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Nullability and Three-Value Logic Comparisons

If you are familiar with the SQL Server data types, you will find similar semantics and precision in the System.Data.SqlTypes namespace in the .NET Framework. There are some differences, however, and this topic covers the most important of these differences.

A primary difference between native common language runtime (CLR) data types and SQL Server data types is that the former do not allow for NULL values, while the latter provide full NULL semantics.

Comparisons are affected by NULL values. When comparing two values x and y, if either x or y is NULL, then some logical comparisons evaluate to an UNKNOWN value rather than true or false.

The System.Data.SqlTypes namespace introduces a SqlBoolean type to represent this 3-value logic. Comparisons between any SqlTypes return a SqlBoolean value type. The UNKNOWN value is represented by the null value of the SqlBoolean type. The properties IsTrue, IsFalse, and IsNull are provided to check the value of a SqlBoolean type.

All arithmetic operators (+, -, *, /, %), bitwise operators (~, &, and |), and most functions return NULL if any of the operands or arguments of SqlTypes are NULL. The IsNull property always returns a true or false value.

Decimal data types in the .NET Framework CLR have different maximum values than those of the numeric and decimal data types in SQL Server. In addition, in the .NET Framework CLR decimal data types assume the maximum precision. In the CLR for SQL Server, however, SqlDecimal provides the same maximum precision and scale, and the same semantics as the decimal data type in SQL Server.

In the .NET Framework CLR, the addition of two very large numbers may not throw an exception. Instead, if no check operator has been used, the returned result may "wrap around" as a negative integer. In System.Data.SqlTypes, exceptions are thrown for all overflow and underflow errors, and divide-by-zero errors.

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