Question

我有以下表格结构：

Table1: BaseObject (ID, Name)

Table2: Instance (BaseObjectID, Type)

Table3: Mapping (Type, Value)

我想

Update the BaseObject.Name = Mapping.Value 
where exists an Instance for the 
BaseObject (Instance.BaseObjectID = BaseObject.ID)
 and Mapping.Type = Instance.Type

无法弄清楚如何在SQL中实现这一目标。

编辑：以下工作，但我不确定它是否是最佳的：

UPDATE BaseObject 
SET BaseObject.Name = (SELECT Mapping.Value 
                        FROM Mapping, Instance
                        WHERE Mapping.Type = Instance.Type
                        AND Instance.BaseObjectId = BaseObject.ID)
WHERE EXISTS (SELECT Instance.BaseObjectID 
              FROM Instance 
              WHERE Instance.BaseObjectID = BaseObject.ID)

Answer 1

UPDATE (SELECT t1.Name name, 
               t3.value value
          FROM BaseObject t1,
               Instance t2,
               Mapping t3
         WHERE t2.BaseObjectId = t1.ID
          AND  t3.Type = t2.Type)
   SET name = value

Answer 2

您可以执行以下操作。请注意，仅当每个Instance最多只有一个BaseObject时，它才有效。否则，您必须根据选择适当的名称来定义逻辑。

update BaseObject b
set name = (
  select m.value 
  from Instance i join Mapping m on (i.Type = m.Type)
  where BaseObjectID = b.ID 
)
where exists (
  select 1 
  from Instance i join Mapping m on (i.Type = m.Type)
  where BaseObjectID = b.ID 
)

查询表中选定行的SQL更新

2 个答案: