Rule 2: Access Rows by Content Only Rule
Users can only retrieve rows by their content, which is the attribute values that exist in each row.
student
sid
lname
fname
class
phone
1
Jones
Allan
2
555-1234
2
Smith
John
3
555-4321
3
Brown
Harry
2
555-1122
5
White
Edward
3
555-3344
enrollment
sid
cno
major
1
101
No
1
108
Yes
2
105
No
3
101
Yes
3
108
No
5
102
No
5
105
No
SQL> SELECT sid, lname FROM student
2> WHERE class = 2;
⇒
sid
lname
1
Jones
3
Brown
Rule 3: The Unique Row Rule
Two tuples in a relation cannot be identical in all column values at once.
Rule 4: Entity Integrity Rule
No column belonging to a primary key of a table T is allowed to take on NULL values for any row in T.
Comments
However, not all databases follow the relational rules.
For example, some databases allow the table rows to be ordered, which should not happen because a table is actually a set and a row is a set element.
Some databases may also allow duplicate rows to exist.