Mysql 添加SQL Case以在更新查询中提供默认值

我正在处理一个查询，该查询将根据用户的lat/long坐标将用户分配到特定的建筑。该查询确实有效，并为关联用户分配了一个建筑id值，但如果用户当前位置不存在建筑，我想分配一个“未知位置”的值

本质上，如果位置已知设置u.building_id=b.building_id，否则设置u.building_id=‘未知位置’

当前查询如下：

UPDATE USERS u 
JOIN BUILDINGS b ON u.latitude BETWEEN (b.latitude - 0.001) AND (b.latitude + 0.001)
    AND u.longitude BETWEEN (b.longitude - 0.001) AND (b.longitude + 0.001)
SET u.building_id = b.building_id

我认为这可以通过添加SQL用例来实现？我应该如何处理这个问题？

您可以尝试以下方法：

UPDATE USERS u JOIN BUILDINGS b ON u.latitude BETWEEN (b.latitude - 0.001) AND (b.latitude + 0.001)
AND
u.longitude BETWEEN (b.longitude - 0.001) AND (b.longitude + 0.001)
SET u.building_id = (case when **location known** then u.building_id = b.building_id ELSE u.building_id = 'unknown location' END)

---

显然，您需要更改逻辑表达式的已知位置

虽然就我个人而言，我会使用

COALESCE

做同样的事情：

UPDATE USERS u
LEFT JOIN BUILDINGS b 
     ON u.latitude BETWEEN (b.latitude - 0.001) AND (b.latitude + 0.001)
     AND u.longitude BETWEEN (b.longitude - 0.001) AND (b.longitude + 0.001)
SET u.building_id = COALESCE(b.building_id, 'unknown location');

将

内部连接

替换为

左连接

将确保更新所有用户，包括没有匹配建筑的用户（因此

空建筑id

）。注意，如果lat/long上有多个匹配项，则建筑物的分配将相当随意

---

进行了更改。对不起，哈

UPDATE USERS u
LEFT JOIN BUILDINGS b 
     ON u.latitude BETWEEN (b.latitude - 0.001) AND (b.latitude + 0.001)
     AND u.longitude BETWEEN (b.longitude - 0.001) AND (b.longitude + 0.001)
SET u.building_id = COALESCE(b.building_id, 'unknown location');

UPDATE USERS u LEFT JOIN BUILDINGS b ON u.latitude BETWEEN (b.latitude - 0.001) AND (b.latitude + 0.001)
AND
u.longitude BETWEEN (b.longitude - 0.001) AND (b.longitude + 0.001)
SET u.building_id = IF( ISNULL(b.building_id), 'unknown', b.building_id);