Q.A table has two candidate keys, one of which is chosen as the primary key. How many alternate keys does this table have ? (A) 0 (B) 1 (C) 2 (D) 3

Database Schema: The Blueprint of a Database

Imagine you are organising a large library. You don't just pile books randomly. First, you decide: What information will we keep about each book? Title, author, ISBN, shelf number. Then you decide: How are these pieces of information related? A book has one shelf, but a shelf holds many books. You also decide rules: Every book must have an ISBN; no two books can share the same ISBN.

That plan — the structure you design before you put a single book on the shelf — is the schema.

The Precise Meaning

A database schema is the formal, logical description of how data is organised in a database. It defines:

• What tables exist (like "Books", "Members", "Loans")
• What columns each table has (e.g., BookID, Title, Author, YearPublished)
• What type of data each column holds (text, number, date, etc.)
• What rules or constraints apply (e.g., "Every loan must refer to an existing member")
• How tables relate to one another (e.g., "A member can borrow many books, but each loan record belongs to one member")

Think of the schema as the skeleton of the database. The actual data — the rows you fill in later — is the flesh and blood. The schema stays mostly unchanged; the data changes constantly.

Why the Schema Matters

Without a schema, data becomes a mess. You might store a customer's phone number in the "Name" column, or lose track of which orders belong to which customer. The schema prevents this by enforcing consistency and integrity.

Consider a bank. The schema says: "Every transaction must have an amount, a date, and a valid account number." If someone tries to enter a transaction without an account number, the database refuses — because the schema says so. This is not a software bug; it is a deliberate design choice built into the structure.

Key Components of a Schema

• Tables – The main containers. Each table represents one kind of thing (Students, Products, Invoices).
• Columns (Fields) – The attributes or properties of that thing (StudentName, DateOfBirth, Email).
• Data Types – What kind of value each column holds (text, whole number, decimal, date).
• Constraints – Rules that data must obey:
  • Primary Key – A unique identifier for each row (like a roll number). No two rows can share it.
  • Foreign Key – A column that links to another table's primary key, creating a relationship.
  • Not Null – A column that must always have a value.
  • Unique – A column where every value must be different.
• Relationships – How tables connect: one-to-one, one-to-many, many-to-many. …