Constraints are rules applied on table columns to control what kind of data can be stored.
They maintain the accuracy, validity, and reliability of data.

The four most important constraints for exams are:

NOT NULL
UNIQUE
CHECK
DEFAULT

⭐ 1. NOT NULL Constraint

NOT NULL ensures that a column cannot have NULL (empty/blank) values.

✔ Important Points:

Value must be provided for this column.
Prevents missing or incomplete data.
Every row must have a valid value in this column.

✔ Example:

AccountNo	Name (NOT NULL)
101	Raj
102	Asha
103	NULL ❌ (Not allowed)

The Name field cannot be empty.

✔ Use in Banking:

Customer Name
Aadhaar Number
Account Number
Transaction Amount

These cannot be NULL.

⭐ 2. UNIQUE Constraint

UNIQUE ensures that all values in the column are different—no duplicates.

✔ Important Points:

Allows only one NULL value (unlike Primary Key).
A table can have multiple UNIQUE constraints.
Ensures distinct values.

✔ Example:

Email (UNIQUE)
a@x.com
b@y.com
a@x.com ❌ (Not allowed)
NULL ✔ (Allowed)

✔ Use in Banking:

Email
Mobile number
PAN number
Aadhaar number (often unique or PK)

👉 UNIQUE ≠ PRIMARY KEY

UNIQUE allows NULL
PK does not allow NULL

⭐ 3. CHECK Constraint

CHECK ensures that the value in a column meets a specific condition.

✔ Important Points:

Used for validation rules.
Helps maintain logical correctness.

✔ Examples:

🔹 Example 1: Age must be above 18

Age CHECK (Age >= 18)

Age
25 ✔
17 ❌ (Not allowed)

🔹 Example 2: Transaction amount must be positive

Amount CHECK (Amount > 0)

Amount
500 ✔
-20 ❌

🔹 Example 3: Gender must be ‘M’, ‘F’, or ‘O’

Gender CHECK (Gender IN ('M','F','O'))

✔ Use in Banking:

Minimum balance rules
Age restrictions
Transaction limits
Salary > 0
Loan tenure between 1 and 30 years

⭐ 4. DEFAULT Constraint

DEFAULT assigns a predefined value** to a column when the user does not provide any value.

✔ Important Points:

Automatically inserts a value.
Prevents NULL values (if default is meaningful).
Useful for commonly used values.

✔ Examples:

🔹 Example 1: Default country = ‘India’

Country DEFAULT 'India'

Name	Country
Raj	India (default)
Asha	USA (given)

🔹 Example 2: Account Status = ‘Active’

Status DEFAULT 'Active'

🔹 Example 3: Current timestamp (auto date)

CreatedAt DEFAULT CURRENT_TIMESTAMP

✔ Use in Banking:

New account status → “ACTIVE”
Opening balance → 0
Country → “India”
Transaction date → system date
Loan status → “PENDING”

📌 Most Important Comparison Table

Constraint	Purpose	Allows NULL?	Ensures	Examples
NOT NULL	Prevents NULL values	❌ No	Data must exist	Customer Name, Account Number
UNIQUE	No duplicates	✔ Yes (only one)	Distinct values	Email, PAN
CHECK	Validates data based on a condition	✔ Yes	Data correctness	Age ≥ 18, Amount > 0
DEFAULT	Auto value if none provided	✔ Yes	Predefined values	Status = Active

🔥 Most Important Differences

✔ NOT NULL vs UNIQUE

NOT NULL	UNIQUE
Cannot be empty	Must be different
Allows duplicates	Cannot have duplicates
Focus: completeness	Focus: uniqueness

✔ UNIQUE vs PRIMARY KEY

UNIQUE	PRIMARY KEY
Allows NULL	Zero NULL
Table can have MANY	Table has only ONE
Ensures uniqueness	Ensures unique identity

✔ CHECK vs DEFAULT

CHECK	DEFAULT
Validates values	Inserts default value
Prevents invalid data	Reduces manual entry

🧠 One-Liner

NOT NULL: Value cannot be left empty.
UNIQUE: All values must be different.
CHECK: Value must satisfy a condition.
DEFAULT: System automatically inserts a default value.

