1. Chapter 9 Constraints

2. Chapter Objectives  Explain the purpose of constraints in a table  Distinguish among PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and NOT NULL constraints and the appropriate use for each constraint  Distinguish between creating constraints at the column level and table level  Explain the purpose of constraints in a table  Distinguish among PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and NOT NULL constraints and the appropriate use for each constraint  Distinguish between creating constraints at the column level and table level

3. Chapter Objectives  Create PRIMARY KEY constraints for a single column and a composite primary key  Create a FOREIGN KEY constraint  Create a UNIQUE constraint  Create a CHECK constraint  Create PRIMARY KEY constraints for a single column and a composite primary key  Create a FOREIGN KEY constraint  Create a UNIQUE constraint  Create a CHECK constraint

4. Chapter Objectives  Create a NOT NULL constraint, using the ALTER TABLE…MODIFY command  Include constraints during table creation  Use DISABLE and ENABLE commands  Use the DROP command  Create a NOT NULL constraint, using the ALTER TABLE…MODIFY command  Include constraints during table creation  Use DISABLE and ENABLE commands  Use the DROP command

5. Constraints  Rules used to enforce business rules, practices, and policies  Rules used to ensure accuracy and integrity of data  Rules used to enforce business rules, practices, and policies  Rules used to ensure accuracy and integrity of data

6. Constraint Types

7. Naming Constraints  Use optional CONSTRAINT keyword during creation to assign a name  Let server name constraint using default format SYS_Cn  Use optional CONSTRAINT keyword during creation to assign a name  Let server name constraint using default format SYS_Cn

8. Creating Constraints  When: –During table creation –Modify existing table  How: –Column level approach –Table level approach  When: –During table creation –Modify existing table  How: –Column level approach –Table level approach

9. General Syntax – Column Level If a constraint is being created at the column level, the constraint applies to the column specified

10. General Syntax – Table Level  Approach can be used to create any constraint type except NOT NULL  Required if constraint is based on multiple columns  Approach can be used to create any constraint type except NOT NULL  Required if constraint is based on multiple columns

11. Enforcement  All constraints are enforced at the table level  If a data value violates a constraint, the entire row is rejected  All constraints are enforced at the table level  If a data value violates a constraint, the entire row is rejected

12. Adding Constraints to Existing Tables  Added to existing table with ALTER TABLE command  Add NOT NULL constraint using MODIFY clause  All other constraints added using ADD clause  Added to existing table with ALTER TABLE command  Add NOT NULL constraint using MODIFY clause  All other constraints added using ADD clause

13. PRIMARY KEY Constraint  Ensures that columns do not contain duplicate or NULL values  Only one per table allowed  Ensures that columns do not contain duplicate or NULL values  Only one per table allowed

14. PRIMARY KEY Constraint for Composite Key List column names within parentheses separated by commas

15. FOREIGN KEY Constraint  Requires a value to exist in referenced column of other table  NULL values are allowed  Enforces referential integrity  Maps to the PRIMARY KEY in parent table  Requires a value to exist in referenced column of other table  NULL values are allowed  Enforces referential integrity  Maps to the PRIMARY KEY in parent table

16. FOREIGN KEY Constraint - Example

17. Deletion of Foreign Key Values  Cannot delete a value in parent table referenced by a row in child table  Use ON DELETE CASCADE keywords when creating FOREIGN KEY constraint – automatically deletes parent row when row in child table is deleted  Cannot delete a value in parent table referenced by a row in child table  Use ON DELETE CASCADE keywords when creating FOREIGN KEY constraint – automatically deletes parent row when row in child table is deleted

18. UNIQUE Constraint  No duplicates allowed in referenced column  NULL values are permitted  No duplicates allowed in referenced column  NULL values are permitted

19. CHECK Constraint Updates and additions must meet specified condition

20. NOT NULL Constraint  Special CHECK constraint with IS NOT NULL condition  Can only be created at column level  Included in output of DESCRIBE command  Can only be added to existing table using ALTER TABLE…MODIFY command  Special CHECK constraint with IS NOT NULL condition  Can only be created at column level  Included in output of DESCRIBE command  Can only be added to existing table using ALTER TABLE…MODIFY command

21. NOT NULL Constraint Example

22. Adding Constraints During Table Creation – Column Level Include in column definition

23. Adding Constraints During Table Creation – Table Level Include at end of column list

24. Viewing Constraints – USER_CONSTRAINTS Can display name, type, and condition of CHECK constraints

25. Disabling/Enabling Constraints Use DISABLE or ENABLE clause of ALTER TABLE command

26. Dropping a Constraint  Constraints cannot be modified, must be dropped and recreated  Actual syntax depends on type of constraint –PRIMARY KEY - just list type of constraint –UNIQUE - include column name –All others - reference constraint name  Constraints cannot be modified, must be dropped and recreated  Actual syntax depends on type of constraint –PRIMARY KEY - just list type of constraint –UNIQUE - include column name –All others - reference constraint name

27. ALTER TABLE…DROP Syntax