What types of data can be used as keys in data structures or databases?

In data structures and databases, data types that possess properties of uniqueness and immutability are usually used as keys. Here are the main types of data that can be used:

• Integer types (int) — often used as identifiers.
• Strings (string) — for example, logins, emails, or other unique text values.
• UUID (Universally Unique Identifier) — unique identifiers, often used for keys.
• Tuples (tuple) — in programming languages, for example, in Python, tuples of immutable elements can be used as keys in dictionaries.

It is important that the key:

• Is unique — there should be no two identical keys.
• Is immutable — so that the key does not change after creation, otherwise it will violate data integrity.

For example, in Python, dictionaries only allow immutable types as keys:

my_dict = {}
my_dict[(1, 2)] = 'value'  # tuple as key
my_dict['key'] = 'value'      # string as key