from: 

String Aggregation Techniques

On occasion it is necessary to aggregate data from a number of rows into a single row, giving a list of data associated with a specific value. Using the SCOTT.EMP table as an example, we might want to retrieve a list of employees for each department. Below is a list of the base data and the type of output we would like to return from an aggregate query.

Base Data:    DEPTNO ENAME---------- ----------        20 SMITH        30 ALLEN        30 WARD        20 JONES        30 MARTIN        30 BLAKE        10 CLARK        20 SCOTT        10 KING        30 TURNER        20 ADAMS        30 JAMES        20 FORD        10 MILLERDesired Output:    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,FORD,ADAMS,SCOTT,JONES        30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD

This article is based on a thread from  and contains several methods to achieve the desired results.

LISTAGG Analystic Function in 11g Release 2

The LISTAGG analytic function was introduced in Oracle 11g Release 2, making it very easy to aggregate strings. The nice thing about this function is it also allows us to order the elements in the concatenated list. If you are using 11g Release 2 you should use this function for string aggregation.

COLUMN employees FORMAT A50SELECT deptno, LISTAGG(ename, ',') WITHIN GROUP (ORDER BY ename) AS employeesFROM   empGROUP BY deptno;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 ADAMS,FORD,JONES,SCOTT,SMITH        30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD3 rows selected.

WM_CONCAT Built-in Function (Not Supported)

If you are not running 11g Release 2, but are running a version of the database where the WM_CONCAT function is present, then it is a zero effort solution as it performs the aggregation for you. It is actually an example of a user defined aggregate function described below, but Oracle have done all the work for you.

COLUMN employees FORMAT A50SELECT deptno, wm_concat(ename) AS employeesFROM   empGROUP BY deptno;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,FORD,ADAMS,SCOTT,JONES        30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD3 rows selected.

Note. WM_CONCAT is an undocumented function and as such is not supported by Oracle for user applications (). If this concerns you, use a User-Defined Aggregate Function described below.

User-Defined Aggregate Function

The WM_CONCAT function described above is an example of a user-defined aggregate function that Oracle have already created for you. If you don't want to use WM_CONCAT, you can create your own user-defined aggregate function as described at .

CREATE OR REPLACE TYPE t_string_agg AS OBJECT(  g_string  VARCHAR2(32767),  STATIC FUNCTION ODCIAggregateInitialize(sctx  IN OUT  t_string_agg)    RETURN NUMBER,  MEMBER FUNCTION ODCIAggregateIterate(self   IN OUT  t_string_agg,                                       value  IN      VARCHAR2 )     RETURN NUMBER,  MEMBER FUNCTION ODCIAggregateTerminate(self         IN   t_string_agg,                                         returnValue  OUT  VARCHAR2,                                         flags        IN   NUMBER)    RETURN NUMBER,  MEMBER FUNCTION ODCIAggregateMerge(self  IN OUT  t_string_agg,                                     ctx2  IN      t_string_agg)    RETURN NUMBER);/SHOW ERRORSCREATE OR REPLACE TYPE BODY t_string_agg IS  STATIC FUNCTION ODCIAggregateInitialize(sctx  IN OUT  t_string_agg)    RETURN NUMBER IS  BEGIN    sctx := t_string_agg(NULL);    RETURN ODCIConst.Success;  END;  MEMBER FUNCTION ODCIAggregateIterate(self   IN OUT  t_string_agg,                                       value  IN      VARCHAR2 )    RETURN NUMBER IS  BEGIN    SELF.g_string := self.g_string || ',' || value;    RETURN ODCIConst.Success;  END;  MEMBER FUNCTION ODCIAggregateTerminate(self         IN   t_string_agg,                                         returnValue  OUT  VARCHAR2,                                         flags        IN   NUMBER)    RETURN NUMBER IS  BEGIN    returnValue := RTRIM(LTRIM(SELF.g_string, ','), ',');    RETURN ODCIConst.Success;  END;  MEMBER FUNCTION ODCIAggregateMerge(self  IN OUT  t_string_agg,                                     ctx2  IN      t_string_agg)    RETURN NUMBER IS  BEGIN    SELF.g_string := SELF.g_string || ',' || ctx2.g_string;    RETURN ODCIConst.Success;  END;END;/SHOW ERRORSCREATE OR REPLACE FUNCTION string_agg (p_input VARCHAR2)RETURN VARCHAR2PARALLEL_ENABLE AGGREGATE USING t_string_agg;/SHOW ERRORS

The aggregate function is implemented using a type and type body, and is used within a query.

COLUMN employees FORMAT A50SELECT deptno, string_agg(ename) AS employeesFROM   empGROUP BY deptno;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,FORD,ADAMS,SCOTT,JONES        30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD3 rows selected.

