如何在Oracle中自定义WMCONCAT函数

在Oracle中，您可以使用用户自定义聚合函数（User-Defined Aggregate Functions，UDAF）来实现类似于WM_CONCAT的功能

1. 创建一个聚合类型：

CREATE OR REPLACE TYPE my_wm_concat_type AS OBJECT (
  concatenated_string VARCHAR2(32767),

  STATIC FUNCTION ODCIAggregateInitialize(sctx IN OUT my_wm_concat_type) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateIterate(self IN OUT my_wm_concat_type, value IN VARCHAR2) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateTerminate(self IN my_wm_concat_type, returnValue OUT VARCHAR2, flags IN NUMBER) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateMerge(self IN OUT my_wm_concat_type, sctx2 IN my_wm_concat_type) RETURN NUMBER
);
/

2. 为聚合类型创建实现：

CREATE OR REPLACE TYPE BODY my_wm_concat_type IS
  STATIC FUNCTION ODCIAggregateInitialize(sctx IN OUT my_wm_concat_type) RETURN NUMBER IS
  BEGIN
    sctx := my_wm_concat_type(NULL);
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateIterate(self IN OUT my_wm_concat_type, value IN VARCHAR2) RETURN NUMBER IS
  BEGIN
    IF self.concatenated_string IS NULL THEN
      self.concatenated_string := value;
    ELSE
      self.concatenated_string := self.concatenated_string || ',' || value;
    END IF;
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateTerminate(self IN my_wm_concat_type, returnValue OUT VARCHAR2, flags IN NUMBER) RETURN NUMBER IS
  BEGIN
    returnValue := self.concatenated_string;
    RETURN ODCIConst.Success;
  END;

  MEMBER FUNCTION ODCIAggregateMerge(self IN OUT my_wm_concat_type, sctx2 IN my_wm_concat_type) RETURN NUMBER IS
  BEGIN
    IF sctx2.concatenated_string IS NOT NULL THEN
      IF self.concatenated_string IS NULL THEN
        self.concatenated_string := sctx2.concatenated_string;
      ELSE
        self.concatenated_string := self.concatenated_string || ',' || sctx2.concatenated_string;
      END IF;
    END IF;
    RETURN ODCIConst.Success;
  END;
END;
/

3. 创建用户自定义聚合函数：

CREATE OR REPLACE FUNCTION my_wm_concat(input VARCHAR2) RETURN VARCHAR2
PARALLEL_ENABLE AGGREGATE USING my_wm_concat_type;
/

现在您可以在SQL查询中像使用WM_CONCAT一样使用MY_WM_CONCAT函数：

SELECT deptno, MY_WM_CONCAT(ename)
FROM emp
GROUP BY deptno;

请注意，这个实现是简单的，并没有处理所有WM_CONCAT的特性，例如排序、分隔符等。您可以根据需要对聚合类型进行修改以实现更多功能。