Front Page Applet ATSQL UC TUC SEQ NSEQ System Guidelines Requirements Doc Support Information Contribution Warranty Related Work Contact Future Work | Temporal upward compatibility (TUC) ensures that legacy applications continue to be operational even if relations are changed to become temporal. TUC ensures a harmonious coexistence of legacy application code and new, temporally-enhanced application code. To illustrate the problem, assume that the new temporal model is in place. If an application needs temporal support a snapshot relation must be changed to become a temporal relation. Clearly, it is undesirable (or even impossible) to change the (legacy) application code that accesses the snapshot relation that has become temporal. TUC ensures that this is unnecessary. Essentially, tables can be rendered temporal without changing any other statements. The TUC "language level" provides only very few sysntactic extensions to SQL. As a matter of fact it is only possible to add time dimensions to nontemporal tables. For this purpose the ALTER statement of SQL-92 has been extended.
      ALTER TABLE p ADD VT;
      INSERT INTO p VALUES (6);
      DELETE FROM p VALUES (8);
      COMMIT;
      SELECT * FROM p;
The first statement extends p to capture valid time by making it a valid-time table. The insert statement adds 6 ensuring that the past is not changed and that 6 will also be there as time passes by. Similarly, 8 is deleted witout changing the past. The select statement returns current but not past (and future) knowledge. The valid time may not be returned.