The most common attributes one specifies for columns are NOT NULL, DEFAULT, and CONSTRAINT.
The NOT NULL attribute requires a value for that column for every row that one adds to the table. By default,
a column may contain a null value.
The DEFAULT attribute allows one to provide an expression that will create a value for a column, if a value
is not otherwise provided when one inserts a new row. For instance, the following column declaration specifies
the default value for the state column to be ???NY???:
State Char(2) DEFAULT 'NY',
148 DATABASE [CHAP. 8
There are four constraints that can be specified: PRIMARY KEY, FOREIGN KEY, UNIQUE and CHECK.
The primary key constraint identifies the column or columns that comprise the primary key.
A foreign key constraint identifies a column that contains values of a primary key in a different table.
Foreign keys are the mechanism for creating relationships among rows (entities) in different tables.
A unique constraint requires all rows in the table to have unique values for the column or set of columns
specified in the constraint. A unique constraint is sometimes called a candidate key, because the unique
column(s) could be used as a primary key for the table, in place of the chosen primary key.
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