keys uniquely identify records;Attributes represent data characteristics, classified as simple or composite, with associated data types. Constraints maintain data integrity

1. Keys , Attributes and Constraints

2. Keys
• Keys play an important role in the relational database.
• It is used to uniquely identify any record or row of data from
the table. It is also used to establish and identify relationships
between tables.
• Types of keys:
• Primary key
• It is the first key used to identify one and only one instance of an
entity uniquely. An entity can contain multiple keys, as we saw in
the PERSON table. The key which is most suitable from those lists
becomes a primary key.

4. Super Key
Super key is an attribute set that can uniquely identify a tuple. A
super key is a superset of a candidate key.

5. Candidate key
• A candidate key is an attribute or set of attributes that can uniquely
identify a tuple.
• Except for the primary key, the remaining attributes are considered a
candidate key. The candidate keys are as strong as the primary key.
• For example: In the EMPLOYEE table, id is best suited for the
primary key. The rest of the attributes, like SSN, Passport_Number,
License_Number, etc., are considered a candidate key.

7. Foreign key
• Foreign keys are the column of the table used to point to the primary
key of another table.
• Every employee works in a specific department in a company, and
employee and department are two different entities. So we can't store
the department's information in the employee table. That's why we link
these two tables through the primary key of one table.
• We add the primary key of the DEPARTMENT table, Department_Id,
as a new attribute in the EMPLOYEE table.
• In the EMPLOYEE table, Department_Id is the foreign key, and both
the tables are related.

9. Alternate key
• There may be one or more attributes or a combination of attributes that
uniquely identify each tuple in a relation.
• These attributes or combinations of the attributes are called the
candidate keys.
• One key is chosen as the primary key from these candidate keys, and
the remaining candidate key, if it exists, is termed the alternate key.

11. Composite key
• Whenever a primary key consists of more than one attribute, it is
known as a composite key. This key is also known as Concatenated
Key.

13. Artificial key
• The key created using arbitrarily assigned data are known as artificial
keys. These keys are created when a primary key is large and complex
and has no relationship with many other relations. The data values of
the artificial keys are usually numbered in a serial order.

14. Atrributes
Types of Attributes
Simple Attribute
Simple attributes are atomic values, which cannot be divided
further.
Attributes are the properties which define the entity type. For
example, Roll_No, Name, DOB, Age, Address, Mobile_No are the
attributes which defines entity type Student. In ER diagram,
attribute is represented by an oval.
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15. Key Attribute
◦ The attribute which uniquely identifies each entity in the entity set is called
key attribute. For example, Roll_No will be unique for each student. In ER
diagram, key attribute is represented by an oval with underlying lines.
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16. Composite Attribute
◦ An attribute composed of many other attribute is called as composite
attribute. For example, Address attribute of student Entity type
consists of Street, City, State, and Country. In ER diagram, composite
attribute is represented by an oval comprising of ovals.
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17. Multivalued Attribute
◦ An attribute consisting more than one value for a given entity. For
example, Phone_No (can be more than one for a given student). In
ER diagram, multivalued attribute is represented by double oval
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18. Derived Attribute
◦ An attribute which can be derived from other attributes of the entity
type is known as derived attribute. e.g.; Age (can be derived from
DOB). In ER diagram, derived attribute is represented by dashed oval
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19. Constraints in DBMS
• Constraints enforce limits to the data or type of data that can be
inserted/updated/deleted from a table. The whole purpose of constraints is
to maintain the data integrity during an update/delete/insert into a table. In
this tutorial we will learn several types of constraints that can be created in
RDBMS.
Types of constraints
• NOT NULL
• UNIQUE
• DEFAULT
• CHECK
• Key Constraints – PRIMARY KEY, FOREIGN KEY
• Domain constraints
• Mapping constraints

20. NOTNULL
NOT NULL constraint makes sure that a column does not hold
NULL value. When we don’t provide value for a particular
column while inserting a record into a table, it takes NULL value
by default. By specifying NULL constraint, we can be sure that a
particular column(s) cannot have NULL values.

21. UNIQUE:
• UNIQUE Constraint enforces a column or set of columns to have
unique values. If a column has a unique constraint, it means that
particular column cannot have duplicate values in a table.
DEFAULT:
• The DEFAULT constraint provides a default value to a column when
there is no value provided while inserting a record into a table.
CHECK:
• This constraint is used for specifying range of values for a particular
column of a table. When this constraint is being set on a column, it
ensures that the specified column must have the value falling in the
specified range.