SQL and Relational Theory, 2nd Edition

RELATIONAL ASSIGNMENT

Relational assignment in general works by assigning a relation value, denoted by some relational expression, to a relation variable, denoted by a relvar reference (where a relvar reference is basically just the pertinent relvar name). Here’s a Tutorial D example:

     S := S WHERE NOT ( CITY = 'Athens' ) ;

Now, it’s easy to see that this particular assignment is logically equivalent to the following DELETE statement:

     DELETE S WHERE CITY = 'Athens' ;

More generally, the Tutorial D DELETE statement

     DELETE R WHERE bx ;

(where R is a relvar name and bx is a boolean expression) is shorthand for, and hence logically equivalent to, the following relational assignment:

     R := R WHERE NOT ( bx ) ;

Alternatively, we might say it’s shorthand for this one (either way, it comes to the same thing):

R := R MINUS ( R WHERE bx ) ;

Turning to INSERT, the Tutorial D INSERT statement

     INSERT R rx ;

(where R is again a relvar name and rx is a relational expression—typically but not necessarily a relation selector invocation) is shorthand for:

     R := R UNION rx ;

For example, the INSERT statement—

     INSERT SP RELATION { TUPLE { SNO 'S5' , PNO 'P6' , QTY 700 } } ;

—effectively inserts a single tuple into the shipments relvar SP.

Finally, the Tutorial D UPDATE statement also corresponds to a certain relational assignment. However, the details are a little more complicated in this case than they are for INSERT and DELETE, and I’ll defer them to Chapter 7.

D_INSERT and I_DELETE

I’ve said the INSERT statement ...