SQL Transformer

The purpose of the SQLTransformer is to query a database and translate the result to XML. To retrieve the information from the database, you are not restricted to use simple SQL statements (eg select, insert, update), it is also possible to use stored procedures. In combination with other transformers (eg FilterTransformer), this one can be very powerful.

To be able to query a database, we need XML that describes exactly what we want to do. The general structure of this input XML is as follows:

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Nothing prevents you from putting other XML aroung the page element. If you do, it will stay untouched. The format of the SQL statement or the stored procedure is exactly the same as if you would call it directly from java with a prepared statement or a callable statement.

The query element has the following optional attributes:

  1. name: Naming a query implicates naming the corresponding rowset (see below). When you have a sequence of queries you want to execute, it can be handy give them a name. To process the retrieved data of a certain query, you can use another transformer to check the name of the rowset and to execute the necessary business logic on it.
    usage: <query name="myName">
  2. isstoredprocedure: When you want to use stored procedures, you have to explicitely add this attribute to the query element. By default, the transformer assumes that you want to execute a SQL statement.
    usage: <query isstoredprocedure="true">

Here is an example of how the input XML might look like:

Hello This is my first Cocoon2 page filled with sql data! select id,name from department_table ]]>

You can use the file generator to retrieve the XML from the filesystem. To invoke the SQLTransformer you have to add following to the sitemap:

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The "use-connection" parameter defines which connection, defined under the datasources element in cocoon.xconf, the SQLTransformer has to use to retrieve the data.

The 'show-nr-of-rows' instructs the transformer to count the number of rows in the resultset explicitely and to set the result as attribute to the rowset element. This attribute is only usefull in combination with an sql statement, not with stored procedures. If a stored procedure returns a resultset and you want to know how many rows it contains, you have to count the number of rows in another transformer or your stored procedure has to rerutn it also (last solution is the best one)

The output XML will look as follows:

Hello This is my first Cocoon2 page filled with sql data! 1 Programmers 2 Loungers ]]>

If you use this in combination with the "simple-sql2html" XSL stylesheet,

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you will get a more visually attractive page.

See below for a more in depth example with stored procedures.

By now you should be able to use the SQLTransformer, but there are some more options you might find usefull...

Sometimes you need more information before you can execute a query, eg. the name of the user that is currently logged on your site. This information is only available at runtime and hence can only be substituted in the query when available.

To pass this information to the SQL statement, the input XML has to look like this:

select id,name from employee_table where name = '' ]]>

The substitution is done by the SQLTransformer before it executes the query (before it calls the method prepareStatement!). For this, the transformer has to be given the necessary values via the sitemap (as parameter):

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Whenever the transformer encounters a 'substitute-value' element for which the attribute 'name' contains the value 'username', it will replace this element with the value 'Stefano Mazzocchi' (without the single quotes!).

The output XML will be as follow:

2 Stefano Mazzocchi ]]>

It is also possible to use substitution in combination with stored procedures.

This functionallity is best described by a simple example.

Take following input XML:

select id,name from department_table select id,name from employee_table where department_id = ]]>

The first query will retrieve all id's and name's from the department_table table. For each id that comes from the department_table, the second query, in which the 'ancestor-value' element will be replaced by the id, will be executed. The above example will be transformed to the following XML:

1 Programmers 1 Donald Ball 2 Stefano Mazzocchi 2 Loungers 3 Pierpaolo Fumagalli ]]>

Stored procedures can return data as a parameter. To make use of this functionallity in java, you have to register these parameters as 'out parameters'. Since this information is application specific, the SQLTransformer uses reflection to retrieve the data in the right format. For this, an extra element is needed in the input XML:

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where:

  1. nr: The targeted parameter number that will return data of a certain type.
  2. type: The type of data that will be returned (defined in java.sql.Types or in database specific drivers, eg oracle.jdbc.driver.OracleTypes). Once the stored procedure returns data in the parameters, the stored procedure tries to process them. If the returned parameter is an instance of ResultSet, it will be translated to XML as we saw before. In all the other situations, the SQLTransformer will convert the parameter to a string.

This is an example of how to call an oracle stored procedure and process it with the SQLTransformer:

begin QUICK_SEARCH.FIND_NAME('',?,?,?); end; ]]>

The SQLTransformer will create 3 elements, respectively 'code', 'nrofrows' and 'resultset' under the element 'namesearch'. Since the type oracle.jdbc.driver.OracleTypes.CURSOR' corresponds to a ResultSet, a 'rowset' element will be created, containing all the data of the resultset. It is also possible to use an 'in-parameter' element, eg. <in-parameter sql:nr="1" sql:value="1"/>. This functionallity is only provided to be complete, because it is available in java itself. You can also use the 'in-parameter' in combination with a SQL statement. Used in combination with an out-parameter, a ?-parameter can be an in-parameter and an out-parameter at the same time.

When you query a database and it returns too many rows too process at once, you might want to take a block of elements, process this block and ignore the rest for now. You can best compare it to a search on Google: they only return 10 results in one time, for more results you have to click on another block (page). It wouldn't be wise to process more than 10 elements in the pipeline if you only need to display 10 elements.

Assume that a query returns 56 row elements (by using the SQLTransformer) and that you only want to display the first 10 elements:

Output XML from the SQLTransformer:

... ]]>

By adding following lines to the sitemap, just under the SQLTransformer, you restrict the results to 10 elements in the first block:

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output XML:

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To make it more dynamically, put something like {reqCount} and {reqBlock} in the values for count and blocknr respectively. These can be parameters from the request and they can be passed to the sitemap with an action.

The FilterTransformer is a standalone component, you don't need to use it in combination with the SQLTransformer.

If you only use the FilterTransformer in combination with the SQLTransformer, you have to query the database each time the user wants to see another part of the result. You can better store the result in the session after the first request and retrieve the result from the session for the subsequent requests. This can be done by using a selector, which checks if the data is available in the session or not.

WriteDOMSessionTransformer can build a DOM starting from a given element (which will be the root of the DOM tree) and store it in the session. If you want to store the result of a query, you have to add following to the sitemap:

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The transformer will build a DOM tree with rowset as root element and will store it in the session with the name "DBresult".

Note: most of the times, it is not smart to keep the output XML of the SQLTransformer in the session. Check if it is better to do the necessary transformations first, so that you get a smaller DOM, and then put the result in the session. You probably will be able to use the FilterTransformer on the transformed XML also.

The WriteDOMSessionTransformer is a standalone component, you don't need to use it in combination with the SQLTransformer.

Simply transforms a DOM to SAX-events, which can be used further on in the pipeline. Once you stored the result of a query in the session with the WriteDOMSessionTransformer, you can read it again with the ReadDOMSessionTransformer:

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In this example, the SAX-events that came from the DOM tree that is stored in the session with name DBresult will be added after the users element. This means as soon that the transformer encounters the end-element 'users', it will start to generate SAX-events from the DOM tree. There are three possible positions, 'before','in' and 'after':

  1. 'before' means that when the transformer encounters the 'users' element, it will FIRST translate the DOM tree to SAX-events and THEN it will continue to forward the other SAX-events (starting with 'users').
  2. 'in' means that the transformer will forward the startElement event for 'users' and that it IMMEDIATELY starts to generate SAX-events from the DOM-tree. After that, it will continue to forward the child elements of users and then all the other elements.
  3. 'after' means that the transformer starts to generate SAX-events from the DOM-tree just after it has forwarded the end-element 'users'.

The ReadDOMSessionTransformer is a standalone component, you don't need to use it in combination with the WriteDOMSessionTransformer.

That's it,

Sven Beauprez