CHECK constraint in SQL - Time & Space Complexity

Time Complexity: CHECK constraint

O(n)

Understanding Time Complexity

When we use a CHECK constraint in SQL, the database checks if data meets certain rules before saving it.

We want to know how the time to check data grows as we add more rows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

CREATE TABLE Employees (
  ID INT PRIMARY KEY,
  Age INT,
  Salary DECIMAL(10,2),
  CHECK (Age >= 18 AND Salary >= 0)
);

INSERT INTO Employees (ID, Age, Salary) VALUES (1, 25, 50000);
INSERT INTO Employees (ID, Age, Salary) VALUES (2, 17, 40000); -- This will fail

This code creates a table with a CHECK constraint to ensure Age is at least 18 and Salary is not negative.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The database checks the constraint for each new row inserted or updated.
• How many times: Once per row operation, so the check runs for every row added or changed.

How Execution Grows With Input

Each time we add a row, the database runs the CHECK condition once.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of checks grows directly with the number of rows inserted or updated.

Final Time Complexity

Time Complexity: O(n)

This means the time to check constraints grows in a straight line as you add more rows.

Common Mistake

[X] Wrong: "The CHECK constraint runs once for the whole table regardless of rows."

[OK] Correct: The database checks each row individually when it is inserted or updated, so the time grows with the number of rows.

Interview Connect

Understanding how constraints affect performance helps you design databases that keep data correct without slowing down too much.

Self-Check

"What if we added a CHECK constraint that involves a subquery? How would the time complexity change?"

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