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Data definition language

Data definition language

(also DDL)

Data definition language definition

Data definition language is a subset of SQL (structured query language) that deals with the structure and schema of the database rather than the data itself. It gives commands to define, modify, or delete database structures, like tables, schemas, indexes, and constraints.

DDL does not handle individual data. Instead, it focuses on the overall schema and architecture of the database. When a DDL statement is executed, it updates the database’s system catalog, which stores the schema of the database, but no data is manipulated.

Here are key DDL commands:

  • CREATE: Used to establish new database objects.
  • ALTER: Modifies existing database structures.
  • DROP: Removes database structures.
  • TRUNCATE: Removes all records from a table but maintains its structure.

See also: database replication

DDL advantages

  • DDL enables the systematic design and modification of database schema.
  • It ensures uniformity in database structure definitions.
  • With proper precautions, DDL allows users to change the database structure without affecting the data within.

DDL disadvantages

  • Some DDL operations, such as DROP, can’t be rolled back and may lead to data loss.
  • Executing certain DDL statements, especially on large datasets or complex structures, can take a lot of time.
  • Altering a table structure can lead to errors if other objects or applications in the database depend on it.

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