Keys in SQL

1. Primary Key
2. Candidate Key
3. Foreign Key
4. Alternate Key
5. Composite or Compound Key
6. Unique Key

Candidate Key :

A Column with

• Unique values
• No null values

[A Table can have more than one column with above attributes. All these columns are candidate keys]

Primary Key:

A Column with

• Unique values
• No null values

[A Table can have more than one column with above attributes. But one of these column can be a primary key]

(Info: A table can have only one primary key but any number of candidate keys)

Alternate Key

When a table contains more than one candidate key (say 3 candidate keys), after selecting a column as a Primary key, other 2 columns are called as an Alternate Key

Unique Key:

A Column

• with Unique values
• can have null values

(Info: A table may not necessarily contain all the keys, but at least a primary key)

Composite Key:

Two or more columns collectively used to identify a row

Example: 

1. Identifying a student in a student table using his name and his father name

2. Identifying a student in a student table using his name, father name & Birth-date

Foreign Key:

• Presence of a column in a table which is a primary key for another table.
• This column is responsible for establishing relationship between two tables.