建一个Employee表，有两个列，一个是name，一个是所属于的部门(dept)
CREATE TABLE Employee(name VARCHAR(15),dept VARCHAR(15));
insert into Employee values('Jack','L3');
insert into Employee values('Lily','Quality');
insert into Employee values('Mark','ID');
insert into Employee values('Lee','L3');
insert into Employee values('Serge','Solutions');
insert into Employee values('John','Development');
insert into Employee values('Miso','Solutions');
insert into Employee values('Berni','Solutions');

select * from Employee;
 NAME  DEPT
 ----- -----------
 Jack  L3
 Lily  Quality
 Mark  ID
 Lee   L3
 Serge Solutions
 John  Development
 Miso  Solutions
 Berni Solutions

现在想写一个SQL，把一个部门的员工给做统计，每个部门一行
数据变成下面的样子
 DEPT        NAMES
 ----------- ----------------
 Development John
 ID          Mark
 L3          Jack,Lee
 Quality     Lily
 Solutions   Berni,Miso,Serge

实现的SQL
SELECT Dept
    ,SUBSTR(Names, 1, LENGTH(names) - 1)
FROM (
    SELECT Dept
        ,REPLACE(REPLACE(XMLSERIALIZE(CONTENT XMLAGG(XMLELEMENT(NAME a, NAME) ORDER BY NAME) AS VARCHAR(60)), ' ', ''), ' ', ',') AS Names
    FROM Employee
    GROUP BY Dept
    ) AS X; 解释几个DB2 XML方法的含义
XMLELEMENT是把标量转成XML的格式
select Dept,XMLELEMENT(NAME a, NAME) from Employee;
 DEPT        2
 ----------- ------------
 L3          Jack
 Quality     Lily
 ID          Mark
 L3          Lee
 Solutions   Serge
 Development John
 Solutions   Miso
 Solutions   Berni

XMLAGG把多个XML进行聚合，这里要给出分组的列(Dept)，并且每个组里，以NAME进行排序
select Dept,XMLAGG(XMLELEMENT(NAME a, NAME) ORDER BY NAME) from Employee GROUP BY Dept;
 DEPT        2
 ----------- -----------------------------------
 Development John
 ID          Mark
 L3          Jack Lee
 Quality     Lily
 Solutions   Berni Miso Serge

XMLSERIALIZE()的作用是把XML转换成为一个String类型
select Dept,XMLSERIALIZE(CONTENT XMLAGG(XMLELEMENT(NAME a, NAME) ORDER BY NAME) AS VARCHAR(60)) from Employee GROUP BY Dept;
 DEPT        2
 ----------- -----------------------------------
 Development John
 ID          Mark
 L3          Jack Lee
 Quality     Lily
 Solutions   Berni Miso Serge

到了这个地方就很简单了，把 干掉，把 转化成,即可

后来，出现了XMLGROUP，使用起来也比较方便 SELECT Dept
    ,XMLGROUP(',' || NAME AS a ORDER BY NAME)
FROM Employee
GROUP BY Dept

 DEPT        2
 ----------- ----------------------------------------------------------------------------------------
 Development ,John
 ID          ,Mark
 L3          ,Jack ,Lee
 Quality     ,Lily
 Solutions   ,Berni ,Miso ,Serge

SELECT Dept
    ,XMLCAST(XMLGROUP(',' || NAME AS a ORDER BY NAME) AS VARCHAR(60))
FROM Employee
GROUP BY Dept

 DEPT        2
 ----------- -----------------
 Development ,John
 ID          ,Mark
 L3          ,Jack,Lee
 Quality     ,Lily
 Solutions   ,Berni,Miso,Serge

SELECT Dept
    ,SUBSTR(XMLCAST(XMLGROUP(',' || NAME AS a ORDER BY NAME) AS VARCHAR(60)), 2) AS Names
FROM Employee
GROUP BY Dept

 DEPT        NAMES
 ----------- ----------------
 Development John
 ID          Mark
 L3          Jack,Lee
 Quality     Lily
 Solutions   Berni,Miso,Serge

到了DB2 9.7.4之后，这个问题得到了完美的解决
SELECT Dept,
LISTAGG(name, ',')
WITHIN GROUP (ORDER BY name)
FROM Employee
GROUP BY Dept;

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