A column can be assigned a default value. When a new row is created and no values are specified for some of the columns, those 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 having to know what that value is. (Details about data manipulation commands are in 6장.)
If no default value is declared explicitly, the default value is the null value. This usually makes sense because a null value can be considered 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, name text, price numeric DEFAULT 9.99 );
The default value can be an expression, which will be evaluated whenever the default value is inserted (not when the table is created). A common example is for a timestamp column to have a default of CURRENT_TIMESTAMP, so that it gets set to the time of row insertion. Another common example is generating a "serial number" for each row. In PostgreSQL this is typically done by something like:
CREATE TABLE products ( product_no integer DEFAULT nextval('products_product_no_seq'), ... );
where the nextval() function supplies successive values from a sequence object (see 9.16절). This arrangement is sufficiently common that there's a special shorthand for it:
CREATE TABLE products ( product_no SERIAL, ... );
The SERIAL shorthand is discussed further in 8.1.4절.