Why CHECK constraint in SQL? - Purpose & Use Cases

The Big Idea

What if your database could catch mistakes before they happen, saving you hours of fixing errors?

The Scenario

Imagine you are managing a guest list for a party using a simple spreadsheet. You want to make sure that the age of each guest is always 18 or older, but you have to check every entry manually to catch mistakes.

The Problem

Manually checking each age is slow and easy to forget. Mistakes slip through, like entering 15 or 120 by accident. This causes confusion and errors later when you try to use the data.

The Solution

The CHECK constraint in a database automatically ensures that data meets rules like "age must be 18 or older". It stops wrong data from being saved, so you never have to hunt for errors later.

Before vs After

✗ Before

INSERT INTO guests (name, age) VALUES ('Alice', 15); -- No check, wrong data allowed

✓ After

CREATE TABLE guests (name VARCHAR(50), age INT CHECK (age >= 18));

What It Enables

It lets you trust your data is correct and consistent without extra work.

Real Life Example

In a hospital database, a CHECK constraint can ensure patient ages are realistic, preventing impossible values like negative numbers or ages over 150.

Key Takeaways

CHECK constraints automatically enforce rules on data.

They prevent invalid data from entering the database.

This saves time and avoids errors in data management.

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