Oracle SQL XML

Dynamic in-line queries

I don’t know of a better name for this feature. I think this name describes the function well, even if it may sound overly ambitious. This whole post will be based on a post by Laurent Schneider where he uses XML features to do a select count(*) from different tables and return the result in one result table. It is like a auto generated number of selects that are unioned together.

You can find Laurent’s post here. It’s a wonderful piece of SQL to do something most of us would probably not have thought of using a single SQL statement for.

The version Laurent uses is:

select table_name
                                   ('select count(*) c from '||table_name)
                ) count
  from user_tables;

Let’s first review what happens in this query. The inner most function is a call to dbms.getxml with a query that will return one row and one value. This sql stateement is created on the fly based on the table name the outer select retrieves from user_tables. The function will of course turn the query result into XML text. The xmltype that wraps the function will convert the XML text into an XML object so XPATH functions can be used. The extractvalue function pulls out the node ROWSET/ROW/C where the two first are given as that is how getxml will return the data. C is the name of the column or in this case the alias assigned in the select statement as the resulting “column” from the count(*) function. The last thing that happens is that the extracted value is converted to a number and it is then returned to the outer select as the result from this function.

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