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 6.)
In a table definition, default values are listed after the column data type. For example:
CREATE TABLE products ( product_no integer, 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).