🎯 Simple Real

✔ Opening a bank account

CustomerName → NOT NULL
PAN → UNIQUE
Age → CHECK (Age >= 18)
AccountStatus → DEFAULT = ‘ACTIVE’

✔ ATM Transactions

Amount → CHECK (Amount > 0)
Timestamp → DEFAULT CURRENT_TIMESTAMP

✔ Customer Contact

Email → UNIQUE
Mobile → UNIQUE

✅ MCQs on NOT NULL, UNIQUE, CHECK, DEFAULT Constraints

🔶 SECTION A — NOT NULL Constraint (Very Important)

1. NOT NULL constraint ensures that:

A. Column cannot have duplicate values
B. Column cannot have empty values
C. Column cannot store numeric data
D. Column must be a key
Answer: B

2. A column defined as NOT NULL:

A. Can have exactly one NULL
B. Cannot have any NULL
C. Can only store numbers
D. Must be a Primary Key
Answer: B

3. NOT NULL is used to maintain:

A. Entity Integrity
B. Referential Integrity
C. Domain Integrity
D. Logical Integrity
Answer: A

4. Which of the following MUST be NOT NULL in banking databases?

A. Customer Name
B. Loan Amount
C. Account Number
D. All of the above
Answer: D

5. NOT NULL constraint is applied:

A. At table level
B. At column level
C. On entire database
D. On index only
Answer: B

🔶 SECTION B — UNIQUE Constraint

6. UNIQUE constraint ensures:

A. No NULL values allowed
B. No duplicate values allowed
C. Only numeric values allowed
D. Only one row allowed
Answer: B

7. How many NULL values can a UNIQUE column have?

A. None
B. One
C. Unlimited
D. Depends on DBMS
Answer: B (Standard SQL allows only 1 NULL)

8. A table can have:

A. Only one UNIQUE constraint
B. Only one Primary Key
C. Multiple UNIQUE constraints
D. No UNIQUE constraint
Answer: C

9. Difference between PRIMARY KEY and UNIQUE:

A. PK allows NULL but UNIQUE does not
B. UNIQUE allows NULL but PK does not
C. PK allows duplicates
D. Both allow NULL
Answer: B

10. UNIQUE ensures:

A. Logical correctness
B. Uniqueness of data
C. Data encryption
D. Physical storage
Answer: B

🔶 SECTION C — CHECK Constraint

11. CHECK constraint is used to:

A. Assign default values
B. Validate data based on a condition
C. Ensure uniqueness
D. Remove NULL values
Answer: B

12. Which constraint enforces Age > 18 rule?

A. NOT NULL
B. UNIQUE
C. CHECK
D. DEFAULT
Answer: C

13. CHECK(Field >= 0) ensures:

A. Field cannot be negative
B. Field must be unique
C. Field must be a key
D. Field is always NULL
Answer: A

14. CHECK is used for:

A. Verifying range
B. Verifying pattern
C. Verifying conditions
D. All of the above
Answer: D

15. CHECK constraint helps maintain:

A. Domain integrity
B. Backup
C. Encryption
D. Indexing
Answer: A

🔶 SECTION D — DEFAULT Constraint

16. DEFAULT constraint:

A. Inserts automatic value when no value is provided
B. Removes NULL values
C. Ensures uniqueness
D. Creates relationship
Answer: A

