Implementation of MLS/DBMS Models - Multilevel Security for Relational Databases - page 46

3.4.3 Update Operation Procedure
The SQL statement for the update operation is described as follows:
UPDATE R
SET [A 1 = a 1 ,A 2 = a 2 ,...,A n = a n ]
WHERE P
where R is an MLS relation, A 1 ,A 2 ,...,A n are attributes from R,
a 1 ,a 2 ,...,a n are values from domains of A 1 ,A 2 ,...,A n , and P is
an update condition that defines the tuples that are to be updated.
If a subject with the security class level L runs a command to update
an MLS relation R, for all t ∈ R, if t satisfies P, the update operation is
implemented as follows [32]:
• Step 1: get the security level of the subject that runs the update
operation.
• Step 2: update all tuples that satisfy update condition P in the
update statement and have a security level equal to the secu-
rity level of the user.
Figure 3.8 illustrates the flow chart for the update operation in the
Smith-Winslett model.
3.4.4 Delete Operation Procedure
The SQL statement for the delete operation is described as follows:
DELETE
FROM R
WHERE P
where R is an MLS relation and P is a delete condition that defines
the tuples that are to be deleted.
If a subject with the security class level L runs a command to delete
tuples from MLS relation R, for all t ∈ R, if t satisfies P, the delete
operation is implemented as follows [32]:
• Step 1: get the security level of the subject that runs the delete
operation.
• Step 2: delete all tuples that satisfy delete condition P in the
delete statement and have a security level equal to the security
level of the user.