1. CS122 Using Relational Databases and SQL
1/2/2019
CS122 Using Relational Databases and SQL
9. Constraints
Behzad Parviz
Department of Computer Science
California State University, Los Angeles

2. Outline Create and drop primary key constraints
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Outline
Create and drop primary key constraints
Create and drop unique constraints
Create and drop foreign key constraints
9. Constraints CS122_W2014

3. 1/2/2019
Constraints
A constraint is any kind of restriction that is placed on the data that is inserted into a table
Entity constraints ensure the value in a column meets some criteria compared to other rows in the table
Primary Key, Unique
Domain constraints ensure the value in a column meets some particular criteria without respect to any other row in the table
Default, Check
Referential integrity constraints ensure the value in a column matches a value in another column in a different table or (sometimes) the same table
Foreign Key
9. Constraints CS122_W2014

4. Constraint Naming Conventions
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Constraint Naming Conventions
Begin the constraints with two letters that indicate the type of constraint
(pk for primary key, fk for foreign key, and un for unique)
Follow that with the table name
If it is a unique constraint, follow the table name with the column being constrained
If it is a foreign key constraint, follow the table name with the name of the table it refers to
9. Constraints CS122_W2014

5. Primary Key Constraint With Alter Table
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Primary Key Constraint With Alter Table
Primary keys are unique identifiers for each record in a database table
Syntax:
ALTER TABLE tablename ADD PRIMARY KEY (fieldname1, fieldname2) Or
ALTER TABLE tablename ADD CONSTRAINT ConstraintName PRIMARY KEY (fieldname1, fieldname2)
9. Constraints CS122_W2014

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Example
Set the primary key of the tblContracts table to ArtistID and ContractDate
ALTER TABLE tblContracts ADD CONSTRAINT pk_tblContracts PRIMARY KEY (ArtistID, ContractDate)
9. Constraints CS122_W2014

7. Primary Key Constraint With Create Table
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Primary Key Constraint With Create Table
Two ways with Create Table
First form cannot create multi-field PK
First form does not allow for naming constraint
First way
Create Table tablename (
fieldname1 datatype null | not null Primary Key,
fieldname2 datatype null | not null )
9. Constraints CS122_W2014

8. Primary Key Constraint With Create Table (Cont.)
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Primary Key Constraint With Create Table (Cont.)
Second way
Create Table tablename (
Fieldname1 datatype null | not null,
Fieldname2 datatype null | not null,
Constraint constraintName Primary Key (fieldname1,fieldname2) )
9. Constraints CS122_W2014

9. 1/2/2019
Example
Create a table called tblContracts with the ArtistID from the Artists table and a ContractDate field. Set the primary key to ArtistID and ContractDate.
Create Table tblContracts (
ArtistID Integer Not Null,
ContractDate DateTime Not Null,
Constraint pk_tblcontracts Primary Key (ArtistID,ContractDate) )
9. Constraints CS122_W2014

10. Unique Constraint With Alter Table
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Unique Constraint With Alter Table
Sometimes we want to make sure a non-primary key column has unique values
Sometimes they are called alternate keys because they could be used as the primary key if you wished
Syntax
Alter Table tablename Add Constraint constraintname Unique (fieldname1, fieldname2)
9. Constraints CS122_W2014

11. Example Add a constraint to the Titles table so that UPC is unique.
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Example
Add a constraint to the Titles table so that UPC is unique.
Alter Table Titles Add Constraint un_titles_upc Unique (UPC)
Add a constraint to the Titles table so that the combination of ArtistID and Title is unique.
Alter Table Titles Add Constraint un_titles_artistid_title Unique (ArtistID, Title)
9. Constraints CS122_W2014

12. Unique Constraint With Create Table
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Unique Constraint With Create Table
Two ways with Create Table
First form cannot create multi-field constraint
First form does not allow for naming constraint
Syntax
Create Table tablename (
Fieldname1 datatype null | not null Primary key,
Fieldname2 datatype null | not null Unique )
9. Constraints CS122_W2014

13. Unique Constraint With Create Table (Cont.)
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Unique Constraint With Create Table (Cont.)
Second way: syntax
Create Table tablename (
Fieldname1 datatype null | not null,
Fieldname2 datatype null | not null,
Constraint constraintname Unique (fieldname1, fieldname2) )
9. Constraints CS122_W2014

