A column can be assigned a default value. When a new row is created and no values are specified for some of the columns, the columns will be filled with their respective default values. A data manipulation command can also request explicitly that a column be set to its default value, without knowing what this value is. (Details about data manipulation commands are in Chapter 3.)
If no default value is declared explicitly, the null value is the default value. This usually makes sense because a null value can be thought to represent unknown data.
In a table definition, default values are listed after the column data type. For example:
CREATE TABLE products ( product_no integer PRIMARY KEY, name text, price numeric DEFAULT 9.99 );
The default value may be a scalar expression, which will be evaluated whenever the default value is inserted (not when the table is created).