XQuery自定义函数my-remove-elements报错err:XPTY0004求解

Let's break down those frustrating err:XPTY0004 errors and get your my-remove-elements function working correctly to keep only first_name and last_name attributes in your employees.xml document.

Understanding the Errors #

First, let's unpack what each error means:

• "is not a sub-type of element()": This usually happens when your function expects an element node as input, but you're passing something else (like attribute nodes, text nodes, or a sequence that includes non-element items). Or you might be trying to perform element-specific operations on non-element nodes inside the function.
• "The actual cardinality for parameter 1 does not match the cardinality in the function's signature": Cardinality refers to how many items a sequence contains (zero, one, or many). This error pops up when your function expects, say, a single element (element()) but you pass a sequence of multiple elements (element()*), or vice versa.

Correct Function Implementation #

Here's a revised version of your function that addresses both issues. It's designed to process a sequence of elements, retain only the first_name and last_name attributes, and preserve the original element structure (plus any child nodes if you need them):

declare function local:my-remove-elements($elements as element()*) as element()* {
  for $elem in $elements
  return
    element { node-name($elem) } {
      -- Keep only the desired attributes
      $elem/@first_name,
      $elem/@last_name,
      -- Optional: retain child nodes of the original element (remove this line if you don't need them)
      $elem/node()
    }
};

-- Example call (assuming your XML has a root <employees> with <employee> child elements)
local:my-remove-elements(/employees/employee)

Fixing the First Error ("not a sub-type of element()") #

The most common cause here is passing the wrong kind of node to your function:

• Double-check your function call: Make sure you're passing element nodes (like /employees/employee) not attribute nodes (like /employees/employee/@*). If you pass attributes directly to a function expecting elements, you'll get this type mismatch error.
• Verify your function parameter type: If you accidentally declared the parameter as attribute() instead of element(), that would also trigger this error. The code above uses element()* to accept any number of element nodes.

Fixing the Cardinality Mismatch Error #

This is all about matching the number of items your function expects vs. what you pass:

• If your original function declared the parameter as element() (single element only), but you're passing multiple employee elements (a sequence), that's the problem. Changing the parameter type to element()* (zero or more elements) fixes this, as shown in the code above.
• Similarly, if your function's return type was element() (single element) but you're generating multiple elements, update it to element()* to match the output cardinality.

Quick Test with Sample XML #

To verify, use a small snippet of your employees.xml like this:

Running the function call above will return each <employee> element with only the first_name and last_name attributes, stripping out the others.

内容的提问来源于stack exchange，提问作者Judah Flynn