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SQL Tables and Constraints Example

SQL Tables and Constraints Example. The Database. A hockey league with rinks, teams, and players: Rink ( name , phone, capacity) Team ( tname, city , color, wins, losses, tie, rname FK references Rink(name))

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SQL Tables and Constraints Example

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  1. SQL Tables and Constraints Example

  2. The Database • A hockey league with rinks, teams, and players: • Rink (name, phone, capacity) • Team (tname, city, color, wins, losses, tie, rname FK references Rink(name)) • Player (id, name, num, pos, tname, tcity, FK (tname, tcity) references Team (tname, city)) • All relations are in BCNF • The only FDs are PKall other fields • Constraint: All players with uniform number 9 must be goalies.

  3. Unpreserved Dependencies • Suppose the users now decide that all rinks in the same city have the same capacity: • city  capacity • While trying to remain calm, you realize that: • This FD doesn’t exist in any single relation, so a join is required to check it each time we add or change a capacity value. • The tradeoff: • Expensive to check, but • may not be checked often enough to justify creating a dependency-preserving decomposition.

  4. Summary • Keys can be specified: • With the attribute, for single-field keys • At end of “create table” statement (all keys) • In a separate “alter table” statements (all keys) • “Check” constraints very useful • SQL Server: can only involve one relation • SQL Standard: more general • Triggers, assertions for IC (FD) enforcement • SQL Server doesn’t support assertions • Must be careful, in general, with triggers

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