Databases

Imagine a school library club that keeps one list of borrowers on paper, another list of books, and another of fines. The same member’s name and class are copied into every list. This causes problems: the same fact can be written differently in two lists (inconsistency: the same fact is not the same everywhere), a change must be made many times, and mistakes creep in (the data stops being correct and valid, which is a loss of integrity). A database fixes this by keeping data in tables with a fixed structure. A table has rows, called records, and columns, called fields. A primary key is a field with a different value in every row, such as MemberID, so that a record can be found without doubt. Names are not good keys because two members may share a name. There are several database models: hierarchical (a tree: each item has one parent), network (items can have many links) and relational (separate tables linked by a field they share). The relational model is the most common. A database management system (DBMS) is the program that stores the data, controls who may read or change it, and keeps it safe. In a relational database each fact is stored once, in the right table, and other tables refer to it by its key.