联合子查询中的MySQL外查询引用不起作用

Question

任何人都可以帮助我重建以下MySQL查询，以便它起作用：

SELECT c.id, COUNT(DISTINCT d.student_id) FROM contract c,  (
    SELECT student_id FROM ilc_course ic, contract_seat cs2
        WHERE c.id = cs2.contract
        AND cs2.id = ic.contract_seat
        AND ic.valid = 1 AND cs2.valid = 1
        AND (ic.finished IS NULL OR ic.finished > CURRENT_TIMESTAMP)
    UNION
    SELECT student_id FROM glc_to_user_profile_student gtups, glc_course gc, contract_seat cs2
        WHERE c.id = cs2.contract
        AND gtups.contract_seat = cs2.id AND gtups.glc_course = gc.id
        AND gtups.valid = 1 AND gc.valid = 1 AND cs2.valid = 1
        AND (gtups.left_date IS NULL OR gtups.left_date > CURRENT_TIMESTAMP)
        AND (gc.end_date IS NULL OR gc.end_date > CURRENT_TIMESTAMP)
) d GROUP BY c.id;

该查询应该统计来自两个来源的所有不同学生，并将他们按合同分组。

问题是对c.id的子查询引用。 显然，子查询对合同表一无所知。

我非常感谢您的帮助，以使其能够正常工作。

谢谢！

Answer 1

您需要在子查询中选择cs2.contract ，然后在外部查询中将其与c.id结合在一起。

SELECT c.id, COUNT(d.student_id) 
FROM contract c
JOIN (
    SELECT cs2.contract, student_id 
    FROM ilc_course ic
    JOIN contract_seat cs2 ON cs2.id = ic.contract_seat
    WHERE ic.valid = 1 AND cs2.valid = 1
        AND (ic.finished IS NULL OR ic.finished > CURRENT_TIMESTAMP)
    UNION
    SELECT cs2.contract, student_id 
    FROM glc_to_user_profile_student gtups
    JOIN glc_course gc ON gtups.glc_course = gc.id
    JOIN contract_seat cs2 ON gtups.contract_seat = cs2.id 
    WHERE gtups.valid = 1 AND gc.valid = 1 AND cs2.valid = 1
        AND (gtups.left_date IS NULL OR gtups.left_date > CURRENT_TIMESTAMP)
        AND (gc.end_date IS NULL OR gc.end_date > CURRENT_TIMESTAMP)
) d ON c.id = d.contract
GROUP BY c.id;

此外，由于UNION会从子查询中删除重复项，因此您无需在COUNT()使用DISTINCT 。