Implementation of MLS/DBMS Models - Multilevel Security for Relational Databases - page 50

3.5.3 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 [33]:
• Step 1: get the security level of the subject that runs the update
operation.
• Step 2: if no attribute of the primary key is in the SET clause,
update all tuples in multilevel relations that satisfy the update
condition P in the update statement and have a security level
equal to the security level of the user. Also, all borrowed
tuples by higher level users that satisfy update condition P in
the update statement will be updated.
• Step 3: if some attribute of the primary key is in the SET
clause, update all tuples in multilevel relations that satisfy
update condition P in the update statement and have a secu-
rity level equal to the security level of the user. Delete all
borrowed tuples by higher level users that satisfy update con-
dition P in the update statement.
Figure 3.11 illustrates the flow chart for the update operation in the
MLR model.
3.5.4 Delete Operation Procedure
The SQL statement for the delete operation is described as follows:
DELETE
FROM R
WHERE P