Question

这是我的疑问：

SELECT a.id, a.hostname, a.asset_tag, d.model, b.location,
        c.type, a.earmarked, a.earmarked_office, a.earmarked_user,
        a.earmarked_date, a.earmarked_by
        FROM laptops a
        JOIN locations b ON a.location = b.id
        JOIN types c ON a.type = c.id
        JOIN models d ON a.model = d.id
        WHERE b.stock = 1

现在，a.earmarked_office字段实际上是locations表中ID的引用，我们已经用它来填充b.location。如何在同一语句中检索earmarked_office的实际位置名称？

Answer 1

两次加入桌子：

SELECT lt.id, lt.hostname, lt.asset_tag, d.model, l.location,
        c.type, lt.earmarked, lt.earmarked_office, lt.earmarked_user,
        lt.earmarked_date, lt.earmarked_by,
        lteo.??
FROM laptops lt JOIN
     locations l
     ON lt.location = l.id JOIN
     types t
     ON lt.type = t.id JOIN
     models m
     ON lt.model = m.id JOIN
     locations leo
     ON lt.earmarked_office = leo.id
WHERE l.stock = 1;

注意：

使用表别名很好。但是，使用表别名可以更容易地遵循查询的逻辑。
我不知道你想从第二张表中找到哪些列。
使用as重命名列，这样他们就不会与来自l的列发生冲突。
如果有不匹配的值，则可能需要LEFT JOIN。

Answer 2

您可以多次加入同一张桌子：

SELECT a.id, a.hostname, a.asset_tag, d.model, b.location,
        c.type, a.earmarked, a.earmarked_office, a.earmarked_user,
        a.earmarked_date, a.earmarked_by
        FROM laptops a
        JOIN locations b ON a.location = b.id
        JOIN types c ON a.type = c.id
        JOIN models d ON a.model = d.id
        JOIN locations e ON a.earmarked_office = e.id
        WHERE b.stock = 1

MySQL在两个地方加入两个表

2 个答案: