It’s a simple requirement. We want to transform this:
SQL> select deptno, ename
2 from emp
3 order by 1,2;
DEPTNO ENAME
---------- ----------
10 CLARK
10 KING
10 MILLER
20 ADAMS
20 FORD
20 JONES
20 SCOTT
20 SMITH
30 ALLEN
30 BLAKE
30 JAMES
30 MARTIN
30 TURNER
30 WARD
into this:
DEPTNO MEMBERS
---------- -------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
Dinosaurs like myself remember the terrible trouble we used to go to in order to solve this seemingly simple problem. We could use the MODEL clause,
SQL> select deptno , rtrim(ename,',') enames
2 from ( select deptno,ename,rn
3 from emp
4 model
5 partition by (deptno)
6 dimension by (
7 row_number() over
8 (partition by deptno order by ename) rn
9 )
10 measures (cast(ename as varchar2(40)) ename)
11 rules
12 ( ename[any]
13 order by rn desc = ename[cv()]||','||ename[cv()+1])
14 )
15 where rn = 1
16 order by deptno;
DEPTNO ENAMES
---------- ----------------------------------------
10 CLARK,KING,MILLER
20 ADAMS,FORD,JONES,SCOTT,SMITH
30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD
or we could use hierarchical trickery,
SQL> select deptno,
2 substr(max(sys_connect_by_path(ename, ',')), 2) members
3 from (select deptno, ename,
4 row_number ()
5 over (partition by deptno order by empno) rn
6 from emp)
7 start with rn = 1
8 connect by prior rn = rn - 1
9 and prior deptno = deptno
10 group by deptno
11 /
DEPTNO MEMBERS
---------- ---------------------------------------------------------
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
20 SMITH,JONES,SCOTT,ADAMS,FORD
10 CLARK,KING,MILLER
or we could build our own aggregation routine from the ground up,
SQL> create or replace type string_agg_type as object
2 (
3 total varchar2(4000),
4
5 static function
6 ODCIAggregateInitialize(sctx IN OUT string_agg_type )
7 return number,
8
9 member function
10 ODCIAggregateIterate(self IN OUT string_agg_type ,
11 value IN varchar2 )
12 return number,
13
14 member function
15 ODCIAggregateTerminate(self IN string_agg_type,
16 returnValue OUT varchar2,
17 flags IN number)
18 return number,
19
20 member function
21 ODCIAggregateMerge(self IN OUT string_agg_type,
22 ctx2 IN string_agg_type)
23 return number
24 );
25 /
Or we had some sort of personality disorder 
then we could resort to manipulating some XML via XMLDB.
SQL> select deptno,
2 xmltransform
3 ( sys_xmlagg
4 ( sys_xmlgen(ename)
5 ),
6 xmltype
7 (
8 '<?xml version="1.0"?><xsl:stylesheet version="1.0"
9 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
10 <xsl:template match="/">
11 <xsl:for-each select="/ROWSET/ENAME">
12 <xsl:value-of select="text()"/>,</xsl:for-each>
13 </xsl:template>
14 </xsl:stylesheet>'
15 )
16 ).getstringval() members
17 from emp
18 group by deptno;
DEPTNO MEMBERS
---------- --------------------------------------------------------
10 CLARK,MILLER,KING,
20 SMITH,FORD,ADAMS,SCOTT,JONES,
30 ALLEN,JAMES,TURNER,BLAKE,MARTIN,WARD,
Thankfully all of these were solved once we made it to 11g with the simple LISTAGG function
SQL> select deptno,
2 listagg( ename, ',')
3 within group (order by empno) members
4 from emp
5 group by deptno;
DEPTNO MEMBERS
---------- -----------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
I’ve always liked LISTAGG because the function reads like the business requirement we are trying to meet:
“AGG-regate a LIST of ENAMEs in sequence of EMPNO within the grouping of DEPTNO”
But there has been once problem with LISTAGG since it’s arrival in 11g, and that is what to do with duplicate data. Duplicates can either just look messy, for example, when I swap out ENAME for JOB:
SQL> select deptno
2 listagg(job,',') within group ( order by job) as jobs
3 from scott.emp
4 group by deptno
5 order by 1;
DEPTNO JOBS
--------- --------------------------------------------------
10 CLERK,MANAGER,PRESIDENT
20 ANALYST,ANALYST,CLERK,CLERK,MANAGER
30 CLERK,MANAGER,SALESMAN,SALESMAN,SALESMAN,SALESMAN
Or they could be even worse in the situation where the number of duplicates results in data exceeding allowing database limits. In the example below, there are hundreds of objects each with the same object type within a schema, and hence the aggregation blows the length limit for a varchar2.
SQL> select owner
2 listagg(object_type,',') within group
3 ( order by object_id ) as types
4 from all_ojects
5 group by owner
6 order by 1;
ERROR:
ORA-01499: result of string concatenation is too long
With 19c, our (technical) prayers have been answered with the long awaited arrival of the DISTINCT extension to the syntax.
SQL> select deptno,
2 listagg(distinct job,',') within group ( order by job ) as jobs
3 from scott.emp
4 group by deptno
5 order by 1;
DEPTNO JOBS
---------- ------------------------------------------------------------
10 CLERK,MANAGER,PRESIDENT
20 ANALYST,CLERK,MANAGER
30 CLERK,MANAGER,SALESMAN
3 rows selected.
SQL> select owner,
2 listagg(distinct object_type,',') within group ( order by object_type ) as types
3 from all_objects
4 group by owner
5 order by 1;
OWNER
------------------------------
TYPES
------------------------------------------------------------------------------------------------------------
------------
APPQOSSYS
SYNONYM,TABLE
AUDSYS
INDEX PARTITION,LIBRARY,PACKAGE,PACKAGE BODY,TABLE,TABLE PARTITION,VIEW
CTXSYS
FUNCTION,INDEX,INDEXTYPE,LIBRARY,OPERATOR,PACKAGE,PACKAGE BODY,PROCEDURE,SEQUENCE,TABLE,TYPE,TYPE BODY,VIEW
etc
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