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 MILLER
Desired Output:
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,FORD,ADAMS,SCOTT,JONES
30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD
LISTAGG Analytic 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 A50
SELECT deptno, LISTAGG(ename, ',') WITHIN GROUP (ORDER BY ename) AS employees
FROM emp
GROUP BY deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 ADAMS,FORD,JONES,SCOTT,SMITH
30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD
3 rows selected.
WM_CONCAT Built-in Function (Not Supported)
If you are not running 11g Release 2 or above, 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 A50
SELECT deptno, wm_concat(ename) AS employees
FROM emp
GROUP BY deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,FORD,ADAMS,SCOTT,JONES
30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD
3 rows selected.
WM_CONCAT is an undocumented function and as such is not supported by Oracle for user applications (
MOS Note ID 1336219.1). If this concerns you, use a User-Defined Aggregate Function described below.
Also,
WM_CONCAT has been removed from 12c onward, so you can't pick this option.
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
asktom.oracle.com. Thanks to
Kim Berg Hansen for some corrections in comments.
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 ERRORS
CREATE 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 := SUBSTR(SELF.g_string, 2);
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 ERRORS
CREATE OR REPLACE FUNCTION string_agg (p_input VARCHAR2)
RETURN VARCHAR2
PARALLEL_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 A50
SELECT /*+ PARALLEL(2) */ deptno, string_agg(ename) AS employees
FROM emp
GROUP BY deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,FORD,ADAMS,SCOTT,JONES
30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD
3 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 VARCHAR2
IS
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 A50
SELECT deptno,
get_employees(deptno) AS employees
FROM emp
GROUP by deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
3 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 A50
SELECT e.deptno,
get_employees(e.deptno) AS employees
FROM (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 VARCHAR2
IS
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 expression is used to allow a query to be passed to the function as a ref cursor, as shown below.
COLUMN employees FORMAT A50
SELECT e1.deptno,
concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employees
FROM emp e1
GROUP BY e1.deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
3 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 A50
SELECT deptno,
concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employees
FROM (SELECT DISTINCT deptno
FROM emp) e1;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
3 rows selected.
ROW_NUMBER() and SYS_CONNECT_BY_PATH functions in Oracle 9i
An example on
williamrobertson.net 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 employees
FROM (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 deptno
CONNECT BY prev = PRIOR curr AND deptno = PRIOR deptno
START WITH curr = 1;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 ADAMS,FORD,JONES,SCOTT,SMITH
30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD
3 rows selected.
COLLECT function in Oracle 10g
An example on
oracle-developer.net 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 A50
SELECT deptno,
tab_to_string(CAST(COLLECT(ename) AS t_varchar2_tab)) AS employees
FROM emp
GROUP BY deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,JONES,SCOTT,ADAMS,FORD
30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
3 rows selected.
