Data definition language


A data definition language or
 data description language
 (DDL) is a syntax similar to a computer programming 
language for defining
 data structures, especially
database schemas.
Many data description languages use a declarative syntax to define columns and data types. Structured query language (e.g., SQL), however, uses a collection of imperative verbs whose effect is to modify the schema of the database by adding, changing, or deleting definitions of tables or other elements. These statements can be freely mixed with other SQL statements, making the DDL not a separate language.

CREATE statementEdit

The create command is used to establish a new database, table, index, or stored procedure.
The CREATE statement in SQL creates a component in a relational database management system (RDBMS). In the SQL 1992 specification, the types of components that can be created are schemas, tables, views, domains, character sets, collations, translations, and assertions. Many implementations extend the syntax to allow creation of additional elements, such as indexes and user profiles. Some systems, such as PostgreSQL and SQL Server, allow CREATE, and other DDL commands, inside a database transaction and thus they may be rolled back.

CREATE TABLE statementEdit

A commonly used CREATE command is the CRE
The column definitions are:
  • A comma-separated list consisting of any of the following
  • Column definitiona 
  • Primary key definition: PRIMARY KEY 
  • Constraints: 
  • RDBMS specific functionality
An example statement to create a table named employees with a few 
Some forms of CREATE TABLE DDL may incorporate DML (data manipulation language)-like constructs, such as the CREATE TABLE AS SELECT (CTAS) syntax of SQL.

DROP statementEdit

The DROP statement destroys an existing database, table, index, or view.
A DROP statement in SQL removes a component from a relational database management system (RDBMS). The types of objects that can be dropped depends on which RDBMS is being used, but most support the dropping of tables, users, and databases. Some systems (such as PostgreSQL) allow DROP and other DDL commands to occur inside of a transaction and thus be rolled back. The typical usage is simply:
For example, the command to drop a table named employees is:
The DROP statement is distinct from the DELETE and TRUNCATE statements, in that DELETE and TRUNCATE do not remove the table itself. For example, a DELETE statement might delete some (or all) data from a table while leaving the table itself in the database, whereas a DROP statement removes the entire table from the database.

ALTER statementEdit

The ALTER statement modifies an existing database object.
An ALTER statement in SQL changes the properties of an object inside of a relational database management system (RDBMS). The types of objects that can be altered depends on which RDBMS is being used.
For example, the command to add (then remove) a column named bubbles for an existing table named sink is:

RENAME statementEdit

The RENAME statement is used to rename a database.

TRUNCATE statementEdit

The TRUNCATE statement is used to delete all data from a table.

Referential integrity statementsEdit

Another type of DDL sentence in SQL is used to define referential integrity relationships, usually implemented as primary key and foreign key tags in some columns of the tables. These two statements can be included in a CREATE TABLE or an ALTER TABLE sentence.

Comments

Post a Comment

8368456574

Popular posts from this blog

KBC 2017: Rules Season 9 KBC Rules

Image
KBC 2017: Rules Season 9 KBC 2017 Rules Season 9 –  Kaun Banega Crorepati Kaun Banega Crorepati, being India’s top notch favourite game show, have certain rules and regulations which one has to abide by in order to enter the show and meet the host. KBC Season 9 also has certain rules which when followed will give you entry to the show. Following are some of the rules a contestant must abide by KBC Game Rules: All contestants of this show must be above 18 years of age as this isn’t a kid’s show. All the contestants strictly needs to be a citizen of India. All the contestants must not be a participant of KBC former Seasons’. The contestants must fall under Income Tax Act, 1961. The contestants must not be employees of the broadcaster, production company or prize providers. Before the main game, all the contestants will have to participate in the Fastest Finger Contest, where you will be given a question and you have to select the right option in the shortest time....
Read more

history of the Caspian sea

Image
History Edit The earliest hominid remains found around the Caspian Sea are from  Dmanisi  dating back to around 1.8 Ma and yielded a number of skeletal remains of  Homo erectus  or  Homo ergaster . More later evidence for human occupation of the region come from a number of caves in  Georgia  and Azerbaijan such as Kudaro and  Azykh Caves . There is evidence for  Lower Palaeolithic  human occupation south of the Caspian from western Alburz. These are Ganj Par and  Darband Cave  sites. Neanderthal  remains also have been discovered at a cave site in Georgia. Discoveries in the Huto cave and the adjacent Kamarband cave, near the town of  Behshahr ,  Mazandaran  south of the Caspian in Iran, suggest human habitation of the area as early as 11,000 years ago. [ The Caspian area is rich in energy resources. Oil wells were being dug in the region as early as the 10th century to reach o...
Read more