Simple-column-Name DataType
    [ Column-level-constraint ]*
    [ [ WITH ] DEFAULT { ConstantExpression | NULL }
       |generated-column-spec ]
    [ Column-level-constraint ]*

The syntax of Data-Type is described in Data types.

The syntaxes of Column-level-constraint and Table-level constraint are described in CONSTRAINT clause.

Column default

For the definition of a default value, a ConstantExpression is an expression that does not refer to any table. It can include constants, date-time special registers, current schemas, users, and null.

Related reference