Database Reference
In-Depth Information
3.2.3 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 needed 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 [30]:
• Step 1: get the security level of the subject that runs the update
operation.
• Step 2: get all tuples that satisfy update condition P in the
update statement and have a security level equal to or below
the security level of the user.
• Step 3: for each tuple in the tuples in step 2, if the security
level of a primary key is equal to the security level of the user,
the tuple in the single-level relation that contains the attri-
butes in the set clause will be updated.
• Step 4: for each tuple in the tuples in step 2, if the security level
of a primary key is lower than the security level of the user, the
tuple in the single-level relation that contains the attributes in
the set clause will be polyinstantiated at the security level of the
user.
Figure  3.3 illustrates the flow chart for the update operation in the
SeaView model.
3.2.4 Delete Operation Procedure
The SQL statement for the delete operation is described as follows:
DELETE
FROM R
WHERE P
Search WWH ::




Custom Search