Question

任何人都可以告诉我为什么会出现错误：当我使用FULL JOIN时，'where子句'中的未知列'cdu_user_progress.uid'，但是当我使用JOIN时却没有？我正在使用MySQL ......

SELECT 
    cdu_lessons.*, 
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 0 THEN score END ORDER BY date), ',', 1) AS game0_first,
    MAX(CASE WHEN game_id = 0 THEN score END) AS game0_max,
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 1 THEN score END ORDER BY date), ',', 1) AS game1_first,
    MAX(CASE WHEN game_id = 1 THEN score END) AS game1_max,
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 2 THEN score END ORDER BY date), ',', 1) AS game2_first,
    MAX(CASE WHEN game_id = 2 THEN score END) AS game2_max,
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 3 THEN score END ORDER BY date), ',', 1) AS game3_first,
    MAX(CASE WHEN game_id = 3 THEN score END) AS game3_max,
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 4 THEN score END ORDER BY date), ',', 1) AS game4_first,
    MAX(CASE WHEN game_id = 4 THEN score END) AS game4_max,
    SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 5 THEN score END ORDER BY date), ',', 1) AS game5_first,
    MAX(CASE WHEN game_id = 5 THEN score END) AS game5_max     
FROM cdu_user_progress 
FULL JOIN cdu_lessons ON cdu_lessons.id = cdu_user_progress.lesson_id 
WHERE cdu_user_progress.uid = 145 
GROUP BY lesson_id

Answer 1

mysql不支持

FULL JOIN，至少不是直接支持，因此FROM cdu_user_progress FULL JOIN...被解释为FROM cdu_user_progress as FULL。实际上，它用别名FULL替换表名。

Answer 2

MySQL缺乏对FULL OUTER JOIN的支持。

因此，如果您想在MySQL上模拟完全加入，请查看here。

通常建议的解决方法如下所示：

   SELECT 
        cdu_lessons.*, 
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 0 THEN score END ORDER BY date), ',', 1) AS game0_first,
        MAX(CASE WHEN game_id = 0 THEN score END) AS game0_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 1 THEN score END ORDER BY date), ',', 1) AS game1_first,
        MAX(CASE WHEN game_id = 1 THEN score END) AS game1_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 2 THEN score END ORDER BY date), ',', 1) AS game2_first,
        MAX(CASE WHEN game_id = 2 THEN score END) AS game2_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 3 THEN score END ORDER BY date), ',', 1) AS game3_first,
        MAX(CASE WHEN game_id = 3 THEN score END) AS game3_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 4 THEN score END ORDER BY date), ',', 1) AS game4_first,
        MAX(CASE WHEN game_id = 4 THEN score END) AS game4_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 5 THEN score END ORDER BY date), ',', 1) AS game5_first,
        MAX(CASE WHEN game_id = 5 THEN score END) AS game5_max     
    FROM cdu_user_progress 
    LEFT JOIN cdu_lessons ON cdu_lessons.id = cdu_user_progress.lesson_id 
    WHERE cdu_user_progress.uid = 145 
    GROUP BY lesson_id

Union ALL

    SELECT 
        cdu_lessons.*, 
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 0 THEN score END ORDER BY date), ',', 1) AS game0_first,
        MAX(CASE WHEN game_id = 0 THEN score END) AS game0_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 1 THEN score END ORDER BY date), ',', 1) AS game1_first,
        MAX(CASE WHEN game_id = 1 THEN score END) AS game1_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 2 THEN score END ORDER BY date), ',', 1) AS game2_first,
        MAX(CASE WHEN game_id = 2 THEN score END) AS game2_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 3 THEN score END ORDER BY date), ',', 1) AS game3_first,
        MAX(CASE WHEN game_id = 3 THEN score END) AS game3_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 4 THEN score END ORDER BY date), ',', 1) AS game4_first,
        MAX(CASE WHEN game_id = 4 THEN score END) AS game4_max,
        SUBSTRING_INDEX(GROUP_CONCAT(CASE WHEN game_id = 5 THEN score END ORDER BY date), ',', 1) AS game5_first,
        MAX(CASE WHEN game_id = 5 THEN score END) AS game5_max     
    FROM cdu_user_progress 
    RIGHT JOIN cdu_lessons ON cdu_lessons.id = cdu_user_progress.lesson_id 
    WHERE cdu_user_progress.uid = 145 
    GROUP BY lesson_id

简单的SQL连接查询（我认为）

2 个答案: