SQL in a Nutshell, 2nd Edition

Scope

Constraints may be applied at the column-level or the table-level:

Column-level constraints

Declared as part of a column definition and apply only to that column.

Table-level constraints

Declared independently from any column definitions (traditionally, at the end of a CREATE TABLE statement) and may apply to one or more columns in the table. A table constraint is required when you wish to define a constraint that applies to more than one column.