Error Handling (XQuery)

The W3C specification allows type errors to be raised statically or dynamically, and defines static, dynamic, and type errors.

Compilation errors are returned from syntactically incorrect Xquery expressions and XML DML statements. The compilation phase checks static type correctness of XQuery expressions and DML statements, and uses XML schemas for type inferences for typed XML. It raises static type errors if an expression could fail at run time because of a type safety violation. Examples of static error are the addition of a string to an integer and querying for a nonexistent node for typed data.

As a deviation from the W3C standard, XQuery run-time errors are converted into empty sequences. These sequences may propagate as empty XML or NULL to the query result, depending upon the invocation context.

Explicit casting to the correct type allows users to work around static errors, although run-time cast errors will be transformed to empty sequences.

Static errors are returned by using the Transact-SQL error mechanism. In SQL Server, XQuery type errors are returned statically. For more information, see XQuery and Static Typing.

In XQuery, most dynamic errors are mapped to an empty sequence ("()"). However, these are the two exceptions: Overflow conditions in XQuery aggregator functions and XML-DML validation errors. Note that most dynamic errors are mapped to an empty sequence. Otherwise, query execution that takes advantages of the XML indexes may raise unexpected errors. Therefore, to provide an efficient execution without generating unexpected errors, SQL Server Database Engine maps dynamic errors to ().

Frequently, in the situation where the dynamic error would occur inside a predicate, not raising the error is not changing the semantics, because () is mapped to False. However, in some cases, returning () instead of a dynamic error may cause unexpected results. The following are examples that illustrate this.

Example: Using the avg() Function with a String

In the following example, the avg function is called to compute the average of the three values. One of these values is a string. Because the XML instance in this case is untyped, all the data in it is of untyped atomic type. The avg() function first casts these values to xs:double before computing the average. However, the value, "Hello", cannot be cast to xs:double and creates a dynamic error. In this case, instead of returning a dynamic error, the casting of "Hello" to xs:double causes an empty sequence. The avg() function ignores this value, computes the average of the other two values, and returns 150.

DECLARE @x xml
SET @x=N'<root xmlns:myNS="test">
SELECT @x.query('avg(//*)')

Example: Using the not Function

When you use the not function in a predicate, for example, /SomeNode[not(Expression)], and the expression causes a dynamic error, an empty sequence will be returned instead of an error. Applying not() to the empty sequence returns True, instead of an error.

Example: Casting a String

In the following example, the literal string "NaN" is cast to xs:string, then to xs:double. The result is an empty rowset. Although the string "NaN" cannot successfully be cast to xs:double, this cannot be determined until runtime because the string is first cast to xs:string.

SET @x = ''
SELECT @x.query(' xs:double(xs:string("NaN")) ')

In this example, however, a static type error occurs.

SET @x = ''
SELECT @x.query(' xs:double("NaN") ')

Implementation Limitations

The fn:error() function is not supported.

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