Transcript Data Definition and Integrity Constraints

Data Definition
and Integrity Constraints
Reading: C&B, Chap 7
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
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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)
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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
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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)
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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
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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
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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]
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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;
Result
SMI
12075 1990
• ANSI SQL supports many scalar functions...
• See CB, Table 6.2, p163
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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
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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
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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?
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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);
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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?
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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...
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Simplified Data Model of a DBMS
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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
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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
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