17. DEFAULT ensures:

A. Auto-filling of values
B. Primary Key creation
C. Sorting
D. Indexing
Answer: A

18. DEFAULT is helpful for:

A. Auto date entries
B. Auto status entries
C. Auto country names
D. All of the above
Answer: D

19. DEFAULT value is applied when:

A. User enters a NULL
B. User enters nothing
C. User enters invalid data
D. User enters duplicate values
Answer: B

20. DEFAULT can be used with:

A. Strings
B. Numbers
C. Dates
D. All of the above
Answer: D

🔶 SECTION E — COMBINED CONSTRAINT

21. Which constraint prevents NULL values?

A. UNIQUE
B. NOT NULL
C. CHECK
D. DEFAULT
Answer: B

22. Which constraint can enforce conditions like “Salary > 0”?

A. CHECK
B. UNIQUE
C. NOT NULL
D. DEFAULT
Answer: A

23. Which constraint ensures “Email must be unique”?

A. CHECK
B. DEFAULT
C. UNIQUE
D. NOT NULL
Answer: C

24. Which constraint auto-fills “Active” when no value is provided?

A. UNIQUE
B. DEFAULT
C. CHECK
D. NOT NULL
Answer: B

25. NOT NULL + UNIQUE together =

A. Primary Key
B. Foreign Key
C. Composite Key
D. Default Key
Answer: A

🔶 SECTION F — TABLE BASED

26. Consider Table:

PAN (UNIQUE)	Age (CHECK Age > 18)	Country (DEFAULT ‘India’)

Which entry is INVALID?
A. PAN = ABC12, Age = 20, Country = India
B. PAN = XYZ99, Age = 17, Country = India
C. PAN = LMN55, Age = 25, Country = NULL
D. PAN = NULL, Age = 26, Country = India
Answer: B (Age < 18 violates CHECK)

27. Which row violates UNIQUE constraint?

Email (UNIQUE)
a@gmail.com
b@gmail.com
a@gmail.com

A. First row
B. Second row
C. Third row
D. None
Answer: C

28. Which row will get DEFAULT automatically?

Status DEFAULT ‘Active’
NULL
‘Blocked’
‘Active’

A. First row
B. Second row
C. Third row
Answer: A

🔶 SECTION G — BANKING SCENARIO

29. Minimum balance rule (Balance >= 1000) uses:

A. DEFAULT
B. UNIQUE
C. CHECK
D. NOT NULL
Answer: C

30. Customer ID must be filled during account creation. Which constraint applies?

A. UNIQUE
B. NOT NULL
C. CHECK
D. DEFAULT
Answer: B

31. Every account number must be different:

A. CHECK
B. DEFAULT
C. UNIQUE
D. NOT NULL
Answer: C

32. Account status should be ‘Active’ if no value is provided.

A. UNIQUE
B. NOT NULL
C. DEFAULT
D. CHECK
Answer: C

🔶 SECTION H

33. A column defined as UNIQUE and NOT NULL becomes:

A. Foreign Key
B. Primary Key
C. Super Key
D. Composite Key
Answer: B

34. CHECK constraint is evaluated:

A. Before data insertion
B. After data insertion
C. Only during deletion
D. Only during updates
Answer: A

35. In a table, DEFAULT is applied:

A. Before constraints
B. After constraints
C. After NOT NULL check but before INSERT
D. None
Answer: C

36. CHECK can be applied at:

A. Column level
B. Table level
C. Both
D. None
Answer: C

37. NOT NULL is stored in:

A. Data dictionary
B. Index
C. Buffer
D. Log file
Answer: A

🔶 SECTION I — TRUE / FALSE

38. UNIQUE allows multiple NULL values.

False (Only one allowed)

39. DEFAULT value is used only when NULL is provided.

False (Used when user provides nothing)

