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All the knowledge that have faced
在 Oracle 中,USING 子句通常与 MERGE 语句一起使用,用于指定用于比较和匹配的源数据。USING 子句可以包含一个或多个数据源,但至少需要一个。在某些情况下,如果没有其他合适的数据源可用,可以使用 dual 表作为 USING 子句的数据源。
例如,以下是一个使用 MERGE 语句和 USING dual 的示例:
MERGE INTO target_table t
USING (SELECT 'X' FROM dual) src
ON (t.id = src.id)
WHEN MATCHED THEN
UPDATE SET t.value = 'UPDATED'
WHEN NOT MATCHED THEN
INSERT (id, value) VALUES (src.id, 'INSERTED');
在这个例子中,USING (SELECT 'X' FROM dual) 提供了一个虚拟的数据源,因为在这个 MERGE 语句中并没有实际的数据表来作为源。这种用法通常用于执行与 dual 表无关的操作,只是为了满足语法上的要求。
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