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集合：两个结果集的字段个数和字段类型必须相同，才能使用集合操作。

--UNION 并集 重复行会去重
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'C','D' FROM DUAL)
UNION
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'E','F' FROM DUAL
);

--UNION ALL 全集 包含所有记录 不去重
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'C','D' FROM DUAL)
UNION ALL
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'E','F' FROM DUAL
);

--INTERSECT 交集 只展示两结果集中都存在的记录
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'C','D' FROM DUAL)
INTERSECT
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'E','F' FROM DUAL
);

--MINUS 差集 结果集A中有的结果集B中没有的记录
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'C','D' FROM DUAL)
MINUS
(SELECT 'A','B' FROM DUAL
UNION
SELECT 'E','F' FROM DUAL
);