pid | p_name  | p_uid
----|---------|--------
xp1 | PHP     | p1
xp2 | SQL     | p1
xp3 | JS      | j1
xp4 | CSS     | b1

TABLE_USER_FILES

fid | f_uid  | f_url   | f_path
----|--------|---------|----------
fa1 | p1     | ax.png  | gallery
fb2 | p1     | bc.png  | gallery
bc3 | j1     | cc.png  | gallery
fd4 | f1     | cx.png  | gallery
fe5 | j1     | qd.png  | gallery

詢問

SELECT post.*, user.u_name, files.f_url
FROM TABLE_USER_POST post

INNER JOIN TABLE_USERS user
ON user.uid = post.p_uid

LEFT JOIN (
    SELECT f_url, f_uid
        FROM TABLE_USER_FILES
        WHERE f_path = "gallery"
        ORDER BY fid DESC
    LIMIT 1
) files
ON files.f_uid = post.p_uid

ORDER BY post.pid DESC 
LIMIT 0, 20

預期結果

pid | p_name  | p_uid  | u_name  | f_url
----|---------|--------|---------|---------
xp1 | PHP     | p1     | Peter   | bc.png
xp2 | SQL     | p1     | Peter   | bc.png
xp3 | JS      | j1     | John    | qd.png
xp4 | CSS     | b1     | Bar     | NULL

Answer 1

請改用這個。

SELECT post.*, user.u_name, files.f_url FROM TABLE_USER_POSTS post LEFT JOIN TABLE_USER_FILES files ON files.f_uid = post.p_uid AND files.fid = (SELECT MAX(fid) FROM TABLE_USER_FILES WHERE f_uid = files.f_uid) INNER JOIN TABLE_USERS user ON user.uid = post.p_uid ORDER BY post.pid DESC;

謝謝！

Answer 2

這里有很多解決方案。 例如，MySQL 8.0.14+ 中的LATERAL

SELECT post.*, user.u_name, files.f_url
FROM TABLE_USER_POST post

INNER JOIN TABLE_USERS user
ON user.uid = post.p_uid

LEFT OUTER JOIN LATERAL (
    SELECT f_url, f_uid
    FROM TABLE_USER_FILES tuf
    WHERE f_path = "gallery"
      AND tuf.f_uid = post.p_uid
    ORDER BY fid DESC
    LIMIT 1
) files ON true

ORDER BY post.pid DESC 
LIMIT 0, 20

如果只需要 TABLE_USER_FILES 中的一列，則 SELECT 子句中的查詢：

SELECT post.*, user.u_name,
  ( SELECT f_url
    FROM TABLE_USER_FILES tuf
    WHERE f_path = "gallery"
      AND tuf.f_uid = post.p_uid
    ORDER BY fid DESC
    LIMIT 1
  ) AS f_url
FROM TABLE_USER_POST post

INNER JOIN TABLE_USERS user
ON user.uid = post.p_uid

ORDER BY post.pid DESC 
LIMIT 0, 20

db<>小提琴