oracle wm_concat()函数的替代方案有哪些

在Oracle中，wm_concat()函数用于将多行数据合并为一个字符串

使用LISTAGG()函数：

SELECT LISTAGG(column_name, ',') WITHIN GROUP (ORDER BY column_order) AS concatenated_string
FROM table_name;

使用COLLECT()和CAST()函数：

SELECT CAST(COLLECT(column_name ORDER BY column_order) AS string_table_type) AS concatenated_string
FROM table_name;

这里，string_table_type是一个自定义的字符串表类型。首先需要创建一个类型：

CREATE TYPE string_table_type AS TABLE OF VARCHAR2(4000);

使用自定义聚合函数：

创建一个自定义聚合函数，例如my_concat()，然后使用它来合并多行数据：

SELECT my_concat(column_name) AS concatenated_string
FROM table_name;

创建自定义聚合函数的示例代码：

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

  STATIC FUNCTION ODCIAggregateInitialize(sctx IN OUT my_concat_impl) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateIterate(self IN OUT my_concat_impl, value IN VARCHAR2) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateTerminate(self IN my_concat_impl, returnValue OUT VARCHAR2, flags IN NUMBER) RETURN NUMBER,
  MEMBER FUNCTION ODCIAggregateMerge(self IN OUT my_concat_impl, ctx2 IN my_concat_impl) RETURN NUMBER
);
/

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

  MEMBER FUNCTION ODCIAggregateIterate(self IN OUT my_concat_impl, 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_concat_impl, 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_concat_impl, ctx2 IN my_concat_impl) RETURN NUMBER IS
  BEGIN
    IF self.concatenated_string IS NULL THEN
      self.concatenated_string := ctx2.concatenated_string;
    ELSIF ctx2.concatenated_string IS NOT NULL THEN
      self.concatenated_string := self.concatenated_string || ',' || ctx2.concatenated_string;
    END IF;
    RETURN ODCIConst.Success;
  END;
END;
/

CREATE FUNCTION my_concat(input VARCHAR2) RETURN VARCHAR2 PARALLEL_ENABLE AGGREGATE USING my_concat_impl;
/

以上三种替代方案都可以实现类似于wm_concat()函数的功能，将多行数据合并为一个字符串。根据实际情况和需求选择合适的方法。

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