nullable typeとは 意味・読み方・使い方

In programming, nullable types are a feature of some statically-typed programming languages which allows a data type to be set to the special value NULL instead of their common range of possible values. For value types or built-in data types like integers and booleans however, such behavior is mostly not possible. Nullable type support allows for the programmer to make also these value types NULL. This can be useful in general and also when working with databases. A field in a Relational database like SQL may have an entry that is NULL (or empty) instead of containing a value. A language with nullable type support can then return a NULL value and correctly represent the behavior of the database.