Oracle string row column (string aggregation technology)

Oracle string row column (string aggregation technology)

1 String Aggregation Techniques String Aggregation technology On occasion it is necessary to aggregate data from a number ofrows 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 ofemployees for each department. below is a list of the base data and the type ofoutput we wowould like to return from an aggregate query.
Occasionally, we need to aggregate multiple data rows into one row to provide a list of specified data values. The following uses the EMP table as an example. We want to aggregate all employees under the table by department.

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

This article is based on a thread from asktom.oracle.com and contains several methods toachieve the desired results.
We will use the following methods:
Listmediaanalytic Function in 11g Release 2 WM_CONCAT Built-in Function (Not Supported) User-Defined Aggregate Function Specific Function Generic Function using Ref Cursor ROW_NUMBER () and SYS_CONNECT_BY_PATH functions in Oracle 9i COLLECT function in Oracle 10g1. 1 listparts AnalysticFunction in 11g Release 2

The listmediaanalyticfunction was introduced in Oracle 11g Release 2, making it very easy toaggregate strings. the nice thing about this function is it also allows us toorder the elements in the concatenated list. if you are using 11g Release 2 youshoshould use this function for string aggregation.

The listparts function is introduced by 11gr2, Making string aggregation easier. And allows us to set sorting rules.

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.

1.2 WM_CONCAT Built-inFunction (Not Supported)

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

If we use a version earlier than 11g r2, you can use the WM_CONCAT built-in function.

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.

Note. WM_CONCAT isan undefinented function and as such is not supported by Oracle for userapplications (MOS Note ID 1336219.1). If this concerns you, use a User-Defined Aggregate Function described below.

However, note that WM_CONCAT is a non-document function and applications using this function will not be supported by ORACLE (Oracle is not liable for any exceptions ). If this is bothering you, you can select the following user-defined aggregate function.

1.3 User-Defined AggregateFunction

The WM_CONCAT functiondescribed abve is an example of a user-defined aggregate function that Oraclehave already created for you. if you don't want to use WM_CONCAT, you can create your ownuser-defined aggregate function as described at asktom.oracle.com.

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 := 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 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.

This function is implemented using TYPE and type body.

COLUMN employees FORMAT A50

SELECT 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.

1.4 Specific Function

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

One path is to write a specific function. For example, the following get_employees method:

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 ininitialized 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 cballs to the function, and thereby improveperformance, we might want to filter the rows in advance.

To reduce the number of function calls, we can filter repeated values 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.

1.5 Generic Function usingRef Cursor

An alternative approach is to write a function to concatenatevalues passed using a ref cursor. This is essential to the same as the previusexample, couldn't that the cursor is passed in making it generic, as shown below.

General functions that use cursor variables. The cursor variable is used as the function input parameter. This method is similar to the above method, except that the cursor variable is used.

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 function is used to allow a query to be passed to thefunction as a ref cursor, as shown below. This allows The query statement to be passed as The CURSOR variable parameter.

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 cballs can be partitioned CED byfiltering the distinct values, rather than calling the function for each row.

Similarly, in order to reduce the number of function calls, we must filter repeated values in advance.

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.

1.6 ROW_NUMBER () andSYS_CONNECT_BY_PATH functions in Oracle 9i

An example on William amrobert tson.net uses the ROW_NUMBER () and SYS_CONNECT_BY_PATH functionsto achieve the same result without the use of PL/SQL or additional typedefinitions.

In another example, we use the ROW_NUMBER () function and the SYS_CONNECT_BY_PATH hierarchical query function.

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.

1.7 COLLECT function inOracle 10g

An example on oracle-developer.net uses the COLLECT functionin Oracle 10g to get the same result. this method requires a table type and afunction to convert the contents of the table type to a string. I 've alteredhis method slightly to bring it in line with this article.

Finally, this example uses the COLLECT function introduced by Oracle10g. To use this method, you must first define a table type and a function to convert the table type to a string.

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 functionin 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.

 

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

Dylan Present.