A table in MySQL can have multiple UNIQUE keys, and each one enforces uniqueness on different columns or column combinations. This improves data integrity but also has performance and storage implications.

1. Ensures Multiple Sets of Values Are Unique

1
Each UNIQUE key enforces its own independent uniqueness rule.
2
Example: email must be unique AND username must be unique in the same table.
3
This allows multiple business rules for avoiding duplicates.

2. Each UNIQUE Key Creates an Index

1
Every UNIQUE constraint automatically creates a UNIQUE index.
2
More indexes → more disk usage.
3
More indexes → slower INSERT, UPDATE, DELETE operations because MySQL must update all related indexes.

3. Read Performance May Improve

1
Queries filtering by unique columns use UNIQUE indexes, improving lookup speed.
2
SELECT queries benefit from having more indexed columns to search with.

4. Composite UNIQUE Keys and Overlapping Columns

1
Unique keys can be single-column or multi-column.
2
Overlapping unique indexes (e.g., UNIQUE(a), UNIQUE(a,b)) increase storage and maintenance cost.
3
MySQL does not automatically optimize or merge redundant unique indexes.

5. Can Affect Foreign Key Relationships

1
A foreign key can reference any UNIQUE key (not only the primary key).
2
Having multiple unique keys gives flexibility for relational design.

In summary, multiple UNIQUE keys improve data integrity and query performance but increase index maintenance overhead and storage usage.