Monday, November 25, 2013

Junction table

From Wikipedia, the free encyclopedia
In database management systems following the relational model, a junction table is a database table that contains common fields from two or more other database tables within the same database. It is on the many side of a one-to-many relationship with each of the other tables. Junction tables are known under many names, among them cross-reference tablebridge table,join tablemap tableintersection tablelinking tablemany-to-many resolverlink tablepairing tablepivot tabletransition table, or association table.
Junction tables are employed when dealing with many-to-many relationships in a database. A practical use of a junction table would be to assign permissions to users. There can be multiple users, and each user can be assigned 0 or more permissions.
A visual depiction of the table schema described, with relationships indicated

    UserLogin VARCHAR(50) PRIMARY KEY,
    UserPassword VARCHAR(50) NOT NULL,
    UserName VARCHAR(50) NOT NULL
CREATE TABLE Permissions (
    PermissionKey VARCHAR(50) PRIMARY KEY,
    PermissionDescription VARCHAR(500) NOT NULL
-- This is the junction table.
CREATE TABLE UserPermissions (
    UserLogin VARCHAR(50) REFERENCES Users (UserLogin),
    PermissionKey VARCHAR(50) REFERENCES Permissions (PermissionKey),
    PRIMARY KEY (UserLogin, PermissionKey)

Using junction tables[edit]

SELECT-statement on a junction table usually involves joining the main table with the junction table:
JOIN UserPermissions USING (UserLogin);
This will return a list of all users and their permissions.
Inserting into a junction table involves two steps: first inserting into the main table (for example, a new User), then updating the junction table.
-- Creating a new User
INSERT INTO Users (UserLogin, UserPassword, UserName)
VALUES ('SomeUser', 'SecretPassword', 'UserName');
-- Creating a new Permission
INSERT INTO Permissions (PermissionKey, PermissionDescription)
VALUES ('TheKey', 'A key used for several permissions');
-- Finally, updating the junction
INSERT INTO UserPermissions (UserLogin, PermissionKey)
VALUES ('SomeUser', 'TheKey');
Using foreign keys, the database will automatically dereference the values of the UserPermissions table to their own table.

See also[edit]


oakleyses said...

louis vuitton handbags, oakley sunglasses, louboutin, longchamp outlet, nike shoes, louis vuitton outlet stores, chanel handbags, burberry outlet, prada outlet, jordan shoes, tiffany and co, michael kors outlet, tory burch outlet, louis vuitton outlet, longchamp handbags, nike free, true religion jeans, michael kors outlet, kate spade outlet, polo ralph lauren outlet, tiffany and co, prada handbags, polo ralph lauren outlet, michael kors outlet, michael kors outlet, longchamp handbags, oakley sunglasses, ray ban sunglasses, kate spade handbags, burberry outlet, louis vuitton outlet, louboutin outlet, louboutin, coach factory outlet, air max, air max, coach outlet, gucci outlet, christian louboutin shoes, michael kors outlet, coach purses, ray ban sunglasses, michael kors outlet, louis vuitton, coach outlet store online, true religion jeans, oakley sunglasses cheap

oakleyses said...

ralph lauren, lululemon, air max, hollister, north face, nike air max, polo lacoste, vanessa bruno, timberland, vans pas cher, louboutin, louis vuitton, oakley pas cher, air max pas cher, nike roshe run, air max, true religion outlet, barbour, sac longchamp, air force, hollister, sac louis vuitton, nike free, polo ralph lauren, nike trainers, louis vuitton uk, nike roshe, sac hermes, longchamp, michael kors, sac burberry, sac guess, mulberry, new balance pas cher, converse pas cher, sac louis vuitton, hogan outlet, nike tn, north face, true religion outlet, ray ban pas cher, michael kors, air jordan, nike blazer, nike free pas cher, michael kors pas cher, abercrombie and fitch, ray ban sunglasses

oakleyses said...

mac cosmetics, mont blanc, marc jacobs, canada goose outlet, nike huarache, vans shoes, soccer jerseys, hollister, giuseppe zanotti, beats by dre, abercrombie and fitch, longchamp, insanity workout, celine handbags, bottega veneta, ghd, nfl jerseys, north face outlet, chi flat iron, ugg boots, birkin bag, ugg australia, canada goose, herve leger, ugg pas cher, rolex watches, valentino shoes, canada goose uk, canada goose, ferragamo shoes, canada goose, ugg boots, uggs outlet, north face jackets, soccer shoes, asics running shoes, new balance shoes, p90x, lululemon outlet, canada goose jackets, mcm handbags, instyler, babyliss pro, ugg, wedding dresses, jimmy choo outlet, reebok outlet, nike roshe run

oakleyses said...

parajumpers, karen millen, air max, converse, pandora charms, moncler, louboutin, moncler, links of london, lancel, juicy couture outlet, oakley, hollister, pandora charms, supra shoes, thomas sabo, canada goose, gucci, wedding dresses, timberland boots, swarovski crystal, air max, coach outlet store online, moncler, ray ban, canada goose, moncler, ugg, louis vuitton, swarovski, hollister, montre homme, moncler, hollister clothing store, ralph lauren, rolex watches, moncler outlet, moncler, iphone 6 cases, baseball bats, juicy couture outlet, toms shoes, vans, pandora jewelry, ugg, converse shoes