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A database management system, or DBMS, is a software program that facilitates the storage and retrieval of potentially large amounts of information A DBMS contains methods for inserting, updating, and removing data; these functions can be used by computer programs and software applications A DBMS also usually contains authentication and access control, thereby permitting the control over which users may access what data There are three principal types of DBMSs in use today: relational, object, and hierarchical, described in this section
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Relational database management systems (rDBMSs) represent the most popular model used for database management systems A relational database permits the design of a logical representation of information Many relational databases are accessed and updated through the SQL (Structured Query Language) computer language Standardized in ISO and ANSI standards, SQL is used in many popular relational database management system products Basic Concepts A relational database consists of one or more tables A table can be thought of as a simple list of records, like lines in a data file The records in a table are often called rows The different data items that appear in each row are usually called fields A table often has a primary key This is simply one of the table s fields, whose values are unique For example, a table of healthcare patient names can include each patient s Social Security number, which can be made the primary key for the table One or more indexes can be built for a table An index facilitates rapid searching for specific records in a table based upon the value of one of the fields other than the primary key For instance, a table that contains a list of assets that includes their serial numbers can have an index of the table s serial numbers One of the most powerful features of a relational database is the use of foreign keys Here, a foreign key is a field in a record in one table that can reference a primary key in another table For example, a table that lists sales orders includes fields that are foreign keys, each of which references records in other tables This is shown in Figure 5-6
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Fields in a sales order table point to records in other tables
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Relational databases enforce referential integrity This means that the database will not permit a program (or user) to delete rows from a table if there are records in other tables whose foreign keys reference the row to be deleted The database instead will return an error code that will signal that there are rows in other tables that would be stranded if the row was deleted Using the example in Figure 5-6, a relational database will not permit a program to delete salesperson #2 or #4 since there are records in the sales order table that reference those rows The power of relational databases comes from their design and from the SQL language Queries are used to find one or more records from a table using the SELECT statement An example statement is
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SELECT * FROM Orders WHERE Price > 100 ORDER BY Customer
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One powerful feature in relational databases is a special query called a join, where records from two or more tables are searched in a single query An example join query is
SELECT SalespersonName, count(*) AS Orders FROM Salesperson JOIN Salesperson_ Number ON SalespersonNumber = OrdersSalesperson GROUP BY SalespersonName
This query will produce a list of salespersons and the number of orders they have sold Relational Database Security Relational databases in commercial applications need to have some security features Three primary security features are: Access controls Most relational databases have access controls at the table and field levels This means that a database can permit or deny a user the ability to read data from a specific table or even a specific field In order to enforce access controls, the database needs to authenticate users so that it knows the identity of each user making access requests Encryption Sensitive data such as financial or medical records may need to be encrypted Some relational databases provide field-level database encryption that permits a user or application to specify certain fields that should be encrypted Audit logging Database management systems provide audit logging features that permit an administrator or auditor to view some or all activities that take place in a database Audit logging can show precisely the activities that take place, including details of database changes and the person who made those changes The audit logs themselves can be protected so that they resist tampering, which can make it difficult for someone to make changes to data and erase their tracks Database administrators can also create views, which are stored queries accessible as virtual tables Views can simplify viewing data by aggregating or filtering data They can improve security by exposing only certain records or fields to users
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