Download Constraints

Constraints
cis 407
Types of Constraints & Naming
Key Constraints
Unique Constraints
Check Constraints
Default Constraints
Misc
Rules and Defaults
Triggers
What is a Constraint
• A constraint is a restriction. Placed at
either column or table level, a constraint
ensures that your data meets certain data
integrity rules.
Types of Constraints
• Domain Constraints deal with one (or
more) columns, e.g., “unit price > 0”
– The domain of unit price is the positive
integers. Dom(unitPrice)
• Entity Constraints:
– Each row must have unique column or
combination of columns called Keys.
• Referential Integrity Constraints
– E.g., all parts must be supplied by a valid
supplier
Constraint Naming
• You really should NAME each constraint
you create (though SQL Server does not
require this). See example for my naming
convention or book pg 155.
– E.g. CKPriceExceedsCost
• Prefer to create separately tables and
constraints (scripts are easier to manage).
Key Constraints
• Primary Key Constraint for a table
– Used with relating that table to another
• Relational databases relate tables by primary and
foreign keys.
• Can create primary key during table
creation (see example 156)
• Prime key implies unique
Prime Key on Existing Table
• Use Accounting
• Alter Table Employees
add constraint PK_EmployeeID
PRIMARY KEY (employeeID)
• Note use of constraint name
PK_EmployeeID
– Constraint can be addressed later for
alternation or deletion
• Note naming convention (from MS)
Foreign Key
• Create TABLE orders (
orderID int identity not null,
customerNO int not null )
• Alter table orders
add constraint FK_EmployeeCreatesOrder
FOREIGN KEY (employeeID)
references employees (employeeID)
Cascading Actions
• Be careful with foreign key constraints!
• Default SQL Server behavior is to restrict
parent row from being deleted if any child
rows exist
– E.g. cannot delete supplies until all supplied
parts deleted.
• However, can set thing up to auto delete
child rows if parent row deleted
• Be careful of dependency chains!
Cascade (pg 162)
• Referential integrity action
• Alter table orderDetails
add constraint FK_orderContainsDetails
foreign key (orderid)
references orders(orderid)
on update no action
no delete cascade
UNIQUE Constraint
• Unique does not automatically prevent null
– must choose not null, but can only have one
null in that column.
• Can add unique constraint on existing
table, but if any rows already in table, may
have problems
• Note exec sp_helpconstraint
Check Constraint
• Cross check within row
• Alter table customers add constraint
CN_customerDateInSystem
check (dateInSystem <= getDate() )
Default
• Default value if one not offered in insert
statement
• Alter table customers
add constraint
CN_customerDefaultDateInSystem
default getdate() for dateInSystem
• Alter table customers
add constraint CN_customerAddress
default ‘unknown’ for address1
Disabling Constraint
• Constraints can be temp turned off when
doing bulk data loads
– Must generate exception report and clean up
data
– Then can turn constraint back on
Constraints v. Exception Reports
• Can use exception reports instead of
constraint
– Constraint forces only good data to be
entered in the system
• Slows down entry, customer service, etc
– Exception report finds bad data
• You then allow bad data to be entered
– e.g., part for which no supplier
• Must have gang of people to clean up mess
• MUCH BETTER TO ONLY HAVE CLEAN
DATA IN DATABASE
Triggers
• Triggers can be used to create integrity
constraints
• Triggers are like a stored procedure but
fired when certain conditions in database
met.
• Covered in a later chapter