LU02c - SQL and its sub-types

SQL (Structured Query Language) is a standard language for managing and manipulating relational databases. It provides a set of commands for defining, querying, manipulating, and controlling data.

In detail, SQL consists of four sub-languages, each of which is responsible for one specific set of operations:

• DDL = Data Definition Language: DDL commands are used to create, modify, and delete database structures, e.g. to create database instances and tables, to alter table structures or or to ftop databases and tables.
• DQL = Data Query Language: DQL commands are used to retrieve data from the database, e.g. to select table contents with and without filters and in a specific order.
• DML = Data Manipulation Language: DML commands are used to insert, update and delete data in tables. DML commands are therefore used to manage table content: to insert new content or to update and delete using certain filters.
• DCL = Data Control Language: DCL commands are used to control access to the database. This includes the management of authorisations and the administration of user profiles. The central questions here are: „Which user is allowed to access which database object?“

CRUD operations is to be applied in all three layers of a three-layers-web-architecure (presentation layer, logic layer/server, data layer). Within the data layer the CRUD operations are realised as follows:

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