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.The LISTAGG analyticfunction 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 youshould use this function for string aggregation.
LISTAGG函數由11gr2引入，使得字符串聚合操作更加容易。並且允許我們制定排序規則。
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.
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.
如果我們使用的是11g r2之前版本，那麼可以選擇使用WM_CONCAT內置函數。
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 undocumented 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.
不過注意WM_CONCAT屬於非文檔型函數並且使用該函數的應用程序將不被ORACLE支持（出了事Oracle不負任何責任）。如果這一點困擾到你，那麼可以選擇下面的用戶自定義聚合函數。
The WM_CONCAT functiondescribed above 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.
此函數使用TYPE和TYPE BODY實現。
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.
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.
一條路是寫一個特定的函數。例如下面的get_employees方法：
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 improveperformance, we might want to filter the rows in advance.
為了減少函數調用次數，我們可以提前做好重復值過濾：
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.
An alternative approach is to write a function to concatenatevalues passed using a ref cursor. This is essentially the same as the previousexample, except that the cursor is passed in making it generic, as shown below.
使用游標變量的通用型函數。游標變量作為函數入參。這個方法類似上面方法，只不過是使用游標變量而已。
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. 這樣就允許查詢語句作為游標變量參數傳遞。
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 byfiltering the distinct values, rather than calling the function for each row.
同樣，為了減少函數調用次數，我們提前進行重復值過濾。
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.
An example on williamrobertson.net uses the ROW_NUMBER() and SYS_CONNECT_BY_PATH functionsto achieve the same result without the use of PL/SQL or additional typedefinitions.
另外一個例子使用ROW_NUMBER()函數和SYS_CONNECT_BY_PATH層次查詢函數實現。
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.
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.
最後這個例子使用Oracle10g引入的COLLECT函數。使用這個方法要求先定義一個表類型和一個用於將表類型轉換為字符串的函數。
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.
