What happens if you try to insert a duplicate value into a UNIQUE key column?

Difficulty: 4/10

UNIQUE constraints, error handling, transaction behavior

What Happens When You Insert a Duplicate into a UNIQUE Key?

A UNIQUE key ensures that all values in the key column (or column group) remain distinct. If you insert a duplicate value, MySQL rejects the operation and raises an error because the uniqueness rule is violated.

1. How UNIQUE Key Enforces Uniqueness

1
MySQL checks the UNIQUE index before inserting or updating a row.
2
If the value already exists, MySQL blocks the operation.
3
This prevents duplicate values and ensures data integrity at the key level.

Example: UNIQUE Key Definition

Attempting Duplicate Insert

The second insert fails with a duplicate key error.

MySQL Error

2. Additional Points About UNIQUE Keys

1
UNIQUE key columns can store multiple NULL values because NULL is not considered equal to any value.
2
In a composite UNIQUE key, the combination must be unique even if individual columns repeat.

Scenario Questions

0-2 years experience

2-5 years experience

1During a recent feature rollout you start seeing duplicate‑key errors in the logs. Walk me through how you'd investigate the root cause.

Follow-up Questions