Question

我不确定如何编写符合这些条件的查询：

SELECT
    gems.*, metals.*

FROM
    `locations`

JOIN
    gems ON locations.locatable_id = gems.id AND locations.locatable_type = 'Gem'

JOIN
    metals ON locations.locatable_id = metals.id AND locations.locatable_type = 'Metal'

WHERE
    locations.latitude BETWEEN 0.0000 AND 1.0000 AND locations.longitude BETWEEN -1.0000 AND -2.0000

如果WHERE语句中的关联gems结果是locatable_type locations，则结果将选择一些Gem，如果metals，则选择SELECT locations属于Metal类型。

是否有可能产生这样的混合结果？

Answer 1

您需要JOIN locations表两次，因为locations.locatable_type不能同时是'Gem'和'Metal'，例如：

SELECT
    gems.*, metals.*

FROM
    `locations` l1

JOIN
    gems ON l1.locatable_id = gems.id AND l1.locatable_type = 'Gem'

JOIN
    users ON locations.locatable_id = metals.id AND locations.locatable_type = 'Metal'

WHERE
    l1.latitude BETWEEN 0.0000 AND 1.0000 AND l1.longitude BETWEEN -1.0000 AND -2.0000



UNION



SELECT
    gems.*, metals.*

FROM
    `locations` l=2

JOIN
    users ON l2.locatable_id = metals.id AND l2.locatable_type = 'Metal'

WHERE
    l2.latitude BETWEEN 0.0000 AND 1.0000 AND l2.longitude BETWEEN -1.0000 AND -2.0000

Answer 2

您只需要重新构建一下查询。以下应该有效...

  currency   price   new_price
  SGD        100     72.99
  USD        80      80
  EUR        75      65.22

具有多个连接的MySQL查询，基于连接类型进行选择

2 个答案: