Associative entity

An associative entity is a term used in relational and entity–relationship theory. A relational database requires the implementation of a base relation (or base table) to resolve many-to-many relationships. This kind of base relation is called an associative table.

As mentioned above, associative entities are implemented in a database structure using associative tables, which are tables that can contain references to columns from the same or different database tables within the same database.

An associative (or junction) table maps two or more tables together by referencing the primary keys of each data table. In effect, it contains a number of foreign keys, each in a many-to-one relationship from the junction table to the individual data tables. The PK of the associative table is typically composed of the FK columns themselves.

Associative tables are colloquially known under many names, including association table, bridge table, cross-reference table, crosswalk, intermediary table, intersection table, join table, junction table, link table, linking table, many-to-many resolver, map table, mapping table, pairing table, or transition table.

Using associative tables

An example of the practical use of an associative table would be to assign permissions to users. There can be multiple users, and each user can be assigned zero or more permissions. Individual permissions may be granted to one or more users. 

CREATE TABLE User (
    UserLogin varchar(50) PRIMARY KEY,
    UserPassword varchar(50) NOT NULL,
    UserName varchar(50) NOT NULL
)

CREATE TABLE Permission (
    PermissionKey varchar(50) PRIMARY KEY,
    PermissionDescription varchar(500) NOT NULL
)

-- This is the junction table.
CREATE TABLE UserPermission (
    UserLogin varchar(50) REFERENCES Users (UserLogin),
    PermissionKey varchar(50) REFERENCES Permission (PermissionKey),
    PRIMARY KEY (UserLogin, PermissionKey)
)

A SELECT-statement on a junction table usually involves joining the main table with the junction table:

SELECT * FROM User
JOIN UserPermission USING (UserLogin);

This will return a list of all users and their permissions.

Inserting into a junction table involves multiple steps: first inserting into the main table(s), then updating the junction table.

-- Creating a new User
INSERT INTO User (UserLogin, UserPassword, UserName)
VALUES ('SomeUser', 'SecretPassword', 'UserName');

-- Creating a new Permission
INSERT INTO Permission (PermissionKey, PermissionDescription)
VALUES ('TheKey', 'A key used for several permissions');

-- Finally, updating the junction
INSERT INTO UserPermission (UserLogin, PermissionKey)
VALUES ('SomeUser', 'TheKey');

Using foreign keys, the database will automatically dereference the values of the UserPermission table to their own tables.

See also

References

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