What happens if you try to insert a duplicate value into a UNIQUE key column?
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
MySQL checks the UNIQUE index before inserting or updating a row.
If the value already exists, MySQL blocks the operation.
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
UNIQUE key columns can store multiple NULL values because NULL is not considered equal to any value.
In a composite UNIQUE key, the combination must be unique even if individual columns repeat.