如何将父级的值传递给子查询或子子查询

Question

请参考下面的示例查询::

    SELECT
    a.id,
    a.status,
    a.updated_at,
    b.id,
    b.updated_at,
    IF((a.status = 1 OR a.status = 2), 
   (IF(a.status = 1), 
         (SELECT COUNT(*) FROM coconut c WHERE c.datetime BETWEEN a.updated_at AND b.updated_at) , 
         (SELECT COUNT(*) FROM coconut c WHERE c.datetime BETWEEN a.updated_at AND NOW()) ), 0) AS 'ab'
    FROM apple a
    LEFT JOIN banana b

IF 条件中的子查询是给我错误消息的问题：“未知列 a.updated_at”和“未知列 b.updated_at”，当我试图从表apple和表banana中获取数据时。

有没有其他方法可以将表apple和表banana的值传递到子查询中？

Answer 1

您可以通过CASE语句（本质上是IF-THEN-ELSE 语句）运行计数，该语句比IF function 更灵活，因为它允许多个分支（从上一个问题中阅读更多关于它的信息）。 另外，不要忘记在LEFT JOIN之后包含您的JOIN条件。

SELECT
a.id,
a.status,
a.updated_at,
b.id,
b.updated_at,
CASE WHEN (a.status = 1) THEN
     (SELECT COUNT(*) FROM coconut c WHERE c.datetime BETWEEN a.updated_at AND b.updated_at)
     WHEN (a.status = 2) THEN 
     (SELECT COUNT(*) FROM coconut c WHERE c.datetime BETWEEN a.updated_at AND NOW())
     ELSE 0
END AS 'ab'
FROM apple a
LEFT JOIN banana b ON a.id = b.id