oracle提供了聚合函数的API可以让我们方便的自己定义聚合函数。
详细看oracle官方文档：
http://docs.oracle.com/cd/B14117_01/appdev.101/b10800/dciaggref.htm
http://docs.oracle.com/cd/B14117_01/appdev.101/b10800/dciaggfns.htm#g1008306
wmsys.wm_concat(合并行函数)是一个很好的例子：
FUNCTION wm_concat(P1 VARCHAR2) RETURN CLOB AGGREGATE USING WMSYS.WM_CONCAT_IMPL ;
TYPE wm_concat_impl AUTHID CURRENT_USER AS OBJECT
(
CURR_STR VARCHAR2(32767),
CURR_STR_C CLOB,
STATIC FUNCTION ODCIAGGREGATEINITIALIZE(SCTX IN OUT WM_CONCAT_IMPL) RETURN NUMBER,
MEMBER FUNCTION ODCIAGGREGATEITERATE(SELF IN OUT WM_CONCAT_IMPL, P1 VARCHAR2) RETURN NUMBER,
MEMBER FUNCTION ODCIAGGREGATETERMINATE(SELF WM_CONCAT_IMPL, RETURNVALUE OUT CLOB, FLAGS NUMBER) RETURN NUMBER,
MEMBER FUNCTION ODCIAGGREGATEMERGE(SELF IN OUT WM_CONCAT_IMPL, SCTX2 WM_CONCAT_IMPL) RETURN NUMBER
);
TYPE BODY wm_concat_impl IS
STATIC FUNCTION ODCIAGGREGATEINITIALIZE(SCTX IN OUT WM_CONCAT_IMPL) RETURN NUMBER IS
BEGIN
SCTX := WM_CONCAT_IMPL(NULL,NULL) ;
RETURN ODCICONST.SUCCESS;
END;
MEMBER FUNCTION ODCIAGGREGATEITERATE(SELF IN OUT WM_CONCAT_IMPL, P1 VARCHAR2) RETURN NUMBER IS
BEGIN
IF (CURR_STR_C IS NULL AND (CURR_STR IS NULL OR LENGTH(CURR_STR)<29950)) THEN
IF(CURR_STR IS NOT NULL) THEN
CURR_STR := CURR_STR || ',' || P1;
ELSE
CURR_STR := P1;
END IF;
ELSE
IF (CURR_STR_C IS NULL) THEN
CURR_STR_C := CURR_STR ;
CURR_STR := NULL ;
END IF ;
CURR_STR_C := CURR_STR_C || ',' || P1;
END IF ;
RETURN ODCICONST.SUCCESS;
END;
MEMBER FUNCTION ODCIAGGREGATETERMINATE(SELF WM_CONCAT_IMPL, RETURNVALUE OUT CLOB, FLAGS NUMBER) RETURN NUMBER IS
BEGIN
IF (CURR_STR IS NOT NULL) THEN
RETURNVALUE := CURR_STR ;
ELSE
RETURNVALUE := CURR_STR_C ;
END IF ;
RETURN ODCICONST.SUCCESS;
END;
MEMBER FUNCTION ODCIAGGREGATEMERGE(SELF IN OUT WM_CONCAT_IMPL, SCTX2 WM_CONCAT_IMPL) RETURN NUMBER IS
BEGIN
IF(SCTX2.CURR_STR IS NOT NULL) THEN
SELF.CURR_STR := SELF.CURR_STR || ',' || SCTX2.CURR_STR ;
END IF;
RETURN ODCICONST.SUCCESS;
END;
END;
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