Implementation of MLS/DBMS Models - Multilevel Security for Relational Databases - page 42

If a subject with the security class level L runs a command to insert
a tuple into an MLS relation R, the insertion operation is imple-
mented as follows [31]:
• Step 1: get the security level of the subject that runs the insert
operation.
• Step 2: if the attribute is included in the attribute list in the
insert statement, this attribute will be set to its value from the
values list in the insert statement.
• Step 3: the security level of all attributes will be equal to the
security level of the subject that runs the insert operation.
• Step 4: insert into the single-level relation, with the security
level equal to the security level of the user, values that are
included in the values list of the insert statement and corre-
spond to the attributes of these single-level relations.
Figure 3.2 illustrates the flow chart for the insertion operation in the
Jajodia-Sandhu model.
3.3.3 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 [31]:
• 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 security levels equal to or below
the security level of the user.