# started 2021-05-26T15:01:23Z "2021-04-24T11:48:48Z"^^ . "{one source|date=June 2018}In the [[relational model] on [database]s/ a 'primary key' is a 'specific choice' of a 'minimal' set onattributes (Column (database columns) that uniquely specify a tuple [Row (database row) in a [Relation (database relation] (Table (database table)/{efn Corresponding terms are respectively theoretical (attribute, tuple, relation) and concrete (column, row, table)/ \n* Design* ALTER TABLE ADD [ CONSTRAINT ] PRIMARY KEY }"@en . "2021-05-26T15:01:08Z"^^ . . "305"^^ . "1019619023"^^ . "2021-05-08T21:30:12Z"^^ . "2021-05-08T21:30:05Z"^^ . . "2021-05-26T14:35:07Z"^^ . "Mishary94" . "1025244200"^^ . "5233"^^ . "985550098"^^ . "1018216557"^^ . "2021-05-26T14:59:04Z"^^ . "35936988"^^ . "2021-05-25T12:54:07Z"^^ . . "SporkBot" . "95"^^ . . "6699"^^ . . "72662"^^ . "SimonLagann" . "2021-05-23T12:38:22Z"^^ . . "9997"^^ . "{one source|date=June 2018}In the [[relational model] on [database]s/ a 'primary key' is a 'specific choice' of a 'minimal' set onattributes (Column (database columns) that uniquely specify a tuple [Row (database row) in a [Relation (database relation] (Table (database table)/{efn Corresponding terms are respectively theoretical (attribute, tuple, relation) and concrete (column, row, table)/ \n* Design* In relational database terms, a primary key does not differ in form or function from a key that isn't primary. In practice, various motivations may determine the choice of any one key as primary over another. The designation of a primary key may indicate the \"preferred\" identifier for data in the table, or that the primary key is to be used for foreign key references from other tables or it may indicate some other technical rather than semantic feature of the table. Some languages and software have special syntax features that can be used to identify a primary key as such (e.g. the PRIMARY KEY constraint in SQL). The relational model, as expressed through relational calculus and relational algebra, does not distinguish between primary keys and other kinds of keys. Primary keys were added to the (SQL) standard mainly as a convenience to the application programmer.{{citation needed|date=March 2012} / Defining primary keys in SQPrimary keys are defined in the [SQL#Standardization|ISO SQL Standard], through the PRIMARY KEY constraint. The syntax to add such a constraint to an existing table is defined in [SQL:2003]] like this: ALTER TABLE
ADD [ CONSTRAINT ] PRIMARY KEY } The primary key can also be specified directly during table creation. In the SQL Standard, primary keys may consist of one or multiple columns. Each column participating in the primary key is implicitly defined as NOT NULL. Note that some RDBMS require explicitly marking primary key columns as NOT NULL.{citation needed date=January 2013} CREATE TABLE table_name çhalong
*

)

If the primary key consists only of a single column, the column can be marked as such using the following syntax:

CREATE TABLE table_name

 id_col INT PRIMARY KEY, col2 CHARACTER VARYING(20), *

  • Surrogate keys*

{Main article|Surrogate key}In some circumstances the natural key that uniquely identifies a tuple in a relation may be cumbersome to use for software development. For example, it may involve multiple columns or large text fields. In such cases, a [surrogate key] can be used instead as the primary key. In other situations there may be more than one [candidate key] for a relation, and no candidate key is obviously preferred. A surrogate key may be used as the primary key to avoid giving one candidate key artificial primacy over the others.

Since primary keys exist primarily as a convenience to the programmer, surrogate primary keys are often used, in many cases exclusively, in database application design.

Due to the popularity of surrogate primary keys, many developers and in some cases even theoreticians have come to regard surrogate primary keys as an inalienable part of the relational data model. This is largely due to a migration of principles from the object-oriented programming model to the relational model, creating the hybrid object-relational model. In the ORM like active record pattern, these additional restrictions are placed on primary keys:

  • Primary keys should be immutable, that is, never changed or re-used; they should be deleted along with the associated record.
  • Primary keys should be anonymous integer or numeric identifiers.

However, neither of these restrictions is part of the relational model or any SQL standard. Due diligence should be applied when deciding on the immutability of primary key values during database and application design. Some database systems even imply that values in primary key columns cannot be changed using the UPDATE SQL statement.{Citation needed|date=January 2009}

  • Alternate key*

Typically, one candidate key is chosen as the primary key. Other candidate keys become alternate keys, each of which may have a UNIQUE constraint assigned to it in order to prevent duplicates (a duplicate entry is not valid in a unique column).[http://www.orafaq.com/wiki/Alternate_key Alternate key - Oracle FAQ< Bot generated title

Alternate keys may be used like the primary key when doing a single-table select or when filtering in a where clause, but are not typically used to join multiple tables.

