An Easy Way to Convert a Hierarchical Query Result to XML
Consider the following classic employees table:
SQL> SELECT employee_id,
2 manager_id,
3 first_name,
4 salary,
5 hire_date
6 FROM employees;
EMPLOYEE_ID MANAGER_ID FIRST_NAME SALARY HIRE_DATE
----------- ---------- -------------------- ---------- ---------
100 Steven 24000 17-JUN-87
101 100 Neena 17000 21-SEP-89
102 100 Lex 17000 13-JAN-93
103 102 Alexander 9000 03-JAN-90
104 103 Bruce 6000 21-MAY-91
105 103 David 4800 25-JUN-97
106 103 Valli 4800 05-FEB-98
107 103 Diana 4200 07-FEB-99
108 101 Nancy 12000 17-AUG-94
109 108 Daniel 9000 16-AUG-94
110 108 John 8200 28-SEP-97
...
The manager_id is also an employee_id. So, we can build the following hierarchical query:
SQL> SELECT
2 rpad(' ', 4 * LEVEL -1, ' ') || first_name tree,
3 LEVEL,
4 employee_id,
5 manager_id,
6 salary,
7 hire_date
8 FROM employees
9 START WITH manager_id IS NULL
10 CONNECT BY PRIOR employee_id = manager_id
11 ORDER siblings BY first_name;
TREE LEVEL EMPLOYEE_ID MANAGER_ID SALARY HIRE_DATE
-------------------- ---------- ----------- ---------- ---------- ---------
Steven 1 100 24000 17-JUN-87
Adam 2 121 100 8200 10-APR-97
Alexis 3 185 121 4100 20-FEB-97
Anthony 3 187 121 3000 07-FEB-99
James 3 131 121 2500 16-FEB-97
Julia 3 186 121 3400 24-JUN-98
Laura 3 129 121 3300 20-AUG-97
Mozhe 3 130 121 2800 30-OCT-97
Nandita 3 184 121 4200 27-JAN-96
TJ 3 132 121 2100 10-APR-99
Alberto 2 147 100 12000 10-MAR-97
...
Now what we want is to transform the above result set to XML. We could use DBMS_XMLGEN.getxml like this:
SQL> SELECT DBMS_XMLGEN.getxml('
2 SELECT
3 employee_id,
4 manager_id,
5 first_name,
6 salary,
7 hire_date
8 FROM employees
9 START WITH manager_id IS NULL
10 CONNECT BY PRIOR employee_id = manager_id
11 ORDER siblings BY first_name
12 ') xml
13 FROM dual;
XML
----------------------------------------------------
<?xml version="1.0"?>
<ROWSET>
<ROW>
<EMPLOYEE_ID>100</EMPLOYEE_ID>
<FIRST_NAME>Steven</FIRST_NAME>
<SALARY>24000</SALARY>
<HIRE_DATE>17-JUN-87</HIRE_DATE>
</ROW>
<ROW>
<EMPLOYEE_ID>121</EMPLOYEE_ID>
<MANAGER_ID>100</MANAGER_ID>
<FIRST_NAME>Adam</FIRST_NAME>
<SALARY>8200</SALARY>
<HIRE_DATE>10-APR-97</HIRE_DATE>
</ROW>
...
</ROWSET>
But this is not what we want. We want the hierarchical structure to be maintained in the XML output. In other words, we would like to have hierarchical XML with recursive elements.
