The document explains various types of keys in SQL, including primary, foreign, composite, candidate, alternate, and super keys, detailing their definitions, use cases, and syntactical representations. Primary keys uniquely identify records and can only exist once per table, while foreign keys establish relationships between tables. Composite keys combine multiple columns to ensure record uniqueness, and alternate and candidate keys can also uniquely identify records, with specific rules for their usage and null values.

1. Keys
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A key in SQL is a column or a combination
of columns used to uniquely identify each
row in a database table.
Keys help to keep data organized, ensure
data accuracy, and create relationships
between different tables.

2. Types of Keys
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2
Primary Key
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Foreign Key
3
4
Composite Key
Candidate Key
Alternate Key
Super Key
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6

3. Primary Key
• A primary key is a column that has a unique and not null value.
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• It uniquely identifies each record in a table.
• Each table can only have one primary key.
• It allows you to quickly find or reference specific records in a
table.

4. Syntax
Create Table Customers(
CustomerID int primary Key,
CustomerNames varchar(40),
Phonenumber varchar(40),
Email varchar(45)
)
Scenario
While Creating Table:
CREATE TABLE table_name (
column1 data_type, PRIMARY
KEY
column2 data_type;
)
Alter Table Customers
ADD Primary Key(CustomerID);
Scenario
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Adding a primary key to an
existing table:
ALTER TABLE table_name
ADD PRIMARY KEY
(column_name);

5. Syntax
Alter Table Customers(
Drop Primary key;
)
Scenario
Drop Primery Key from Table:
Alter TABLE table_name (
DROP PRIMARY KEY;
)
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6. Example scenario
Customers table:
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Products table: Orders table:
PRIMARY KEY PRIMARY KEY PRIMARY KEY

7. Foreign Key
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• A column or a set of columns in one table that refers to the primary key in another table.
• It establishes a relationship between two tables.
• A table can have multiple foreign keys, each linking to different tables.
• Foreign key columns can contain NULL ,meaning that not all records need to relate to
the referenced table.
• We can change the name of a foreign key column in SQL.
• We cannot add new values in a foreign key column that do not exist in the referenced
primary key column.

8. Syntax
CREATE TABLE suppliers (
supplier_id INT PRIMARY KEY,
ProductID int,
supplier_name VARCHAR(100),
supplier_contact VARCHAR(100),
foreign key (ProductID) references
Products(ProductID) );
Scenario
Alter Table suppliers
ADD Foreign Key(ProductID)
references Products(ProductID);
Scenario
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Adding a primary key to an existing table:
ALTER TABLE table_name
ADD FOREIGN KEY (foreign_key_column) REFERENCES
referenced_table(referenced_column);
While Creating Table:
CREATE TABLE table_name (
column1 data_type,
column2 data_type,
FOREIGN KEY(foreign_key_column)
REFERENCES referenced_table(referenced_column)
);

9. Syntax
Alter Table suppliers(
Drop Foreign key
fk_ProductID;
)
Scenario
Drop Foreign Key from Table:
Alter TABLE table_name (
DROP Foreign KEY foreign_key_name;
)
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10. Example scenario
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Products table: Suppliers table:
Foreign KEY
PRIMARY KEY

11. Example scenario
Customers table:
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Products table: Orders table:
PRIMARY KEY PRIMARY KEY FOREIGN
KEYS

12. • It is also a primary key that is consists of two or more columns used together
to uniquely identify a record in a table.
• Composite keys are used when a single column is not sufficient to ensure the
uniqueness of records.
• UNIQUE + NONUNIQUE = COMPOSITE KEY.
• The combination of values in these columns must be unique across the table.
• Composite key columns can not be NULL.
Composite Key
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13. Example scenario
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Products table: Suppliers table:
COMPOSITE KEY
UNIQUE+NONUNIQUE
ProductID+Price
COMPOSITE KEY
NONUNIQUE+UNIQUE
Supplier_name+supplier_contact

14. Example scenario
Customers table:
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Orders table:
COMPOSITE KEY
NONUNIQUE+UNIQUE
CustomerName+Phonenumber
COMPOSITE KEY
UNIQUE+NONUNIQUE
OrderID+OrderStatus

15. • It is not the primary key.
• It includes unique columns.
• An alternate key is a column or a set of columns in a database table that
can be used to uniquely identify records.
• A table can have multiple alternate keys. While there is only one primary
key, any number of alternate keys can exist.
• It can be NULL.
Alternate Key
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16. Example scenario
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Customers table: Suppliers table:
Alternate KEY
Alternate KEY

17. • It also include primary key column.
• It includes all the unique columns.
• A candidate key can consist of a single column or multiple columns
(composite candidate key).
• Candidate keys cannot contain NULL values. Every row must have a valid,
unique value for the candidate key.
Candidate Key
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18. Example scenario
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Customers table: Suppliers table:
CANDIDATE KEY
CANDIDATE KEY

19. • It is a set of key that contains all the keys in it.
• Every super key must have the ability to uniquely identify each row in a
table. This means that no two rows can have the same values in all the
columns that make up the super key.
• A super key can consist of a single column or multiple columns. For
example, a single column like customer_id or a combination of columns
like (first_name, last_name, date_of_birth) can form a super key.
Super Key
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20. Example scenario
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Customers table:
CustomerID
CustomerID+ CustomerNames
CustomerID+CustomerNames+Email
Email
CustomerID+Email