Mandatory Column Values
Columns that are required to have a value (indicated by • in the ERD) should have a NOT NULL constraint applied, following the data type, like so:
Sale_Date timestamp not null,
Default Column Values
Sometimes a column has a sensible default value, which the system can insert automatically if the user does not specify one. These can be a fixed value such as a default bank balance of 0, or a calculation such as using the current date and time for new sales.
To define a default column value, use the DEFAULT keyword followed by the desired expression. This must appear after the data type for the column, but before any constraints such as NOT NULL.
Sale_Date timestamp default current_timestamp not null,