SQL: Structured Query Language Sequel

1
SQL Structured Query Language(Sequel)

• Chapter 5
• (cont.)

2
SQL

• Query Language
• Constraints
• Triggers

3
Integrity Constraints (Review)

• An IC describes conditions that every legal
  instance of a relation must satisfy.
• Inserts/deletes/updates that violate ICs are
  disallowed.
• Can be used to
• ensure application semantics (e.g., sid is a
  key), or
• prevent inconsistencies (e.g., sname has to be a
  string, age must be lt 200)
• Types of ICs
• Fundamental Domain constraints, primary
  key constraints, foreign key constraints
• General constraints Check Constraints, Table
  Constraints and Assertions.

4
Check or Table Constraints
CREATE TABLE Sailors ( sid INTEGER, sname
CHAR(10), rating INTEGER, age REAL, PRIMARY
KEY (sid), CHECK ( rating gt 1 AND rating
lt 10 )

• Can use queries to express constraint.

5
Table Constraints
6
Assertions ( Constraints over Multiple Relations)
CREATE TABLE Sailors ( sid INTEGER, sname
CHAR(10), rating INTEGER, age REAL, PRIMARY
KEY (sid), CHECK ( (SELECT COUNT (S.sid)
FROM Sailors S) (SELECT COUNT (B.bid) FROM
Boats B) lt 100 )
Number of boats plus number of sailors is lt 100

• Awkward !
• Wrong?
• Associated with Sailors.
• If Sailors is empty, the number of Boats tuples
  can be anything!

7
Assertions ( Constraints over Multiple Relations)
Number of boats plus number of sailors is lt 100
CREATE TABLE Sailors ( sid INTEGER, sname
CHAR(10), rating INTEGER, age REAL, PRIMARY
KEY (sid), CHECK ( (SELECT COUNT (S.sid)
FROM Sailors S) (SELECT COUNT (B.bid) FROM
Boats B) lt 100 )

• ASSERTION is the right solution not
  associated with either table.

CREATE ASSERTION smallClub CHECK ( (SELECT
COUNT (S.sid) FROM Sailors S) (SELECT COUNT
(B.bid) FROM Boats B) lt 100 )
8
Triggers (active database)

• Trigger procedure that starts automatically if
  specified changes occur to the DBMS
• Three parts
• Event (activates the trigger)
• Condition (tests whether the triggers should run)
• Action (what happens if the trigger runs)
• A "daemon" that monitors a database
• Row-level trigger statement-level trigger

9
Triggers Example (SQL1999)

• CREATE TRIGGER youngSailorUpdate
• AFTER INSERT ON SAILORS
• REFERENCING NEW TABLE NewSailors
• FOR EACH STATEMENT
• INSERT
• INTO YoungSailors(sid, name, age, rating)
• SELECT sid, name, age, rating
• FROM NewSailors N
• WHERE N.age lt 18

10
Design of Active Consistent Data

• Triggers are flexible and powerful
• But can be hard to understand
• Several triggers (order?)
• Chain triggers (When to stop?)
• Recursive triggers
• Constraints
• IC achieves same goal
• More opportunity for optimization
• Not restricted into insert/delete/update

11
Summary

• SQL allows specification of rich integrity
  constraints
• Triggers respond to changes in the database