Oracle实现字符串拼接和分离功能的方法(LISTAGG函数)，oraclelistagg

字符串拼接(String Aggregation Techniques)是数据处理时经常需要用到一个技术，比如需要按时间顺序拼装一个快递的运输记录，或者将流程中各个环节的处理人拼装为一个字符串。

Oracle中有多种方法来实现这个功能，这里罗列几种，详细用法可以参考下面的文章：

WM_CONCAT函数

LISTAGG函数

自定义聚合函数

本文介绍第二种：LISTAGG函数，这是Oracle11gR2开始正式推出的字符串聚合函数，功能非常强大。

0.测试样例及基本用法

从all_objects视图中取4个表记录和3个视图记录作为测试数据：

SQL> CREATE TABLE T_STRAGG AS

2 select OBJECT_TYPE,CREATED,OBJECT_NAME from ALL_OBJECTS WHERE OBJECT_TYPE='TABLE' AND rownum<5

3 UNION ALL

4 select OBJECT_TYPE,CREATED,OBJECT_NAME from ALL_OBJECTS WHERE OBJECT_TYPE='VIEW' AND rownum<4;

Table created

SQL> select OBJECT_TYPE,TO_CHAR(CREATED,'YYYY-MM-DD HH24:MI:SS') CREATED,OBJECT_NAME from T_STRAGG;

OBJECT_TYPE CREATED OBJECT_NAME

------------------- ------------------- ------------------------------

TABLE 2013-10-09 18:23:43 DUAL

TABLE 2013-10-09 18:23:44 SYSTEM_PRIVILEGE_MAP

TABLE 2013-10-09 18:23:45 TABLE_PRIVILEGE_MAP

TABLE 2013-10-09 18:23:47 STMT_AUDIT_OPTION_MAP

VIEW 2013-10-09 18:23:53 ALL_XML_SCHEMAS

VIEW 2013-10-09 18:23:56 ALL_XML_SCHEMAS2

VIEW 2013-10-09 18:23:54 V_$ADVISOR_CURRENT_SQLPLAN

现在想要根据OBJECT_TYPE分组，将OBJECT_NAME按时间顺序拼装成类似物流信息的字符串：

2013-10-09 18:23:43@DUAL

2013-10-09 18:23:44@SYSTEM_PRIVILEGE_MAP

2013-10-09 18:23:45@TABLE_PRIVILEGE_MAP

实现方法很简单：

SQL> select object_type

2 ,LISTAGG(TO_CHAR(CREATED,'YYYY-MM-DD HH24:MI:SS')||'@'||OBJECT_NAME,CHR(10)) WITHIN GROUP(ORDER BY CREATED) AS TXT

3 FROM T_STRAGG

4 GROUP BY object_type;

OBJECT_TYPE TXT

------------------- --------------------------------------------------------------------------------

TABLE 2013-10-09 18:23:43@DUAL

2013-10-09 18:23:44@SYSTEM_PRIVILEGE_MAP

2013-10-09 18:23:45@TABLE_PRIVILEGE_MAP

2013-10-09 18:23:47@STMT_AUDIT_OPTION_MAP

VIEW 2013-10-09 18:23:53@ALL_XML_SCHEMAS

2013-10-09 18:23:54@V_$ADVISOR_CURRENT_SQLPLAN

2013-10-09 18:23:56@ALL_XML_SCHEMAS2

跟WM_CONCAT相比，LISTAGG有如下特征(优缺点)：

1.返回值的分隔符可以自定义

这一点大大方便了开发人员，只要有需要可以摆出各种姿势^_^

2.返回值类型是VARCHAR2

跟WM_CONCAT相反，速度是快了，但如果数据量大就报错

ORA-01489: result of string concatenation is too long

(字符串连接的结果过长)

SQL> INSERT INTO T_STRAGG select object_type,CREATED,OBJECT_NAME FROM ALL_OBJECTS where rownum<1000;

999 rows inserted

SQL> select object_type

2 ,LISTAGG(TO_CHAR(CREATED,'YYYY-MM-DD HH24:MI:SS')||'@'||OBJECT_NAME,CHR(10)) WITHIN GROUP(ORDER BY CREATED) AS TXT

3 FROM T_STRAGG

4 GROUP BY object_type;

ORA-01489: result of string concatenation is too long

没办法，用回WM_CONCAT？

还是建议用第三个方法：自定义聚合函数

3.不支持DISTINCT

直接报错ORA-30482: DISTINCT option not allowed for this function

(DISTINCT 选项在此函数中禁用)

SQL> select object_type,LISTAGG(DISTINCT OBJECT_NAME) WITHIN GROUP(ORDER BY CREATED) AS TXT

2 FROM T_STRAGG

3 GROUP BY object_type;

ORA-30482: DISTINCT option not allowed for this function

解决方法：

1.先distinct，结果再listagg

2.自定义聚合函数来取代listagg函数

结论

listagg能满足的情况下，用listagg，不要用wm_concat listagg无法满足的情况下，还是网上拿一个自定义聚合函数好了

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