Column-Level Constraints and Table-Level Constraints
Constraints can be specified for:
• Individual columns as part of the column specification (column-level constraints)
• Groups of columns as part of the table definition (table-level constraints)
For example:
Column-level constraints:
create table mytable(name char(10) not null,
id integer references idtable(id),
age integer check (age > 0));
Table-level constraints:
create table yourtable(firstname char(20) not null,
lastname char(20) not null,
unique(firstname, lastname));
Note: Multiple column constraints are separated by a space.
Names can be assigned to both column-level and table-level constraints. If the constraint name is omitted, the DBMS assigns one. To drop a constraint (using the ALTER TABLE statement), specify the constraint name. It is advisable to specify a name when creating a constraint; otherwise system catalogs must be queried to determine the name assigned by the DBMS when the constraint was created.
Last modified date: 01/30/2023