Compound key

composite keycomposite primary keyCompound
In database design, a composite key is a candidate key that consists of two or more attributes (table columns) that uniquely identify an entity occurrence (table row).wikipedia
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Candidate key

candidatekey candidateskeys
In database design, a composite key is a candidate key that consists of two or more attributes (table columns) that uniquely identify an entity occurrence (table row).

Unique key

keykeysprimary key
A compound key is a composite key for which each attribute that makes up the key is a simple (foreign) key in its own right.

Relational database

relational database management systemRDBMSrelational databases
Another common occurrence, especially in regard to N:M cardinality is the composite key.

Foreign key

Foreignforeign key constraintforeign key references
A compound key is a composite key for which each attribute that makes up the key is a simple (foreign) key in its own right.

Surrogate key

Surrogatesurrogate-key
Surrogate keys are also less expensive to join (fewer columns to compare) than compound keys.

Data modeling

data modellingmodelingmodeled
In database design, a composite key is a candidate key that consists of two or more attributes (table columns) that uniquely identify an entity occurrence (table row).

Elementary key normal form

EKNF
This happens when there is more than one unique compound key and they overlap.

Entity integrity

Morphologically, a composite primary key is in a "steady state": If it is reduced, PK will lose its property of identifying every row of its relation but if it is extended, PK will be redundant.

Database normalization

normalizationnormalizednormal form
The Book table has one candidate key, the compound key {Title, Format}.