Database management systems

A Database Management System (DBMS) is software to create and manage databases: users and programs can store, change and retrieve data through it. Examples (open source and commercial): MySQL, Oracle, PostgreSQL, SQL Server, Microsoft Access, MongoDB.

• A DBMS hides how data is actually stored and gives users an abstract view of the data. It is the interface between the database and its users.
• A database is a collection of related tables.

Turning the school's files into a database:

• SName is removed from ATTENDANCE, since the name can be found in STUDENT through the roll number.
• Guardian details move into a new GUARDIAN table, with a unique guardian ID (GUID), and STUDENT keeps only the GUID. Now each fact is stored once, which removes redundancy.

Key terms:

• Database schema: the design of the database: its tables, attributes, data types, constraints and relationships.
• Data constraint: a restriction on the values allowed, such as NOT NULL (a value must be given) or UNIQUE (no repeats). RollNumber can have both. Constraints keep data accurate and reliable.
• Meta-data (also called the data dictionary or catalog): the schema and constraints, stored by the DBMS: data about data.
• Database instance: the data in the database at a given moment. The instance changes as data is added or altered; the schema normally does not.
• Query: a request for information from the database.
• Data manipulation: insertion (adding records), deletion (removing them) and update (changing existing values).
• Database engine: the underlying set of programs the DBMS uses to create the database and handle queries.

Example (new): when a new student joins, adding her row to STUDENT is an insertion; when a family moves house, changing the address once in GUARDIAN is an update; and every student linked to that guardian now shows the new address automatically.