Monday, February 25, 2013

How to create XML with a TSQL query?

In this blog post I will describe how you can generate an XML file using TSQL statements. For instance for data migrations, you need to export data from your SQL database which can be imported via XML in another system.

The solution is really simple. Add ''FOR XML" to your SELECT query.

Example 1:
SELECT res_id, sur_name, first_name
FROM Humres
WHERE Res_id > 0
FOR XML PATH ('Resource')







The „FOR XML‟ always needs to be completed with the „AUTO‟ or „PATH‟ command:
  • When using „AUTO‟ command every column in the SELECT query will be handled as an attribute in a single element per records.
  • When including the „PATH(< path name >)‟ command the XML path can be set. Every records starts with its own parent element having the label as defined in the „PATH‟ command. Every column in the SELECT query will be handled as child element.
In this example the „PATH‟ command is used since this allows better control.
 
Example 2:
The next step would be to include custom column names to be used in the XML elements (rather than using „res_id‟, „sur_name‟, etc.) and include the resource number as an attribute in the „Resource‟ element.
 
SELECT res_id AS '@number',
 RTRIM(sur_name) AS 'LastName',
 RTRIM(first_name) AS 'FirstName'
FROM Humres
WHERE Res_id > 0
FOR XML PATH ('Resource')
 
 
 
 
 
 
 
 
 
 
Explanation:
  • Use a „@‟ in the column name results in an attribute
  • Including the RTRIM command trims the value (removing the extra spaces at the end of the value).
  • In many cases XML is case sensitive. Therefore make sure to use the correct attribute and element names.
Example 3:
A final step in creating a basic XML file would be to include the root element. A root element can be included by simply adding the command „ROOT(< root name >)‟ to the XML command in the SQL query.
 
SELECT res_id AS '@number',
 RTRIM(sur_name) AS 'LastName',
 RTRIM(first_name) AS 'FirstName'
FROM Humres
WHERE Res_id > 0
FOR XML PATH ('Resource'), ROOT('Resources')
 
 
Please note:
Sometimes a second root element is needed. For instance in Exact  (the element). Since the XML formatting of SQL queries only allows one root element, this can only be handled by using sub queries (or adding the root element manually to the output file). The use of sub queries will be explained in the following paragraph.
 
Example 4: Creating child-elements
There are two ways of generating child-elements as part of your parent element (which is defined in the „PATH‟ command and applies to every record).
Example 4.1 Child elements with 1:1 relationship
The first way can only be used in case there is a 1:1 relationship between the parent element (in our example the resource records) and the child element (in the example below the title record). In this case the child-element can be generated by including the element name in the column names (in the SELECT section):

SELECT
h.res_id as '@number',
RTRIM(h.sur_name) as 'LastName',
RTRIM(h.first_name) as 'FirstName',
RTRIM(p.predcode) as 'Title/@code',
p.aan_oms as 'Title/Description',
p.aanhef as 'Title/Salutation'
FROM humres h
LEFT JOIN pred p ON h.predcode = p.predcode
WHERE res_id > 0
FOR XML PATH('Resource'), ROOT('Resources')
 
 
Explanation:
  • A LEFT JOIN on table „pred‟ has been included to get the prefix data.
  • Correlation names „h‟ and „p‟ have been included to easily refer to the correct tables (in this case „humres‟ and „pred‟).
  • By including a forward slash (“/”) in the custom column names, a child element can be generated. The child element name needs to be defined on the left side of the forward slash.
  • Multiple forward slashed can be used in the column names to use deeper child element levels.
Example 4.2 Child elements with 1:N relationship
In case a 1:N relationship exist, such as one customer having multiple contacts, the child elements should be generated based on a sub query which gets all matching records. In our example, using the resources table, the resource is the parent element and the roles are the child elements. The sub query should get all roles linked to the resource and generate the corresponding child elements.
First create the sub query with a XML mark-up to get all roles:
SELECT
r.RoleID as '@code',
r.RoleLevel as '@level',
rd.Description as 'Description'
FROM humres h2
LEFT JOIN HRRoles r ON h2.res_id = r.EmpID
LEFT JOIN HRRoleDefs rd ON r.RoleID = rd.ID
FOR XML PATH('Role'), ROOT('Roles')
 
 

Explanation:
  • The query gets all resources (humres), linked to roles (hrroles) and the role details (hrroledefs).
  • The correlation name for humres is set to h2 since this query will become a sub query in which “h” already exists.
Next, this query needs to be part of the main query. This can be done by including it in the SELECT section of our main query and by making sure the sub query returns only the roles per specific resource.
 
SELECT
h.res_id as '@number',
RTRIM(h.sur_name) as 'LastName',
RTRIM(h.first_name) as 'FirstName',
RTRIM(p.predcode) as 'Title/@code',
p.aan_oms as 'Title/Description',
p.aanhef as 'Title/Salutation',
(
SELECT
r.RoleID as '@code',
r.RoleLevel as '@level',
rd.Description as 'Description'
FROM humres h2
LEFT JOIN HRRoles r ON h2.res_id = r.EmpID
LEFT JOIN HRRoleDefs rd ON r.RoleID = rd.ID
WHERE h2.res_id = h.res_id
FOR XML PATH('Role'), ROOT('Roles'), TYPE
)
FROM humres h
LEFT JOIN pred p ON h.predcode = p.predcode
WHERE res_id > 0
FOR XML PATH('Resource'), ROOT('Resources')
 
 

Explanation:
  • In the WHERE section of the sub query the filter „h2.res_id = h.res_id‟ has been added to make sure only the roles per user are taken.
  • In the FOR XML section the command „TYPE‟ has been added. If left out, the query result of the sub query will be alphanumeric (varchar) instead of XML. This means, with the „TYPE‟, the sub query result will be printed as a text/string.
  • Note that in this case it is “legal” to have multiple records and fields coming from a sub query. In standard (non-XML) SELECT queries having a sub query in the SELECT should only result in one record and field.
Example 5. XML file format
When running a SELECT query with XML commands, the XML output is in „Unicode (UTF-8)‟ coding by default:
 
 

When saving the output file, the file format will be using this encoding. Therefore, make sure to select the correct encoding type supported by the target application. (One of) the encoding type supported by Exact is „Western European (Windows)‟.
 
Enjoy it to create your own XML files via TSQL.

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