Specific Function

One approach is to write a specific function to solve the problems. The get_employees function listed below returns a list of employees for the specified department.

CREATE OR REPLACE FUNCTION get_employees (p_deptno  in  emp.deptno%TYPE)  RETURN VARCHAR2IS  l_text  VARCHAR2(32767) := NULL;BEGIN  FOR cur_rec IN (SELECT ename FROM emp WHERE deptno = p_deptno) LOOP    l_text := l_text || ',' || cur_rec.ename;  END LOOP;  RETURN LTRIM(l_text, ',');END;/SHOW ERRORS

The function can then be incorporated into a query as follows.

COLUMN employees FORMAT A50SELECT deptno,       get_employees(deptno) AS employeesFROM   empGROUP by deptno;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,JONES,SCOTT,ADAMS,FORD        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES3 rows selected.

To reduce the number of calls to the function, and thereby improve performance, we might want to filter the rows in advance.

COLUMN employees FORMAT A50SELECT e.deptno,       get_employees(e.deptno) AS employeesFROM   (SELECT DISTINCT deptno        FROM   emp) e;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,JONES,SCOTT,ADAMS,FORD        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES        3 rows selected.

Generic Function using Ref Cursor

An alternative approach is to write a function to concatenate values passed using a ref cursor. This is essentially the same as the previous example, except that the cursor is passed in making it generic, as shown below.

CREATE OR REPLACE FUNCTION concatenate_list (p_cursor IN  SYS_REFCURSOR)  RETURN  VARCHAR2IS  l_return  VARCHAR2(32767);   l_temp    VARCHAR2(32767);BEGIN  LOOP    FETCH p_cursor    INTO  l_temp;    EXIT WHEN p_cursor%NOTFOUND;    l_return := l_return || ',' || l_temp;  END LOOP;  RETURN LTRIM(l_return, ',');END;/SHOW ERRORS

The CURSOR function is used to allow a query to be passed to the function as a ref cursor, as shown below.

COLUMN employees FORMAT A50SELECT e1.deptno,       concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employeesFROM   emp e1GROUP BY e1.deptno;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,JONES,SCOTT,ADAMS,FORD        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES3 rows selected.

Once again, the total number of function calls can be reduced by filtering the distinct values, rather than calling the function for each row.

COLUMN employees FORMAT A50SELECT deptno,       concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employeesFROM   (SELECT DISTINCT deptno        FROM emp) e1;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,JONES,SCOTT,ADAMS,FORD        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES3 rows selected.

ROW_NUMBER() and SYS_CONNECT_BY_PATH functions in Oracle 9i

An example on  uses the ROW_NUMBER() and SYS_CONNECT_BY_PATH functions to achieve the same result without the use of PL/SQL or additional type definitions.

SELECT deptno,       LTRIM(MAX(SYS_CONNECT_BY_PATH(ename,','))       KEEP (DENSE_RANK LAST ORDER BY curr),',') AS employeesFROM   (SELECT deptno,               ename,               ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) AS curr,               ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) -1 AS prev        FROM   emp)GROUP BY deptnoCONNECT BY prev = PRIOR curr AND deptno = PRIOR deptnoSTART WITH curr = 1;    DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 ADAMS,FORD,JONES,SCOTT,SMITH        30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD3 rows selected.

COLLECT function in Oracle 10g

An example on  uses the COLLECT function in Oracle 10g to get the same result. This method requires a table type and a function to convert the contents of the table type to a string. I've altered his method slightly to bring it in line with this article.

CREATE OR REPLACE TYPE t_varchar2_tab AS TABLE OF VARCHAR2(4000);/CREATE OR REPLACE FUNCTION tab_to_string (p_varchar2_tab  IN  t_varchar2_tab,                                          p_delimiter     IN  VARCHAR2 DEFAULT ',') RETURN VARCHAR2 IS  l_string     VARCHAR2(32767);BEGIN  FOR i IN p_varchar2_tab.FIRST .. p_varchar2_tab.LAST LOOP    IF i != p_varchar2_tab.FIRST THEN      l_string := l_string || p_delimiter;    END IF;    l_string := l_string || p_varchar2_tab(i);  END LOOP;  RETURN l_string;END tab_to_string;/

The query below shows the COLLECT function in action.

COLUMN employees FORMAT A50SELECT deptno,       tab_to_string(CAST(COLLECT(ename) AS t_varchar2_tab)) AS employeesFROM   empGROUP BY deptno;           DEPTNO EMPLOYEES---------- --------------------------------------------------        10 CLARK,KING,MILLER        20 SMITH,JONES,SCOTT,ADAMS,FORD        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES        3 rows selected.

For more information see:

Hope this helps. Regards Tim...