The relation schema describes set of fields (Column heads/Attributes).

The three-schema approach provides for three types of schemas with schema techniques based on formal language descriptions:
→External schema for user views
→Conceptual schema integrates external schemata
→Internal schema that defines physical storage structures

Types of Attributes
Simple attribute − Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits.
Composite attribute − Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name.
Derived attribute − Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived. For another example, age can be derived from data_of_birth.
Single-value attribute − Single-value attributes contain single value. For example − Social_Security_Number.
Multi-value attribute − Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.
Key is an attribute or collection of attributes that uniquely identifies an entity among entity set.