While creating a table, which constraint does not allow insertion of duplicate values in the table ? (A) UNIQU

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Q. While creating a table, which constraint does not allow insertion of duplicate values in the table ? (A) UNIQUE (B) DISTINCT (C) NOT NULL (D) HAVING

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Concept understanding — Unique Not Null

Unique Not Null: A Concept-First Explanation

Think about your Aadhaar number. Every citizen gets exactly one, and no two people share the same number. But here's the catch — you must have one. You cannot leave that field blank. That is the essence of Unique Not Null.

The Everyday Intuition

Imagine you are maintaining a class register. You want to record each student's roll number. Two things are obvious: every student must have a roll number (you cannot leave it empty), and no two students can share the same roll number. That is a Unique Not Null constraint in action.

Now contrast this with a student's phone number. Some students may not have one — that field can be left blank. And two siblings in the same class might share a phone number. So phone number is neither unique nor mandatory. The difference is intuitive once you see it.

The Precise Meaning

Unique means that within a given column (or set of columns), every value must be distinct from every other value. No duplicates allowed. Not Null means that the column cannot be left empty — it must contain a value. When you combine the two, you get a column where:

Every row must have a value (no blanks)
Every value must be different from every other value in that column

Important

Unique Not Null is not the same as a Primary Key, though they look similar. A table can have only one Primary Key, but it can have multiple columns with Unique Not Null. Also, a Primary Key automatically enforces both uniqueness and non-nullness — but the reverse is not true. A Unique Not Null column does not automatically become the table's primary identifier.

Why It Matters

In real-world data, certain pieces of information are both mandatory and naturally unique. Consider these examples:

Employee ID in a company database: every employee must have one, and no two employees can share the same ID
PAN Card number in a tax database: mandatory for taxpayers, and unique by law
Email address in a user registration system: every user must provide one, and no two accounts can share the same email

If you allowed duplicates in such columns, you would create confusion — two employees with the same ID, or two taxpayers with the same PAN. If you allowed blanks, you would have records that cannot be identified at all. …

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Concept: SQL table constraints that enforce data integrity rules at the column or table level.

When defining a table schema, constraints control what values can be inserted. The UNIQUE constraint explicitly prevents duplicate values in a column (or combination of columns), ensuring each entry is distinct. If you attempt to insert a row with a value that already exists in a UNIQUE column, the database rejects the operation. …

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A constraint is a rule enforced at the database level; UNIQUE is the constraint that prevents duplicate values in a column. The answer is (A).

When you design a database table, you want to enforce certain rules on what data can be stored. These rules are called constraints, and they live in the table definition itself — the database engine checks them automatically every time you try to insert or update a row.

The question asks which constraint blocks duplicate values. Let's see what each option actually does.

Understanding the options

UNIQUE — This is a table constraint. When you declare a column (or combination of columns) as UNIQUE, the database refuses any INSERT or UPDATE that would create a duplicate value in that column. For example:
```
CREATE TABLE students (
    roll_no INT UNIQUE,
    name VARCHAR(50)
);
```
Now you cannot have two students with the same roll_no. The database enforces this rule.
DISTINCT — This is not a constraint; it's a keyword used in SELECT queries to filter out duplicate rows from the result set. It doesn't prevent duplicates from being inserted — it only affects what you see when you retrieve data:
```
SELECT DISTINCT city FROM customers;
```
The table can still contain duplicate cities; DISTINCT just shows each city once in the output.
NOT NULL — This constraint ensures a column cannot contain NULL values. It has nothing to do with duplicates. You can have a column that is NOT NULL and still insert the same value multiple times:
```
CREATE TABLE products (
    product_id INT NOT NULL,
    name VARCHAR(100)
);
``` …
```

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Q.While creating a table, which constraint does not allow insertion of duplicate values in the table ? (A) UNIQUE (B) DISTINCT (C) NOT NULL (D) HAVING

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