Null value is "do not know", "absence", the value "meaningless" is, the SQL language allows certain properties of some of the tuple null value under certain circumstances. The following circumstances:
- This should have a value of the property, but they did not know its specific value.
- This property should not have value.
- For some reason inconvenient to fill
Null is a very special value, contain uncertainties .
- Generating null - Except generating null general, the outer connector will produce a null. Relational operators null values will produce a null value.
- Determining a null value - a property value determination is empty, a is null or is not null represented.
- Null constraint - attribute definition (or domain definition) not null constraint has not null value, add unique restriction property can not null value, the code attribute value can not be emptied.
- Null value arithmetic, comparison operations, logic operations - and the other null value (which may be null) the result of arithmetic operation to a null value , the null value and another value (can be null) of comparing the calculation result for the Unknown , with the unknown, the traditional binary logic operations (ture, false) becomes logical to extend our three-valued logic .
In the query, only the where and having clause in the selection criteria is true of tuples was only selected as the output .