Ensuring data presence and uniqueness

You can define columns with the NOT NULL clause to ensure that the required data is present. You can also control the type of data by assigning data types and lengths to column data. For example, you can specify that alphabetic data cannot be entered into a column with one of the numeric data types. You can also specify that the data for a DATE or TIME column must use a specific format.

You must ensure that the data in a column or a set of columns is unique. You can do so by creating a unique index on a column or set of columns.