Database services under Linux that use the software discussed in this chapter are based on a client/server model. Database clients are often used to input data and to query or display query results from the server. You can use the command line or a graphical client to access a running server. Databases generally come in two forms: flat file and relational. A flat file database can be as simple as a text file with a space, tab, or some other character delimiting different parts of the information. One example of a simple flat file database is the Ubuntu /etc/passwd file. Another example could be a simple address book that might look something like this:
Doe~John~505 Some Street~Anytown~NY~12345~555-555-1212
You can use standard UNIX tools such as grep, awk, and perl to search for and extract information from this primitive database. Although this might work well for a small database such as an address book that only one person uses, flat file databases of this type have several limitations:
• They do not scale well Flat file databases cannot perform random access on data. They can only perform sequential access. This means they have to scan each line in the file, one by one, to look for specific information. As the size of the database grows, access times increase and performance decreases.
• Flat file databases are unsuitable for multi-user environments Depending on how the database is set up, it either enables only one user to access it at a time or allows two users to make changes simultaneously, and the changes could end up overwriting each other and cause data loss.
These limitations obviously make the flat file database unsuitable for any kind of serious work in even a small businessmuch less in an enterprise environment. Relational databases, or relational database management systems (RDBMSs) to give them their full name, are good at finding the relationships between individual pieces of data. An RDBMS stores data in tables with fields much like those in spreadsheets, making the data searchable and sortable. RDBMSs are the focus of this chapter.
Oracle, DB2, Microsoft SQL Server, and the freely available PostgreSQL and MySQL are all examples of RDBMSs. The following sections discuss how relational databases work and provide a closer look at some of the basic processes involved in administering and using databases. You will also learn about SQL, the standard language used to store, retrieve, and manipulate database data.
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