Native XML Support in DB2 Databases [DB2 9 for z/OS]

One of the biggest technological advances in DB2 V9 is the ability to combine the management of structured and unstructured data. Basically, V9 will allow you to store data as native XML. This capability has already been introduced into V9 of DB2 for Linux, Unix, and Windows. Many of you may well ask “Hey, what’s the big deal here? Can’t we already use the XML Extender and store XML data in DB2 prior to V9?” Yes, but V9 changes the game. You will be able to search and analyze structured data in a relational data repository and unstructured data in an XML repository without the need to reformat it. So your regular “relational” data gets stored as always; and XML data gets stored in its native format without the need to shove it into a CLOB or shred it into “relational” columns. The approach is novel in that DB2 will now support native XML via dual storage engines – the traditional SQL/relational engine and a new XML engine. DB2 9 for z/OS handles XML as a new data type that is stored in a natural hierarchy - different from relational data. For those of you not familiar with XML, you need to know that there are big differences between XML data and typical DB2 data. Foremost among these differences is that XML data is hierarchical, whereas “relational” DB2 data is basically “flat.” Additionally, XML data is self-describing. XML tags identify and name the data elements in the XML document. This capability concentrates both the data and its structure into a single document. So, in essence, the XML document becomes self-describing. This is important to keep in mind because a single XML document can have many different types of data, whereas “relational” DB2 data is defined in the system catalog by its column definition. And all data in the same column must have the same data type (e.g. you cannot store a name in an integer column). Finally, XML data is ordered, whereas “relational” DB2 data is not. The order in which data items are specified in the XML document is relevant. There is often no other way to specify order within an XML document. For relational data, the order of the rows is not guaranteed unless you specify an ORDER BY clause on one or more columns. OK, now, just how would you support XML data in DB2 V9 then? Think of XML as just another data type. You would use the XML data type in a CREATE TABLE statement to define a column to be of type XML. Each column of type XML can hold one XML document for every row of the table. Even though the XML documents are logically associated with a row, XML and “relational” columns are stored differently. The “relational” columns are stored in the traditional structures we all know and love. The XML data is stored in hierarchical structures. Don’t let that scare you. IBM has seamlessly integrated XML with relational data to simplify application development while optimizing search performance with highly optimized XML indexes. Here is a quick example walkthrough from the DB2 XML Guide manual that creates a simple table with an XML column. First, as with triggers, when you create tables with XML in SPUFI be sure to set the SQL terminator to a character other than a semicolon, for example, the pound sign (#). This is done so that your SQL can have embedded semicolons. Also, you’ll probably want to set CAPS OFF in SPUFI to preserve lower case. Then, create a table like this: CREATE TABLE MYCUSTOMER (CID BIGINT, INFO XML) # This creates a two columns table, the first column as a big integer and the second for the XML data. Next, we’ll build an index over XML data. We will assume that the XML documents to be stored in the INFO column will have a root element named customerinfo with an attribute named Cid. So, here is the DDL for the unique index on the Cid attribute:

CREATE UNIQUE INDEX MYCUT_CID_XMLIDX ON MYCUSTOMER(INFO)
GENERATE KEY USING XMLPATTERN
‘declare default element namespace
   "http://posample.org"; /customerinfo/@Cid’
AS SQL DECFLOAT
#

The XML pattern defining the index is case-sensitive. The element and attribute names in the XML pattern must match the element and attribute names in the XML documents exactly. Now we can insert a couple of XML documents into the INFO column, such as:

INSERT INTO MYCUSTOMER (CID, INFO) VALUES (1000,
’<customerinfo xmlns="http://posample.org" cid="1000">
  <name>Kathy Smith</name>
  <addr country="Canada">
    <street<5 Rosewood</street>
    <city>Toronto</city>
    <prov-state<Ontario</PROV-STATE>
    <pcode-zip<M6W 1E6</PCODE-ZIP>
  </addr>
  <phone type="work">416-555-1358</phone>
 </customerinfo>’)
#

INSERT INTO MYCUSTOMER (CID, INFO) VALUES (1002,
’<customerinfo xmlns="http://posample.org" cid="1002">
  <name>Jim Noodle</name>
  <addr country="Canada">
    <street>25 EastCreek</street>
    <city>Markham</city>
    <prov-state>Ontario</PROV-STATE>
    <pcode-zip>N9C 3T6</PCODE-ZIP>
  </addr>
  <phone type="work">905-555-7258</phone>
  <phone type="cell">905-555-7254</phone>
 </customerinfo>’)
#

