1. Data Definition and Integrity Constraints
Reading: C&B, Chap 7

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 Enhancement Features (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) in ANSI 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', , '12-JAN-1990', 'F'); . . OK
('S0023', 'Jones', , '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]
This is a note
Dept. of Computing Science, University of Aberdeen

9. Dept. of Computing Science, University of Aberdeen
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
Result
SMI
12075
1990
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 Integrity and 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. Dept. of Computing Science, University of Aberdeen
Triggers
Often, real-world business rules cannot be implemented using constraints.
Example: The branch manager is notified by 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. Dept. of Computing Science, University of Aberdeen
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