CHECK constraint in SQL

Introduction

A CHECK constraint helps make sure the data in a table follows simple rules. It stops wrong or unexpected data from being saved.

When you want to make sure a person's age is always 0 or more.

When you want to ensure a product's price is never negative.

When you want to allow only certain values in a column, like 'Male' or 'Female' for gender.

When you want to keep dates in a valid range, like a start date before an end date.

When you want to prevent invalid status values in an order, like only 'Pending', 'Shipped', or 'Cancelled'.

Syntax

SQL

CREATE TABLE table_name (
  column_name datatype CHECK (condition),
  ...
);

-- Or add CHECK constraint after table creation:
ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK (condition);

The condition inside CHECK must be something that returns true or false.

If the condition is false, the database will not allow the data to be saved.

Examples

This makes sure the Age is 18 or older.

SQL

CREATE TABLE Employees (
  ID INT,
  Age INT CHECK (Age >= 18)
);

This prevents negative prices.

SQL

CREATE TABLE Products (
  ProductID INT,
  Price DECIMAL CHECK (Price >= 0)
);

This limits the Status column to only three allowed values.

SQL

ALTER TABLE Orders
ADD CONSTRAINT chk_status CHECK (Status IN ('Pending', 'Shipped', 'Cancelled'));

Sample Program

The first insert works because 85 is between 0 and 100. The second insert fails because 105 is outside the allowed range.

SQL

CREATE TABLE Students (
  StudentID INT,
  Name VARCHAR(50),
  Grade INT CHECK (Grade BETWEEN 0 AND 100)
);

INSERT INTO Students (StudentID, Name, Grade) VALUES (1, 'Alice', 85);
INSERT INTO Students (StudentID, Name, Grade) VALUES (2, 'Bob', 105);

Important Notes

CHECK constraints help keep your data clean and reliable.

They are checked every time you insert or update data.

Not all database systems support complex CHECK constraints equally.

Summary

CHECK constraints enforce simple rules on data in a table.

They prevent invalid data from being saved.

You can add them when creating a table or later with ALTER TABLE.

Practice

1. What is the main purpose of a CHECK constraint in SQL?

easy

A. To automatically generate unique IDs

B. To create a backup of the table data

C. To speed up query performance

D. To enforce rules on column values to prevent invalid data

Solution

1. Step 1: Understand what a CHECK constraint does

   A CHECK constraint sets a rule that data must follow when inserted or updated in a table.

2. Step 2: Identify the purpose from the options

   Only To enforce rules on column values to prevent invalid data describes enforcing rules on data to prevent invalid entries.

3. Final Answer:

   To enforce rules on column values to prevent invalid data -> Option D

4. Quick Check:

   CHECK constraint = enforce data rules [OK]

Hint: CHECK constraints prevent bad data from entering tables [OK]

Common Mistakes:

• Confusing CHECK with indexing or keys
• Thinking CHECK creates backups
• Assuming CHECK improves speed

2. Which of the following is the correct syntax to add a CHECK constraint on a column age to allow only values greater than 18?

easy

A. ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18);

B. ALTER TABLE users CHECK (age > 18);

C. ALTER TABLE users ADD CHECK age > 18;

D. ALTER TABLE users ADD CONSTRAINT chk_age CHECK age > 18;

Solution

1. Step 1: Recall correct syntax for adding CHECK constraint

   The correct syntax uses ADD CONSTRAINT with a name, then CHECK with parentheses around the condition.

2. Step 2: Compare options to syntax

   ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); matches the syntax: ADD CONSTRAINT name CHECK (condition). Options A and C miss the constraint name or parentheses; D misses parentheses.

3. Final Answer:

   ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); -> Option A

4. Quick Check:

   ADD CONSTRAINT name CHECK (condition) = ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); [OK]

Hint: Use ADD CONSTRAINT name CHECK (condition) syntax [OK]

Common Mistakes:

• Omitting parentheses around condition
• Not naming the constraint
• Writing condition outside CHECK()

3. Given the table creation:

CREATE TABLE products (
  id INT,
  price DECIMAL(5,2),
  CONSTRAINT chk_price CHECK (price >= 0)
);

What happens if you run:

INSERT INTO products (id, price) VALUES (1, -10.00);

medium

A. The row is inserted successfully

B. The price is automatically set to 0

C. The insert fails due to CHECK constraint violation

D. The insert causes a syntax error

Solution

1. Step 1: Understand the CHECK constraint on price

   The constraint requires price to be greater than or equal to 0.

2. Step 2: Analyze the insert statement

   The insert tries to add price = -10.00, which violates the CHECK condition.

3. Final Answer:

   The insert fails due to CHECK constraint violation -> Option C

4. Quick Check:

   price < 0 violates CHECK = insert fails [OK]

Hint: CHECK rejects rows violating the condition [OK]

Common Mistakes:

• Assuming negative values are accepted
• Thinking CHECK auto-corrects values
• Confusing CHECK violation with syntax error

4. You try to create a table with this statement:

CREATE TABLE employees (
  id INT,
  salary INT CHECK salary > 0
);

But get a syntax error. What is the likely fix?

medium

A. Add a semicolon after salary

B. Add parentheses around the CHECK condition: CHECK (salary > 0)

C. Change salary to VARCHAR

D. Remove the CHECK constraint entirely

Solution

1. Step 1: Identify syntax for inline CHECK constraints

   Inline CHECK constraints require the condition to be inside parentheses.

2. Step 2: Check the given statement

   The statement misses parentheses around salary > 0, causing syntax error.

3. Final Answer:

   Add parentheses around the CHECK condition: CHECK (salary > 0) -> Option B

4. Quick Check:

   Inline CHECK needs parentheses [OK]

Hint: Always use parentheses around CHECK conditions [OK]

Common Mistakes:

• Omitting parentheses in inline CHECK
• Changing data type unnecessarily
• Ignoring syntax error details

5. You want to ensure a discount column in orders table is between 0 and 50 inclusive. Which CHECK constraint correctly enforces this?

hard

A. CHECK (discount >= 0 AND discount <= 50)

B. CHECK (discount > 0 AND discount < 50)

C. CHECK (discount BETWEEN 1 AND 50)

D. CHECK (discount != 0 OR discount != 50)

Solution

1. Step 1: Understand the required range including boundaries

   The discount must be at least 0 and at most 50, so boundaries are included.

2. Step 2: Evaluate each option

   B: CHECK (discount > 0 AND discount < 50) excludes 0 and 50 (strict inequalities). A: CHECK (discount >= 0 AND discount <= 50) includes boundaries with >= and <=. C: CHECK (discount BETWEEN 1 AND 50) excludes 0 (>=1 <=50). D: CHECK (discount != 0 OR discount != 50) is logically incorrect.

3. Final Answer:

   CHECK (discount >= 0 AND discount <= 50) -> Option A

4. Quick Check:

   Inclusive range needs >= and <= [OK]

Hint: Use >= and <= for inclusive CHECK ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using incorrect bounds with BETWEEN
• Logical errors with OR instead of AND