Then you can issue a SELECT statement against this table and thereby verify that the XML documents were successfully inserted. For example: SELECT CID, INFO FROM MYCUSTOMER # V9 also supports XPath to query elements within an XML document, as well as catalog extensions to support definitions of XML schemas. Furthermore, the IBM DB2 utilities have been extended such that they can be used to administer XML data, too. To my mind, though, one of the problems with XML in DB2 9 for z/OS is the lack of support for XQuery. XQuery is an XML query language capable of traversing XML documents. Just like SQL is the query language for native DB2 data, XQuery is the query language for native XML data. DB2 9 for Linux, Unix, and Windows supports XQuery, but DB2 9 for z/OS does not. For an independent tutorial on XQuery, click on this link or for an IBM tutorial on using XQuery in DB2 LUW click on this link instead. So, how do you retrieve XML data using DB2 9 for z/OS? You can use SQL to retrieve entire XML documents from XML columns just like you would retrieve any other column. But if you need to retrieve portions of that XML document you will need to specify XPath expressions, through SQL with XML extensions. For an independent tutorial on XPath, click on this link . Here is an example of using XPath to identify data within our XML data: DELETE FROM MYCUSTOMER WHERE XMLEXISTS ( ’declare default element namespace "http://posample.org"; /customerinfo/phone[@type="cell"]’ PASSING INFO) # This should DELETE any XML document that has cell phone information, and for the purposes of this example, that would be CID 1002. I do not wish to go into any detailed description of XPath in this blog, but you can use XML functions with XPath expressions to traverse the XML document for data. One final note: some of the IBM documentation could be clearer. For example, I take exception with this paragraph lifted directly out of the “What’s New” manual (GC18-9856-00): “Support for XML capabilities and functions span the entire DB2 family. Version 8 of DB2 for z/OS and Version 8 of DB2 for Linux, Unix, and Windows provide basic support for storing, retrieving, and querying XML documents. DB2 9 for Linux, UNIX and Windows continues the work by delivering rich support of XML, including an XQuery interface to the data. Now, DB2 V9.1 for z/OS expands on similar support by delivering seamless integration of XML data and relational data in the DB2 database.” Anyone reading that paragraph would be completely justified in expecting DB2 V9 for z/OS to include XQuery support. It seems to have been written using intentionally misleading wording in order to avoid admitting that XQuery is not supported on z/OS. At least, that is what it seems like to me, I could be wrong. I’m also interested in how many folks out there in DB2-mainframe-land expect to use the XML capabilities of DB2 for z/OS? Please sign in and leave a comment expressing whether or not you plan to use DB2’s XML support. Thanks, and that is all for today. Keep an eye out for future DB2 9 for z/OS related posts as I plan to continue adding to this series on new V9 features over the course of the next month or so (at least). Cheers!

ORDER BY and FETCH FIRST in Subselects [DB2 9 for z/OS]

Here is another quick post in my series on new features in DB2 9 for z/OS.

Today, we will look at the additional flexibility gained in how the ORDER BY and FETCH FIRST n ROWS ONLY clauses can be specified in V9. Prior to the V9, the only place you could specify these clauses was at the statement level. Indeed, this has been a source of confusion for many DB2 SQL programmers.

A discussion of DB2 SELECT should be broken down into three topics:

1. fullselect,
2. subselect, and
3. select-statement.

The select-statement is the form of a query that can be directly specified in a DECLARE CURSOR statement, or prepared and then referenced in a DECLARE CURSOR statement. It is the thing most people think of when they think of SELECT in all its glory. If so desired, it can be issued interactively using SPUFI. The select-statement consists of a fullselect, and any of the following optional clauses: order-by, fetch-first, update, read-only, optimize-for, isolation and queryno. Well, that is, until V9 which still allows the fetch-first and order-by at this level, but also at the fullselect and subselect level!

A fullselect can be part of a select-statement, a CREATE VIEW statement, or an INSERT statement. Basically, a fullselect specifies a result table. Prior to V9, this sometimes confused folks as they tried to put a FETCH FIRST n ROWS clause or an ORDER BY in a view or as part of an INSERT. That was not allowed!

Finally, a subselect is a component of the fullselect. A subselect specifies a result table derived from the result of its first FROM clause. The derivation can be described as a sequence of operations in which the result of each operation is input for the next.

This is all a bit confusing. Think of it this way: in a subselect you specify the FROM to get the tables, the WHERE to get the conditions, GROUP BY to get aggregation, HAVING to get the conditions on the aggregated data, and the SELECT clause to get the actual columns. In a fullselect you add in the UNION to combine subselects and other fullselects. Finally, you add on any optional order-by, fetch-first, update, read-only, optimize-for, isolation and queryno clauses to get the select-statement.

But, of course, as of V9 you can use the order-by and/or the fetch-first at the subselect or fullselect level. This can be useful if you want to limit the results within a subquery or part of a UNION statement. Also, if you specify ORDER BY with FETCH FIRST n ROWS ONLY, the result is ordered before the fetch-first is applied. (That's a good thing.)

So, that means all of the following are now legal SQL formulations in V9:

(SELECT COL1 FROM T1
UNION
SELECT COL1 FROM T2
ORDER BY 1)
UNION
SELECT COL1 FROM T3
ORDER BY 1;

This example shows how ORDER BY can be applied in fullselects with UNION.

