Data Definition and Integrity Constraints

1. Data Definitionand Integrity Constraints Reading: C&B, Chap 6

2. In this lecture you will learn • the different SQL data types & related scalar functions • how to define new data types with DDL statements • some of the integrity constraints used in DBMSs • SQL's Integrity Enhancement Features (IEF) • how integrity constraints can affect row operations • the notion of schemas Dept. of Computing Science, University of Aberdeen

3. SQL's Integrity EnhancementFeatures (IEF) • So far, we have thought of databases as static repositories. In fact, real databases are often very ‘active’ with 100's of users simultaneously querying and updating the DB. • So database integrity is important • IEFs allow the DB designer to specify & enforce: • domain constraints • required data • entity integrity • referential integrity • enterprise constraints (business rules) Dept. of Computing Science, University of Aberdeen

4. Creating Tables - Data Definition • CREATE TABLE is used to define relational tables • it defines the data type for each column • defines rules for how data may be inserted and deleted CREATE TABLE Staff (StaffNo VARCHAR(5), Lname VARCHAR(20), Salary FLOAT, HireDate DATE); • VARCHAR, FLOAT, and DATE are examples of domains • Domains specify type & range of allowed data values Dept. of Computing Science, University of Aberdeen

5. Built-in Data Types (Domains) inANSI SQL • ANSI SQL supports many data types (vendors often also have own dialects): • CHARACTER (CHAR), CHARACTER VARYING (VARCHAR) • NUMERIC, DECIMAL (DEC), INTEGER (INT), SMALLINT • FLOAT, REAL, DOUBLE PRECISION • DATE, TIME, TIMESTAMP • BOOLEAN, BIT • BINARY LARGE OBJECT, etc. • Some types have an associated size . e.g. CHAR(5) Dept. of Computing Science, University of Aberdeen

6. User-Defined Domains in ANSI SQL CREATE DOMAIN SexType AS CHAR(1) DEFAULT 'M' CHECK (VALUE IN ('M', 'F')); CREATE TABLE Staff (StaffNo VARCHAR(5), Lname VARCHAR(20), Salary FLOAT, HireDate DATE, Sex SexType); INSERT INTO Staff VALUES ('S0057', 'Smith', 12075.50, '12-JAN-1990', 'F'); . . OK INSERT INTO Staff VALUES ('S0023', 'Jones', 14250.50, '14-FEB-1997', 'X'); . . Fails • SexType acts as a constraint on allowed range of values Dept. of Computing Science, University of Aberdeen

7. Required Data& More Domain Constraints • Example: CREATE TABLE Staff ( StaffNo VARCHAR(5) NOT NULL, Lname VARCHAR(20) NOT NULL, Salary FLOAT CHECK (Salary BETWEEN 50 and 20000), HireDate DATE, Sex SexType); • StaffNo & Lname are required - may not be NULL • The CHECK clause gives a domain constraint for Salary • Updates & insertions will fail if constraints not satisfied Dept. of Computing Science, University of Aberdeen

8. Dynamic Domain Constraints • Domains may be defined ‘dynamically’ using values that already exist in the database: CREATE DOMAIN StaffNoDomain AS VARCHAR(5) CHECK (VALUE IN (SELECT StaffNo FROM Staff)); CREATE TABLE PropertyForRent (PropertyNo VARCHAR(5) NOT NULL, StaffNo StaffNoDomain); • This could be used to ensure every StaffNo in PropertyForRent is valid • Domains can be deleted: DROP DOMAIN DomainName [RESTRICT | CASCADE] Dept. of Computing Science, University of Aberdeen

9. Scalar Functions • Scalar functions may be used to convert/manipulate data values (remember aggregates: MIN, MAX, etc?). • Example: SELECT SUBSTRING(Lname FROM 1 TO 3), CONVERT(INTEGER Salary), EXTRACT(YEAR FROM HireDate) FROM Staff; • ANSI SQL supports many scalar functions... • See CB, Table 6.2, p163 Dept. of Computing Science, University of Aberdeen