Starting with Oracle Database 10g Release 1 (10.0.1), we can use DBMS_XMLGEN.newContextFromHierarchy like this:
SQL> SELECT XMLELEMENT
2 ("employees",
3 (SELECT DBMS_XMLGEN.getxmltype
4 (DBMS_XMLGEN.newcontextfromhierarchy
5 ('SELECT level,
6 XMLElement("employee",
7 XMLElement("id", employee_id),
8 XMLElement("name", first_name),
9 XMLElement("salary", salary),
10 XMLElement("hiredate", hire_date))
11 FROM employees
12 START WITH manager_id is null
13 CONNECT BY PRIOR employee_id = manager_id
14 ORDER SIBLINGS BY first_name'))
15 FROM DUAL)) xmldoc
16 FROM DUAL;
XMLDOC
-----------------------------------------------------------------------
<employees>
<employee>
<id>100</id>
<name>Steven</name>
<salary>24000</salary>
<hiredate>1987-06-17</hiredate>
<employee>
<id>121</id>
<name>Adam</name>
<salary>8200</salary>
<hiredate>1997-04-10</hiredate>
<employee>
<id>185</id>
<name>Alexis</name>
<salary>4100</salary>
<hiredate>1997-02-20</hiredate>
</employee>
</employee>
...
</employees>
And that’s what we want.
Here is what the documentation says about the DBMS_XMLGEN.newContextFromHierarchy function:
newContextFromHierarchy (queryString IN VARCHAR2) RETURN ctxHandle;
Parameter: queryString (IN) – the query string, the result of which must be converted to XML. The query is a hierarchical query typically formed using a CONNECT BY clause, and the result must have the same property as the result set generated by a CONNECT BY query. The result set must have only two columns, the level number and an XML value. The level number is used to determine the hierarchical position of the XML value within the result XML document.
Returns: Context handle. Call this function first to obtain a handle that you can use in the getXML() and other functions to get a hierarchical XML with recursive elements back from the result.
Since the function returns a context handle, it is a good practice to close the context to release all resources associated with that context, including the SQL cursor and bind and define buffers, and so on. This can be done using the procedure DBMS_XMLGEN.closeContext and requires the use of PL/SQL:
SQL> CREATE TABLE xml_documents OF XMLTYPE
2 /
Table created.
SQL> DECLARE
2 qryctx DBMS_XMLGEN.ctxhandle;
3 BEGIN
4 qryctx :=
5 DBMS_XMLGEN.newcontextfromhierarchy
6 ('SELECT level,
7 XMLElement("employee",
8 XMLElement("id", employee_id),
9 XMLElement("name", first_name),
10 XMLElement("salary", salary),
11 XMLElement("hiredate", hire_date))
12 FROM employees
13 START WITH manager_id is null
14 CONNECT BY PRIOR employee_id=manager_id
15 ORDER SIBLINGS BY first_name');
16
17 INSERT INTO xml_documents
18 SELECT XMLELEMENT ("emloyees", DBMS_XMLGEN.getxmltype (qryctx))
19 FROM DUAL;
20
21 DBMS_XMLGEN.closecontext (qryctx);
22 END;
23 /
PL/SQL procedure successfully completed.
SQL> SELECT sys_nc_rowinfo$
2 FROM xml_documents
3 /
SYS_NC_ROWINFO$
------------------------------------------------------------------------------
<employees>
<employee>
<id>100</id>
<name>Steven</name>
<salary>24000</salary>
<hiredate>1987-06-17</hiredate>
<employee>
<id>121</id>
<name>Adam</name>
<salary>8200</salary>
<hiredate>1997-04-10</hiredate>
<employee>
<id>185</id>
<name>Alexis</name>
<salary>4100</salary>
<hiredate>1997-02-20</hiredate>
</employee>
</employee>
...
</employees>
A couple of notes:
-
The
XMLTypedatatype was first introduced in Oracle Database 9i Release 1 (9.0.1) -
Oracle XML DB is a feature of the Oracle Database. It provides a high-performance, native XML storage and retrieval technology. You can verify that XML DB has been installed by simply running the SQL below:
SQL> SELECT comp_name
2 FROM dba_registry
3 WHERE comp_name LIKE '%XML%';
COMP_NAME
------------------------------------
Oracle XML Database
Sources and Resources:
Related articles:
- Beware of Comments in SQL
- Producing XML from SQL using cursor expressions
- Cool SQL Analytic Function: NTILE
- How to select an xmltree recursive?
from: http://awads.net/wp/2007/02/20/an-easy-way-to-convert-a-hierarchical-query-result-to-xml/