SELECT EMP_ACT.EMPNO, PROJNO
FROM EMP_ACT
WHERE EMP_ACT.EMPNO IN (SELECT EMPLOYEE.EMPNO
FROM EMP
ORDER BY SALARY DESC
FETCH FIRST 10 ROWS ONLY);

And this example will return the employee number and project number for any projects assigned to employees with one of the top ten salaries.

So, once you move to V9 you will have much more lattitude in how you write your SELECTs. If you are interested in more details, here is a link to the section of the DB2 9 for z/OS SQL Reference manual on building SQL queries.

INTERSECT and EXCEPT [DB2 9 for z/OS]

With this blog entry I am introducing a new series in which I will briefly blog about the new feature of DB2 9 for z/OS. Today's entry will cover the new INTERSECT and EXCEPT keywords.

DB2 for Linux, Unix, and Windows has supported INTERSECT and EXCEPT in SQL SELECT statements for quite some time now, and with V9 the z/OS platform catches up. These two set operations can be used to simplify some SQL statements. Think of them as being similar to the UNION operation.

INTERSECT is used to match result sets between two tables. If the data is the same in both results sets it passes through. When INTERSECT ALL is specified, the result consists of all rows that are in both result sets. If INTERSECT is specified without the ALL option, the duplicates will be removed from the results. For example, the following SQL will show all customers in the USA who are also employees (with no duplicates):

SELECT last_name, first_name, cust_num
FROM CUST
WHERE country = 'USA'
INTERSECT
SELECT last_name, first_name, emp_num
FROM EMP
WHERE country = 'USA';

EXCEPT, on the other hand, combines non-matching rows from two result tables. Some other DBMS implementations refer to this as the MINUS operation. When EXCEPT ALL is specified, the result consists of all rows from the first result table that do not have a corresponding row in the second and any duplicate rows are kept. If EXCEPT is specified without the ALL option, duplicates are eliminated. As an example, the following SQL will return only those items from TABLE1 that are not also in TABLE2:

SELECT item FROM TABLE1
EXCEPT
SELECT item FROM TABLE2;

Both INTERSECT and EXCEPT make it easier to formulate SQL requests...

Mimicking ROWNUM in DB2

I was going through the many random files stored on my PC when I came across this useful nugget. A year or so ago a reader sent me this code. He and his supervisor had come up with a way to simulate the Oracle rownum feature in DB2 and wanted to share it with the world. I think, at the time, I posted it up on TechTarget as I was doing Q+A there on the old search390.com site. But that site is now defunct, so I'm reposting the solution here on my DB2portal blog for all to share.

SELECT DEPTNO, ROW#
FROM DEDBADM.DEPT TB1,
     TABLE (SELECT COUNT(*) + 1 AS ROW#
            FROM DEDBADM.DEPT TB2
            WHERE TB2.DEPTNO < TB1.DEPTNO) AS TDEPT_TAB
WHERE ROW# = 2;

If you decide to use this code, be careful because there is no guarantee that the row numbers will be the same from run to run of the SQL. As long as you get the same access path the row numbers should be the same from run to run, but if the access path changes -- for example, from a table space scan to indexed access or vice versa -- then the row numbers for the runs will differ. That is, each row may have a different number than it had for a previous execution.

All in all, though, a nice solution to the problem!

IBM Annual Report: Good News for DB2 DBAs

I sometimes enjoy reading through the annual reports of companies like IBM (that is, companies whose products I use "on the job"), so I thought maybe readers of my blog might enjoy that, too. With that in mind, IBM's annual report for 2006 is now up and available online.

What nuggets of information did I find in it? Well, I have to admit, I have not scoured it from front to back (and I probably won't), but I did try to track down the status of DB2 within the company. DB2 falls within IBM's Information Management brand, in their Middleware software segment. The annual report tells me that the Middleware software segment is doing better than the overall software category within IBM:

• The Middleware segment grew to $13.891 billion in 2006 from $12.552 billion in 2005. And that represents year over year growth of 10.7%.
• IBM's entire software portfolio posted revenue of $18.161 in 2006 versus $16.83 billion in 2005, representing a year over year growth of 7.9%.

OK, but what about DB2? The best I can do is all of Information Management, which include DB2, as well as IMS and Informix, and all of the data management tools that IBM sells. These products posted a 14% gain in 2006 over 2005. Nice...

In terms of hardware, though, IBM saw a slide in that market segment. The company posted revenue of $21.97 billion in 2006 down from the $23.857 billion posted in 2005. This is a 7.9% drop in annual revenue for hardware. But, you may well ask, we care about the mainframe more than those other platforms, how did it do?

It did well, my friends. Of the four major computer brands sold by IBM, the z series boxes (that is, the mainframes) did the best. System z was up 7.8% over 2005. Its closest competitor was System x (UNIX servers) which was up 3.7%. Bad news for the other two computing platforms: System i (that is, the AS/400s) was down 15% and System p (Windows servers) was down 1.1%.

So, if you are my kinda people - that is, DBAs working on DB2 for z/OS - then IBM's annual report contains good news all around. The DBMS business is healthy and so is the mainframe business.