This document discusses different types of constraints in SQL including primary keys, foreign keys, unique constraints, and check constraints. It provides the syntax for creating each constraint using CREATE TABLE and ALTER TABLE statements. Primary keys enforce uniqueness and can be defined to include one or multiple columns. Foreign keys enforce referential integrity by linking columns in one table to columns in another table. Unique constraints require that the values in a column or set of columns are unique. Check constraints specify conditions that each row must satisfy.

1. LECTURE -4
S T R U C T U R E D Q U E R Y L A N G U A G E
( K E Y S & C O N S T R A I N T S )
D e l i v e r e d B y :
M r . S h u b h a m S h u k l a

2. PRIMARY KEYS
Note
• In Oracle, a primary key can not contain more than 32 columns.
• A primary key can be defined in either a CREATE TABLE statement or an ALTER TABLE
statement.
Create Primary Key - Using CREATE TABLE statement
Syntax
CREATE TABLE table_name
(
column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ... column_n)
);

3. Example:
CREATE TABLE supplier
(
supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id, supplier_name)
);

4. Create Primary Key - Using ALTER TABLE statement
Syntax
ALTER TABLE table_name
ADD CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ...
column_n);
Example
ALTER TABLE supplier
ADD CONSTRAINT supplier_pk PRIMARY KEY (supplier_id);

5. • Drop Primary Key
Syntax
• ALTER TABLE table_name DROP CONSTRAINT constraint_name;
Example:
ALTER TABLE supplier DROP CONSTRAINT supplier_pk;

6. Disable Primary Key
Syntax
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;
Example
ALTER TABLE supplier
DISABLE CONSTRAINT supplier_pk;
Enable Primary Key
Syntax
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
Example
ALTER TABLE supplier
ENABLE CONSTRAINT supplier_pk;

7. FOREIGN KEYS
• A foreign key is a way to enforce referential integrity within your Oracle
database. A foreign key means that values in one table must also appear in
another table.
• The referenced table is called the parent table while the table with the foreign
key is called the child table. The foreign key in the child table will generally
reference a primary key in the parent table.

8. Using a CREATE TABLE statement
Syntax
CREATE TABLE table_name
(
column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT fk_column
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n)
);

9. Example
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
);

10.  In case of multiple column key:
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id, supplier_name)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
CONSTRAINT fk_supplier_comp
FOREIGN KEY (supplier_id, supplier_name)
REFERENCES supplier(supplier_id, supplier_name)
);

11. Using an ALTER TABLE statement
Syntax
ALTER TABLE table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n);
Example:
ALTER TABLE products
ADD CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id, supplier_name)
REFERENCES supplier(supplier_id, supplier_name);

12. Foreign Keys with Cascade Delete
Using a CREATE TABLE statement
Syntax
CREATE TABLE table_name
(
column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT fk_column
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n)
ON DELETE CASCADE
);

13. For example:
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id) );
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
ON DELETE CASCADE );

14. Using an ALTER TABLE statement
Syntax
ALTER TABLE table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n)
ON DELETE CASCADE;
For example:
ALTER TABLE products
ADD CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
ON DELETE CASCADE;

15. Foreign Keys with Set Null on Delete
A foreign key with "set null on delete" means that if a record in the parent table is deleted, then
the corresponding records in the child table will have the foreign key fields set to null. The
records in the child table will not be deleted.
Using a CREATE TABLE statement
Syntax
CREATE TABLE table_name
( column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT fk_column
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n)
ON DELETE SET NULL );

16. Example
CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10),
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
ON DELETE SET NULL
);

17. Using an ALTER TABLE statement
Syntax
ALTER TABLE table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n)
ON DELETE SET NULL;
Example
ALTER TABLE products
ADD CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
ON DELETE SET NULL;

18. • Drop a Foreign Key
Syntax
ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
ALTER TABLE products
DROP CONSTRAINT fk_supplier;

19. • Disable a foreign key
Syntax
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;
For Example:
ALTER TABLE products
DISABLE CONSTRAINT fk_supplier;
• Enable a foreign key
Syntax
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
For Example:
ALTER TABLE products
ENABLE CONSTRAINT fk_supplier;

20. UNIQUE CONSTRAINTS
• A unique constraint is a single field or combination of fields that uniquely defines a record.
Some of the fields can contain null values as long as the combination of values is unique.
 Using a CREATE TABLE statement
CREATE TABLE table_name
(
column1 datatype [ NULL | NOT NULL ],
column2 datatype [ NULL | NOT NULL ],
...
CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);

21. For example:
CREATE TABLE supplier
( supplier_id numeric(10) NOT NULL,
supplier_name varchar2(50) NOT NULL,
contact_name varchar2(50),
CONSTRAINT supplier_unique UNIQUE (supplier_id)
);

22. Using an ALTER TABLE statement
ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);
Example:
ALTER TABLE supplier
ADD CONSTRAINT supplier_unique UNIQUE (supplier_id);
Drop Unique Constraint
ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
Example
ALTER TABLE supplier
DROP CONSTRAINT supplier_unique;

23. Disable Unique Constraint
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;
Example:
ALTER TABLE supplier
DISABLE CONSTRAINT supplier_unique;
Enable Unique Constraint
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
Example:
ALTER TABLE supplier
ENABLE CONSTRAINT supplier_unique;

24. CHECK CONSTRAINTS
• A check constraint allows you to specify a condition on each row in a table.
Note
• A check constraint can NOT be defined on a SQL View.
• The check constraint defined on a table must refer to only columns in that table.
It can not refer to columns in other tables.
• A check constraint can NOT include a SQL Subquery.
• A check constraint can be defined in either a SQL CREATE TABLE
statement or a SQL ALTER TABLE statement.

25.  Using a CREATE TABLE statement
CREATE TABLE table_name
(
column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE]
);
Example
CREATE TABLE suppliers
(
supplier_id numeric(4),
supplier_name varchar2(50),
CONSTRAINT check_supplier_id
CHECK (supplier_id BETWEEN 100 and 9999)
);

26. Using an ALTER TABLE statement
ALTER TABLE table_name
ADD CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE];
Example
ALTER TABLE suppliers
ADD CONSTRAINT check_supplier_name
CHECK (supplier_name IN ('IBM', 'Microsoft', 'NVIDIA'));
Drop a Check Constraint
ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
Example
ALTER TABLE suppliers
DROP CONSTRAINT check_supplier_id;

27. Enable a Check Constraint
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
Example
ALTER TABLE suppliers
ENABLE CONSTRAINT check_supplier_id;
Disable a Check Constraint
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;
Example
ALTER TABLE suppliers
DISABLE CONSTRAINT check_supplier_id;

28. THANK YOU