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Alternate key
Candidate key
Compound key
Primary key
Super key
Foreign key
Basically they are the constraints you can attach on table level as well as field level of the table.
Unique key constraint is added to any field means the field should contain only unique value, it can allow users to insert the same value again for that field in another record.
Not Null constraint, if it is added to the field in table, than you should for sure had to provide value for that field, otherwise it throws an error being a non null field
Primary key is nothing but unique and not null, combination of above two. It is the main key one rdbms table should have for the performance of data retrieval or access for indexing purpose., but not must for every table.
foregin key- it is a relational key between two different tables for the purpose of normalisation to avoid redundancy of data.
etc..
