### 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:
  
  
```plaintext
	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.


```plaintext
Sale_Date timestamp default current_timestamp not null,
```