UNIQUE constraint in SQL - Time & Space Complexity

Time Complexity: UNIQUE constraint

O(log n)

Understanding Time Complexity

When we add a UNIQUE constraint to a database column, the system automatically creates a unique index (typically a B-tree) to enforce uniqueness efficiently.

We want to understand how the time to check uniqueness grows as the table gets bigger.

Scenario Under Consideration

Analyze the time complexity of enforcing a UNIQUE constraint during data insertion.

INSERT INTO users (email) VALUES ('[e-mail]');
-- The database checks the unique index on 'email' for duplicates
-- If not found, it inserts the new row into both table and index
    

This code inserts a new email; the database uses the index to ensure uniqueness efficiently.

Identify Repeating Operations

The database traverses the unique index (B-tree) to check for existing values.

• Primary operation: Tree traversal/comparisons in the B-tree index.
• How many times: Proportional to the height of the tree, logarithmic in the number of rows.

How Execution Grows With Input

As the table grows, the index height increases logarithmically, keeping checks efficient.

Input Size (n)	Approx. Operations (log n)
10	~3-4 comparisons
100	~6-7 comparisons
1000	~9-10 comparisons

Pattern observation: The number of operations grows logarithmically with the number of rows.

Final Time Complexity

Time Complexity: O(log n)

This means the time to check and enforce uniqueness grows logarithmically as the table gets bigger, thanks to the index.

Common Mistake

[X] Wrong: "The UNIQUE check scans all rows linearly, so O(n)."

[OK] Correct: UNIQUE constraints automatically use an index (e.g., B-tree), enabling O(log n) lookups and inserts.

Interview Connect

Understanding how constraints and indexes affect performance helps you design scalable databases and optimize queries.

Self-Check

"What if there was no index on the column? How would the time complexity change?"

Practice

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

easy

A. To automatically generate unique IDs

B. To allow NULL values in a column

C. To speed up query performance

D. To prevent duplicate values in a column or group of columns

Solution

1. Step 1: Understand the UNIQUE constraint and compare to options

   The UNIQUE constraint ensures that all values in a column or a set of columns are different from each other. The other options describe different SQL features, but only "To prevent duplicate values in a column or group of columns" correctly describes the UNIQUE constraint.

2. Final Answer:

   To prevent duplicate values in a column or group of columns -> Option D

3. Quick Check:

   UNIQUE constraint = prevent duplicates [OK]

Hint: UNIQUE means no two rows can have same value [OK]

Common Mistakes:

• Confusing UNIQUE with PRIMARY KEY
• Thinking UNIQUE speeds up queries
• Assuming UNIQUE allows duplicates

2. Which of the following is the correct syntax to add a UNIQUE constraint to an existing column email in a table users?

easy

A. ALTER TABLE users ADD UNIQUE email;

B. ALTER TABLE users ADD CONSTRAINT UNIQUE email;

C. ALTER TABLE users ADD UNIQUE (email);

D. ALTER TABLE users ADD UNIQUE CONSTRAINT (email);

Solution

1. Step 1: Recall ALTER TABLE syntax for UNIQUE and check options

   The correct syntax to add a UNIQUE constraint is: ALTER TABLE table_name ADD UNIQUE (column_name); "ALTER TABLE users ADD UNIQUE (email);" matches exactly. The other options misuse keywords or omit parentheses.

2. Final Answer:

   ALTER TABLE users ADD UNIQUE (email); -> Option C

3. Quick Check:

   ALTER TABLE ADD UNIQUE (column) syntax = ALTER TABLE users ADD UNIQUE (email); [OK]

Hint: Use ADD UNIQUE (column) with parentheses [OK]

Common Mistakes:

• Omitting parentheses around column name
• Using ADD CONSTRAINT UNIQUE without naming constraint
• Misplacing keywords in ALTER TABLE

3. Given the table products with a UNIQUE constraint on product_code, what happens when you run this SQL?

INSERT INTO products (product_code, name) VALUES ('X123', 'Item A');
INSERT INTO products (product_code, name) VALUES ('X123', 'Item B');

medium

A. The second insert fails with a UNIQUE constraint violation error

B. The second insert overwrites the first row

C. Both rows are inserted successfully

D. The database ignores the second insert silently

Solution

1. Step 1: Analyze UNIQUE constraint effect on the insert statements

   The UNIQUE constraint on product_code means no two rows can have the same product_code value. The first insert adds 'X123' successfully. The second insert tries to add the same product_code 'X123', which violates the UNIQUE constraint, causing an error.

2. Final Answer:

   The second insert fails with a UNIQUE constraint violation error -> Option A

3. Quick Check:

   Duplicate insert on UNIQUE column = error [OK]

Hint: Duplicate UNIQUE values cause insert errors [OK]

Common Mistakes:

• Assuming duplicates overwrite existing rows
• Thinking duplicates are ignored silently
• Believing both inserts succeed

4. You tried to add a UNIQUE constraint on column username but got an error. What is the most likely cause?

medium

A. The column username is not indexed

B. The column username contains duplicate values already

C. UNIQUE constraints cannot be added after table creation

D. The table does not exist

Solution

1. Step 1: Understand UNIQUE addition failure and evaluate causes

   Adding a UNIQUE constraint fails if existing data violates uniqueness (duplicates exist). "The column username contains duplicate values already" identifies the cause. A non-existent table is unrelated (assuming it exists); UNIQUE can be added later; indexing is irrelevant or automatic.

2. Final Answer:

   The column username contains duplicate values already -> Option B

3. Quick Check:

   Duplicates prevent adding UNIQUE constraint [OK]

Hint: Check for duplicates before adding UNIQUE [OK]

Common Mistakes:

• Assuming UNIQUE can't be added after creation
• Ignoring existing duplicate data
• Confusing indexing with UNIQUE constraint

5. You want to ensure that the combination of first_name and last_name in the employees table is unique, but duplicates are allowed in each column individually. Which SQL statement correctly enforces this?

hard

A. ALTER TABLE employees ADD UNIQUE (first_name, last_name);

B. ALTER TABLE employees ADD UNIQUE (first_name); ALTER TABLE employees ADD UNIQUE (last_name);

C. ALTER TABLE employees ADD UNIQUE first_name, last_name;

D. ALTER TABLE employees ADD UNIQUE CONSTRAINT employees_unique (first_name);

Solution

1. Step 1: Understand multi-column UNIQUE and analyze options

   A UNIQUE constraint on multiple columns ensures the combination is unique (individual duplicates allowed). "ALTER TABLE employees ADD UNIQUE (first_name, last_name);" correctly adds UNIQUE on the pair. Adding UNIQUE separately disallows individual duplicates; other syntax is wrong or ignores one column.

2. Final Answer:

   ALTER TABLE employees ADD UNIQUE (first_name, last_name); -> Option A

3. Quick Check:

   Multi-column UNIQUE = unique pairs [OK]

Hint: Use UNIQUE on multiple columns for combined uniqueness [OK]

Common Mistakes:

• Adding UNIQUE on each column separately
• Using incorrect syntax without parentheses
• Ignoring the need for combined uniqueness