40. CHECK ensures logical correctness.

True

41. NOT NULL ensures uniqueness.

False

🔶 SECTION J — TRICKY

42. Which constraint ensures that a column must ALWAYS have a value?

A. UNIQUE
B. CHECK
C. DEFAULT
D. NOT NULL
Answer: D

43. DEFAULT is ignored when:

A. User enters NULL explicitly
B. User enters a value
C. User enters duplicate
D. User enters nothing
Answer: B

44. CHECK (Salary <= 50000) rejects:

A. Salary 30000
B. Salary 50000
C. Salary 75000
D. Salary 0
Answer: C

45. A column defined as CHECK (Gender IN (‘M’,’F’,’O’)) rejects:

A. ‘M’
B. ‘A’
C. ‘F’
D. ‘O’
Answer: B

🔶 SECTION K — ADVANCED

46. DEFAULT constraint reduces:

A. Null entries
B. Redundant entries
C. Invalid entries
D. Duplicate entries
Answer: A

47. Which constraint does NOT help with data correctness?

A. CHECK
B. UNIQUE
C. DEFAULT
D. NOT NULL
Answer: C (DEFAULT doesn’t validate correctness)

48. UNIQUE constraint can be applied on:

A. One column only
B. Multiple columns
C. Whole table
D. Only numeric fields
Answer: B

49. NOT NULL + UNIQUE together enforce:

A. Domain integrity
B. Primary Key rules
C. Foreign Key
D. Default value
Answer: B

50. CHECK constraint is evaluated on:

A. INSERT only
B. UPDATE only
C. Both INSERT & UPDATE
D. DELETE only
Answer: C

Constraints: NOT NULL, UNIQUE, CHECK, DEFAULT

⭐ 1. NOT NULL Constraint

✔ Important Points:

✔ Example:

✔ Use in Banking:

⭐ 2. UNIQUE Constraint

✔ Important Points:

✔ Example:

✔ Use in Banking:

⭐ 3. CHECK Constraint

✔ Important Points:

✔ Examples:

🔹 Example 1: Age must be above 18

🔹 Example 2: Transaction amount must be positive

🔹 Example 3: Gender must be ‘M’, ‘F’, or ‘O’

✔ Use in Banking:

⭐ 4. DEFAULT Constraint

✔ Important Points:

✔ Examples:

🔹 Example 1: Default country = ‘India’

🔹 Example 2: Account Status = ‘Active’

🔹 Example 3: Current timestamp (auto date)

✔ Use in Banking:

📌 Most Important Comparison Table

🔥 Most Important Differences

✔ NOT NULL vs UNIQUE

✔ UNIQUE vs PRIMARY KEY

✔ CHECK vs DEFAULT

🧠 One-Liner

🎯 Simple Real

✔ Opening a bank account

✔ ATM Transactions

✔ Customer Contact

✅ MCQs on NOT NULL, UNIQUE, CHECK, DEFAULT Constraints

🔶 SECTION A — NOT NULL Constraint (Very Important)

1. NOT NULL constraint ensures that:

2. A column defined as NOT NULL:

3. NOT NULL is used to maintain:

4. Which of the following MUST be NOT NULL in banking databases?

5. NOT NULL constraint is applied:

🔶 SECTION B — UNIQUE Constraint

6. UNIQUE constraint ensures:

7. How many NULL values can a UNIQUE column have?

8. A table can have:

9. Difference between PRIMARY KEY and UNIQUE:

10. UNIQUE ensures:

🔶 SECTION C — CHECK Constraint

11. CHECK constraint is used to:

12. Which constraint enforces Age > 18 rule?

13. CHECK(Field >= 0) ensures:

14. CHECK is used for:

15. CHECK constraint helps maintain:

🔶 SECTION D — DEFAULT Constraint

16. DEFAULT constraint:

17. DEFAULT ensures:

18. DEFAULT is helpful for:

19. DEFAULT value is applied when:

20. DEFAULT can be used with:

🔶 SECTION E — COMBINED CONSTRAINT

21. Which constraint prevents NULL values?

22. Which constraint can enforce conditions like “Salary > 0”?

23. Which constraint ensures “Email must be unique”?

24. Which constraint auto-fills “Active” when no value is provided?

25. NOT NULL + UNIQUE together =

🔶 SECTION F — TABLE BASED

26. Consider Table:

27. Which row violates UNIQUE constraint?

28. Which row will get DEFAULT automatically?

🔶 SECTION G — BANKING SCENARIO

29. Minimum balance rule (Balance >= 1000) uses:

30. Customer ID must be filled during account creation. Which constraint applies?

31. Every account number must be different:

32. Account status should be ‘Active’ if no value is provided.

🔶 SECTION H

33. A column defined as UNIQUE and NOT NULL becomes:

34. CHECK constraint is evaluated:

35. In a table, DEFAULT is applied:

36. CHECK can be applied at:

37. NOT NULL is stored in:

🔶 SECTION I — TRUE / FALSE