Introduction To SQL
Database Management Systems
Databases have been in use since the earliest days of electronic computing, but the vast majority of these were custom programs written to access custom databases. Unlike modern systems which can be applied to widely different databases and needs, these systems were tightly linked to the database in order to gain speed at the expense of flexibility.
Codd's 12 Rules
Codd's 12 rules are a set of thirteen rules proposed by Edgar F. 'Ted' Codd, a pioneer of the relational model for databases, designed to define what is required from a database management system in order for it to be considered relational, i.e., a RDBMS. Codd produced these rules as part of a personal campaign to prevent his vision of the relational database being diluted, as database vendors scrambled in the early 1980s to repackage existing products with a relational veneer. Rule 12 was particularly designed to counter such a positioning. In fact, however, the rules are so strict that even systems whose only interface is the SQL language fail on some of the criteria.
Database Users and Schemas
Each database has a list of usernames. To access a database, a user must use a database application and attempt a connection with a valid username of the database. Each username has an associated password to prevent unauthorized use.
Databases In SQL
The software used to store, manage, query, and retrieve data stored in a relational database is called a relational database management system (RDBMS). The RDBMS provides an interface between users and applications and the database, as well as administrative functions for managing data storage, access, and performance.
Select Statements
To fully use the power of a relational database we need to communicate with it. The ultimate communication would be to turn to our computer and say, in a clear, distinct voice, `Show me all the left-handed, brown- eyed bean counters who have worked for this company for at least 10 years.`A few of us may already be doing so (talking to our computer, not listing bean counters). Everyone else needs a more conventional way of retrieving information from the database.
Posted August 9, 2022 by Anusha ‐ 5 min read