Null in database columns

This capability is especially useful when value types are boxed and unboxed, providing enhanced compatibility with object types. SqlTypes nulls rather than Nullable. Allowing null values in column definitions introduces three-valued logic into your application. A comparison can evaluate to one of three conditions:. Because null is considered to be unknown, two null values compared to each other are not considered to be equal. In expressions using arithmetic operators, if any of the operands is null, the result is null as well.

Comparison between any System. SqlTypes will return a SqlBoolean. However, you can't prevent different connections from setting null options for that connection. Null values are special, and their storage and assignment semantics differ across different type systems and storage systems. A Dataset is designed to be used with different type and storage systems.

This section describes the null semantics for assigning null values to a DataColumn in a DataRow across the different type systems. Value This assignment is valid for a DataColumn of any type.

Value is coerced into the appropriate strongly typed Null value. Null All System. SqlTypes data types implement INullable. If the strongly typed null value can be converted into the column's data type using implicit cast operators, the assignment should go through. I think the results for AND are what you would expect, but the results you would expect for OR are not.

In many cases when a value is unknown, the uncertainty of the value translates through the expression. Keep in mind we use NULL to denote unknown values. This leaves to concluding the result, in general is unknown. Here is the Truth table for the OR operator.

This is even the case when the other input is NULL. All that matters is one input is TRUE, the other input, albeit unknown, is irrelevant. Please log in again. The login page will open in a new tab. After logging in you can close it and return to this page. September 5, However, you can't prevent different connections from setting null options for that connection. Null values are special, and their storage and assignment semantics differ across different type systems and storage systems.

A Dataset is designed to be used with different type and storage systems. This section describes the null semantics for assigning null values to a DataColumn in a DataRow across the different type systems. Value This assignment is valid for a DataColumn of any type. Value is coerced into the appropriate strongly typed Null value. Null All System. SqlTypes data types implement INullable. If the strongly typed null value can be converted into the column's data type using implicit cast operators, the assignment should go through.

Otherwise an invalid cast exception is thrown. In this case, if a strongly typed null value associated with the derived class is assigned, it is stored as an untyped DbNull. Value , because null storage is always consistent with the DataColumn's data type. The default value for any System. SqlTypes instance is null. Nulls in System. SqlTypes are type-specific and cannot be represented by a single value, such as DbNull. Use the IsNull property to check for nulls. For instance, if the supplier you met with does not have a fax number, you would enter a blank value into that column.

If a Stories on CD supplier gave the company a thousand free CDs to send to the next one thousand customers who placed an order, the RetailPrice column for that CD would contain a zero. In many cases it appears that a blank and a null can be used interchangeably. If a value is truly unknown, enter a null into the column.

You can represent a blank by typing a single space in the field.