Question

我有3张桌子。 items，shops和支点shop_item。

我想检索所有商店，但如果给定商品附加到商店，则需要额外字段attached。

以下内容有效，但并未指定要设置Attached参数的项目。

我在某个地方成像WHERE shop_item.item_id = 1，但我还没有设法让它发挥作用。

如何实现参数？

SELECT shops.*, shop_item.id AS attached FROM shops
LEFT OUTER JOIN shop_item
ON shops.id = shop_item.shop_id

Answer 1

您希望将此条件放在on子句中，而不是where子句中：

SELECT shops.*, shop_item.id AS attached
FROM shops LEFT OUTER JOIN
     shop_item
     ON shops.id = shop_item.shop_id and shopt_item.item_id = 1;

如果你把它放在where条款中，它将＆＃34;撤消＆＃34; LEFT OUTER JOIN，将其变为INNER JOIN。

Answer 2

如果您只想要附加项目，请使用INNER JOIN：

SELECT shops.*, shop_item.id AS attached
FROM shops
    INNER JOIN shop_item ON shops.id = shop_item.shop_id

如果您想要attached的布尔值，那么您需要评估该值，但将其保留为LEFT OUTER JOIN：

SELECT shops.*,
    CASE WHEN shop_item.id IS NULL THEN 0 ELSE 1 END CASE AS attached
FROM shops
    LEFT OUTER JOIN shop_item ON shops.id = shop_item.shop_id

获得所有商店，如果附加了特定商品，则附上“附件”

2 个答案: