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Nullable Structured Types (Entity SQL)

  • 项目

A null instance of a structured type is an instance that does not exist. This is different from an existing instance in which all properties have null values.

This topic describes the nullable structured types, including which types are nullable and which code patterns produce null instances of structured nullable types.

Kinds of Nullable Structured Types

There are three kinds of nullable structure types:

  • Row types.

  • Complex types.

  • Entity types.

Code Patterns that Produce Null Instances of Structured Types

The following scenarios produce null instances:

  • Shaping null as a structured type:

    TREAT (NULL AS StructuredType)

  • Upcasting of a base type to a derived type:

    TREAT (BaseType AS DerivedType)

  • Outer join on false condition:

    Collection1 LEFT OUTER JOIN Collection2
ON FalseCondition

    --or

    Collection1 RIGHT OUTER JOIN Collection2
ON FalseCondition

    --or

    Collection1 FULL OUTER JOIN Collection2
ON FalseCondition

  • Dereferencing a null reference:

    DEREF(NullRef)

  • Obtaining ANYELEMENT from an empty collection:

    ANYELEMENT(EmptyCollection)

  • Checking for null instances of structured types:

    ...
for (int i = 0; i < reader.FieldCount; i++)
{
    if (reader.IsDBNull(i))
    {
        Console.WriteLine(“[NULL]”);
    }
    else
    {
        Console.WriteLine(reader.GetValue(i).ToString());
    }
}

See Also

Concepts

Entity SQL Overview