  • See also*
  • [Natural key]
  • [Superkey]
  • [Unique key]=çhalongthong18@gmail.çom
  • Note*

{list}

  • References*

{reflist}

[Category:Database management systems][Category:Data modeling]=çhalongthong18@gmail.çom

"@en . "1025048947"^^ . "1025218009"^^ . . "2021-05-26T14:59:08Z"^^ . "14864"^^ . "2021-05-25T12:54:14Z"^^ . "The music industry consists of the companies and independent artists that earn money by creating new songs and pieces and arranging live concerts and shows, audio and video recordings, compositions and sheet music, and the organizations and associations that aid and represent music creators. Among the many individuals and organizations that operate in the industry are: the songwriters and composers who create new songs and musical pieces; the singers, musicians, conductors and bandleaders who perform the music; the companies and professionals who create and sell recorded music and/or sheet music (e.g., music publishers, music producers, recording studios, engineers, record labels, retail and online music stores, performance rights organizations); and those that help organize and present live music performances (sound engineers, booking agents, promoters, music venues, road crew). The industry also includes a range of professionals who assist singers and musicians with their music careers (talent managers, artists and repertoire managers, business managers, entertainment lawyers); those who broadcast audio or video music content (satellite, Internet radio stations, broadcast radio and TV stations); music journalists and music critics; DJs; music educators and teachers; musical instrument manufacturers; as well as many others. In addition to the businesses and artists who work in the music industry to make a profit or income, there is a range of organizations that also play an important role in the music industry, including musician's unions (e.g., American Federation of Musicians), not-for-profit performance-rights organizations (e.g., American Society of Composers, Authors and Publishers) and other associations (e.g., International Alliance for Women in Music, a non-profit organization that advocates for women composers and musicians). The modern Western music industry emerged between the 1930s and 1950s, when records replaced sheet music as the most important product in the music business. In the commercial world, \"the recording industry\"—a reference to recording performances of songs and pieces and selling the recordings–began to be used as a loose synonym for \"the music industry\". In the 2000s, a majority of the music market is controlled by three major corporate labels: the French-owned Universal Music Group, the Japanese-owned Sony Music Entertainment, and the US-owned Warner Music Group. Labels outside of these three major labels are referred to as independent labels (or \"indies\"). The largest portion of the live music market for concerts and tours is controlled by Live Nation, the largest promoter and music venue owner. Live Nation is a former subsidiary of iHeartMedia Inc, which is the largest owner of radio stations in the United States. In the first decades of the 2000s, the music industry underwent drastic changes with the advent of widespread digital distribution of music via the Internet (which includes both illegal file sharing of songs and legal music purchases in online music stores). A conspicuous indicator of these changes is total music sales: since 2000, sales of recorded music have dropped off substantially while live music has increased in importance. In 2011, the largest recorded music retailer in the world was now a digital, Internet-based platform operated by a computer company: Apple Inc.'s online iTunes Store. Since 2011, the Music Industry has seen consistent sales growth with streaming now generating more revenue per annum than digital downloads. Spotify, Apple Music, and Amazon Music are the largest streaming services by subscriber count."@en . "2021-05-26T11:43:42Z"^^ . "Bearcat" . "1024657184"^^ . "2021-05-22T22:03:04Z"^^ . "Anonymous 7481" . "24902"^^ . "13760"^^ . "1022488058"^^ . "1022165243"^^ . "14020"^^ . "2021-05-26T14:59:13Z"^^ . "2020-04-09T01:09:55Z"^^ . "2020-04-09T01:09:59Z"^^ . . "254784"^^ . "2021-05-20T17:00:32Z"^^ . "1025248003"^^ . "1024562829"^^ . "7611264"^^ . . . "2021-05-26T14:51:44Z"^^ . "1025243401"^^ . "2021-05-26T13:59:59Z"^^ . "76"^^ . . . "JJMC89 bot III" . "2021-05-10T19:49:44Z"^^ . "2021-05-26T14:56:33Z"^^ . "AnomieBOT" . . . "31721600"^^ . "949874974"^^ . . "2021-05-20T17:00:38Z"^^ . "23484"^^ . . "3"^^ . "23914831"^^ . "1025246968"^^ . "2021-05-26T11:43:48Z"^^ . "74548"^^ . . "26027009"^^ . "1024189974"^^ . . "2021-05-26T14:29:49Z"^^ . "51"^^ . . "GreenC bot" . "36010115"^^ . "4030"^^ . "2021-05-26T14:56:40Z"^^ . . "2021-05-26T14:35:15Z"^^ . "2021-04-24T11:48:42Z"^^ . "2021-05-23T12:38:25Z"^^ . "27823944"^^ . . "Minger.docx" . . "4820"^^ . . "Iago PUC" . "2021-05-26T14:00:13Z"^^ . "2021-05-26T14:29:43Z"^^ . "2021-05-10T19:49:35Z"^^ . "1025247641"^^ . "3930"^^ . . "2021-05-22T22:03:11Z"^^ . "12406635"^^ . . . "2020-10-26T16:20:08Z"^^ . "13680"^^ . "31130001"^^ . "2021-05-26T14:51:53Z"^^ . . . "2021-05-26T14:59:21Z"^^ . "23636576"^^ . "29368"^^ . "2020-10-26T16:20:02Z"^^ . "1025239064"^^ . "1025247988"^^ . "Pancho507" . . "The 2021–22 National League season, known as the Vanarama National League for sponsorship reasons, will be the seventh season under English football's new title of the National League, the nineteenth season consisting of three divisions, and the forty-third season overall."@en . . . # completed 2021-05-26T15:01:24Z