Getting Started with DB2 Express-C

276 Getting Started with DB2 Express-C
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Listing 15.7 - An example using XMLEXISTS
In Listing 15.7, the first line is an SQL clause specifying that you want to retrieve
information in the name column of the clients table.
The WHERE clause invokes the XMLEXISTS function, specifying the XPath expression
that prompts DB2 to navigate to the zip element and check for a value of 95116
The $c/Client/Address clause indicates the path inside the XML document hierarchy
where DB2 can locate the zip element. A dollar sign ($) is used to specify a variable;
therefore “c” is a variable. This variable is then defined by this line: passing
clients.contact as "c". Here, clients is the name of the table and contact is the
name of the column with an XML data type. In other words, we are passing the XML
document to the variable “c”.
DB2 inspects the XML data contained in the contact column, navigates from the root
Client node down to the Address node, then to the zip node and finally determines if
the customer lives in the target zip code. The XMLEXISTS function evaluates to “true” and
DB2 returns the name of the client associated with that row.
Starting with DB2 9.5, the above query could be simplified as shown in Listing 15.8 below.
SELECT name FROM clients
WHERE xmlexists(
'$CONTACT/Client/Address[zip="95116"]'
)
Listing 15.8 - Simplified version of the query shown in Listing 15.7
A variable with the same name as an XML column is created automatically by DB2. In the
above example, the variable CONTACT is created automatically by DB2. Its name matches
the name of the XML column CONTACT.
Example 2
Let’s consider how to solve the problem of how to create a report listing the e-mail
addresses of “Gold” status customers. The query in Listing 15.9 below could be run for this
purpose.
SELECT xmlquery('$c/Client/email' passing contact as "c")
FROM clients
WHERE status = 'Gold'
Listing 15.9 - An example using XMLQUERY
The first line indicates we want to return the email address which is an element of the XML
document (not a relational column). As in the previous example, “$c” is a variable that
contains the XML document. In this example we use the XMLQUERY function which can
be used after a SELECT, while the XMLEXISTS function can be used after a WHERE
clause.