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Example
Create a table called tblX with an integer ID field and an integer anotherfield. Both fields are not null. Make anotherfield unique.
Create Table tblX (
ID int Not Null,
anotherfield int Not Null Unique )
anotherfield int Not Null,
constraint un_tblx_anotherfield Unique(anotherfield)
9. Constraints CS122_W2014

15. Setting Default Constraints
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Setting Default Constraints
A default constraint automatically fills in a value if the user fails to supply a value.
They are only used with INSERTs - once a row exists in a table the default plays no part in setting the value
If the INSERT supplies a value for the column specified by a default, the INSERT’s value is used instead of the default
If the INSERT does not supply a value, the default value will be inserted.
9. Constraints CS122_W2014

16. Adding a default constraint with Alter table
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Adding a default constraint with Alter table
Syntax:
ALTER TABLE tablename
ALTER fieldname
SET DEFAULT value
Value must be a constant
9. Constraints CS122_W2014

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Example
Make a default entry of 0 for the RespParty field of XrefArtistsMembers
ALTER TABLE XrefArtistsMembers
ALTER RespParty
SET DEFAULT 0
9. Constraints CS122_W2014

18. Adding a default constraint with Create table
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Adding a default constraint with Create table
Syntax:
CREATE TABLE tablename(
Fieldname1 datatype,
Fieldname2 datatype DEFAULT value NULL|NOT NULL
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Example
Create a table called tblX with an integer ID field and integer another field. Another field should have a default of 0
CREATE TABLE tblX(
ID int,
Another int DEFAULT 0 NOT NULL )
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20. Foreign Key Constraints
Parent
Table
“One”
Primary Key
Foreign Key
Foreign Key constraint requires that every CD_ID in Child table matches existing CD_ID in Parent table
Child
Table
“Many”

21. Foreign key constraints
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Foreign key constraints
Parent table and child table
Parent table: the table whose primary key is involved in the relationship
Child table: the table whose foreign key is involved in the table
The purpose of foreign key constraints
Make sure that all foreign keys in the child table match the primary keys in the parent table
Which table to create the foreign key constraint?
The child table
9. Constraints CS122_W2014

22. Foreign Key Constraints With Create Table
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Foreign Key Constraints With Create Table
Syntax:
Create Table tablename (
Fieldname1 datatype null | not null,
Fieldname2 datatype null | not null,
Constraint constraintname Foreign Key (child_field1, child_field2) References parent_tablename (parent_field1, parent_field2) )
9. Constraints CS122_W2014

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Example
Create a table called Contracts with the ArtistID from the Artists table and a ContractDate field. Create a foreign key constraint between ArtistID and the ArtistID in the Artists table.
Create Table tblContracts (
ArtistID Integer Not Null,
ContractDate DateTime Not Null,
Constraint fk_tblcontracts_artists Foreign Key (ArtistID) References Artists (ArtistID) )
9. Constraints CS122_W2014

24. Foreign Key Implications
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Foreign Key Implications
Foreign Key constraint requires that every FK field in child table matches existing PK in Parent table
Constraint will not allow "orphan" records
Must delete child records before deleting related parent record
If updating or deleting PK value in parent table
Drop constraint before updating either parent or child
Or use Cascade Update or delete
9. Constraints CS122_W2014

25. Cascading Updates & Deletes
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Cascading Updates & Deletes
Parent
Table
“One”
Cascade Update would change all references in Child table when a PK is changed in Parent table
Cascade Delete would delete all related records in Child table when a record is deleted in Parent table
Avoids orphan records
MySQL supports cascade deletes with the inclusion of On Delete Cascade at the end of the constraint clause
Child
Table
“Many”
9. Constraints CS122_W2014

26. Dropping foreign key constraints
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Dropping foreign key constraints
Syntax:
Alter Table tablename Drop FOREIGN KEY foreign_key_name
Example: Drop the fk_tblcontracts_artsists on the tblContracts table.
Alter Table tblContracts Drop FOREIGN KEY fk_tblcontracts_artists
9. Constraints CS122_W2014

27. Dropping primary key constraints
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Dropping primary key constraints
Syntax:
Alter Table tablename Drop PRIMARY KEY
Example: Drop the primary key of the tblX table.
Alter Table tblX Drop PRIMARY KEY
9. Constraints CS122_W2014