Join

Join operator *

The Join operator A * B combines the matching information in A and B. The result contains all matching combinations of entities from both arguments.

Principles of joins

1. The operands A and B must be join-compatible.
2. The primary key of the result is the union of the primary keys of the operands.

Examples of joins

Example 1 : When the operands have no common attributes, the result is the cross product -- all combinations of entities.

Example 2 : When the operands have common attributes, only entities with matching values are kept.

Example 3 : Joining on secondary attribute.

Properties of join

1. When A and B have the same attributes, the join A * B becomes equivalent to the set intersection A ∩ B. Hence, DataJoint does not need a separate intersection operator.

2. Commutativity: A * B is equivalent to B * A.

3. Associativity: (A * B) * C is equivalent to A * (B * C).