10. Entity Integrity - Primary Keys • Reminder: the primary key of each row in a table must be unique and non-null. • Example: The primary key of the Viewing table is composed of two attributes (composite key): CREATE TABLE Viewing ( ClientNo VARCHAR(5) NOT NULL, PropertyNo VARCHAR(5) NOT NULL, PRIMARY KEY (ClientNo, PropertyNo)); • SQL will reject operations that would violate primary key uniqueness • Can use UNIQUE(Colname) to enforce uniqueness of alternate keys Dept. of Computing Science, University of Aberdeen

11. Referential Integrity - Foreign Keys • Reminders: • A foreign key links a child table to its parent table. • If a foreign key is non-null, it must match an existing row in the parent table. • So... SQL has more keywords for this: CREATE TABLE PropertyForRent (... StaffNo VARCHAR(5) NOT NULL, FOREIGN KEY (StaffNo) REFERENCES Staff); • SQL will reject operations that would violate referential integrity Dept. of Computing Science, University of Aberdeen

12. Referential Integrityand Referential Actions • Question: if a key attribute in the parent table is modified, what should happen in the child table ? - SQL provides 4 alternative referential actions: FOREIGN KEY (Key) REFERENCES Table [ON DELETE | UPDATE Action] • CASCADE - apply changes to child rows • SET NULL - set child keys to NULL • SET DEFAULT - set child keys to DEFAULT value • NO ACTION - reject the operation (default) • Suppose a client is removed from the DreamHome DBMS. What's the most appropriate action to specify for ClientNo in the Viewing table? Dept. of Computing Science, University of Aberdeen

13. Enterprise Constraints(Business Rules) • Sometimes, real-world business rules involve constraints that refer to more than one table. Its useful to define enterprise constraints just once. • Example: A member of staff may manage no more than 100 properties: CREATE ASSERTION StaffNotOverLoaded CHECK (NOT EXISTS (SELECT StaffNo FROM PropertyForRent GROUP BY StaffNo HAVING COUNT (*) > 100)); CREATE TABLE PropertyForRent ( ... CONSTRAINT StaffNotOverLoaded); Dept. of Computing Science, University of Aberdeen

14. Triggers • Often, real-world business rules cannot be implemented using constraints. • Example: The branch manager is notified by e-mail if a client views more than 10 properties. • Different DBMSs often provide a trigger mechanism • Triggers may contain procedural code (if/then/else, function calls) • Triggers can implement complex database operations • However, triggers can add to database complexity (hidden rules) • Triggers are not ANSI standard - should they be? Dept. of Computing Science, University of Aberdeen

15. Putting It All Together - Schemas • A schema is a collection of named DBMS objects: • Tables, Domains, Constraints, Views (later), Triggers, and more ... • A multi-user DMBS may contain multiple schemas: • Each schema is owned by a given user • A Database Administrator (DBA) manages schemas (CREATE, DROP) • Schemas are maintained in special system tables • However, different DBMSs have different ways of managing schemas... Dept. of Computing Science, University of Aberdeen

16. Simplified Data Model of a DBMS Dept. of Computing Science, University of Aberdeen

17. Database Schemas Evolve Over Time • Ideally, a database is created once and then used for many years ... BUT • The data model may be improved (integrity, performance) ... • New features may be added in new releases ... • Enterprise rules may change ... • Therefore, SQL provides many options for changing tables: • See ALTER TABLE, CB Ch. 6, p172 Dept. of Computing Science, University of Aberdeen

18. Summary So Far... • DBs are ‘active’ or ‘alive’ - contents always changing • The structure of a DB can also evolve over time... • DB contents should always be consistent - integrity • ANSI SQL provides several Integrity Enhancement Features (IEFs) • IEF => domain constraints, entity/referential integrity, business rules... • IEFs imply additional design choices for new DBs • One DBMS can manage multiple DBs - notion of schemas & privileges Dept. of Computing Science, University of Aberdeen