标识在两个不同列中具有两个相同值的记录？

Question

我有一张学生证的表格，major1，major2和minor。 我想确定那些注册了同一专业两次的学生的记录。 我需要一个函数来仅选择那些具有相同专业的人，例如在Major1和Major2列中使用“ FIN”。 我到目前为止：

CREATE VIEW A5T5 AS
SELECT (firstname || ' ' || lastname)"FullName", StudentID AS "StudentID", Major1 as "DoubleDipping"
FROM A5
Group by Major1, Major2 ?????
Having count ?????
ORDER BY Major,LastName,FirstName;

Answer 1

为什么不这么简单：

从A5那里选择名字，姓氏Major1 = Major2

Answer 2

我认为您正在为此加倍努力。 如果我正确理解了您的问题，则以下查询将为您提供所需的信息：

SELECT (firstname || ' ' || lastname) AS "FullName",
       StudentID AS "StudentID",
       Major1 as "DoubleDipping"
  FROM A5
  WHERE MAJOR1 = MAJOR2

如果确实需要将此函数用作函数，则返回结果集的通常方法是返回打开的SYS_REFCURSOR ，然后调用者负责关闭它。 例如：

CREATE OR REPLACE FUNCTION DOUBLE_DIPPING_STUDENTS
    RETURN SYS_REFCURSOR
IS
  csr SYS_REFCURSOR;
BEGIN
  OPEN csr FOR SELECT (firstname || ' ' || lastname) AS "FullName",
                      StudentID AS "StudentID",
                      Major1 as "DoubleDipping"
                 FROM A5
                 WHERE MAJOR1 = MAJOR2;

  RETURN csr;
END DOUBLE_DIPPING_STUDENTS;

可以从PL/SQL块中调用上述函数，如下所示：

DECLARE
  csr       SYS_REFCURSOR;
  strNAME   VARCHAR2(2000);
  nID       A5.STUDENTID%TYPE;
  strMAJOR  A5.MAJOR1%TYPE;
BEGIN
  csr := DOUBLE_DIPPING_STUDENTS;

  LOOP
    FETCH csr
      INTO NAME, ID, MAJOR;

    WHEN csr%NOTFOUND THEN EXIT;

    DBMS_OUTPUT.PUT_LINE(strNAME || '  ' || nID || '  ' || strMAJOR);
  END LOOP;

  CLOSE csr;
END;