Database Manage and Organization
A database is a collection of information that is
organized so that it can be easily accessed, managed and updated.
Data is organized into
rows, columns and tables, and it is indexed to make it easier to find relevant
information. Data gets updated, expanded and deleted as new information is
added. Databases process workloads to create and update themselves, querying the
data they contain and running applications against it.
Database in Terminology
·
Database
A database is a named
collection of tables. (see table).
A database can also contain views, indexes, sequences, data types, operators,
and functions. Other relational database products use the term catalog.
·
Record
A field is an area in a
fixed or known location in a unit of data such as a record, message header, or computer instruction that has a purpose and usually a fixed size. In some
contexts, a field can be subdivided into smaller fields. Here are some examples:
·
Command
A command is a string that you
send to the server in hopes of having the server do something useful. Some
people use the word statement to
mean command.
The two words are very similar in meaning and, in practice, are
interchangeable.
·
Query
A query is a type of command
that retrieves data from the server.
·
Table (relation, file,
class)
A table is a collection of
rows. A table usually has a name, although some tables are temporary and exist
only to carry out a command. All the rows in a table have the same shape (in
other words, every row in a table contains the same set of columns). In other
database systems, you may see the terms relation, file,
or even class?these
are all equivalent to a table.
·
Column (field, attribute)
A column is the smallest unit
of storage in a relational database. A column represents one piece of
information about an object. Every column has a name and a data type. Columns
are grouped into rows, and rows are grouped into tables. In Figure 1.1, the
shaded area depicts a single column.
·
Row (record, tuple)
A row is
a collection of column values. Every row in a table has the same shape (in
other words, every row is composed of the same set of columns). If you are
trying to model a real-world application, a row represents a real-world object.
For example, if you are running an auto dealership, you might have a vehicles table.
Each row in the vehicles table
represents a car (or truck, or motorcycle, and so on). The kinds of information
that you store are the same for all vehicles (that is,
every car has a color, a vehicle ID, an engine, and so on). In Figure 1.2, the
shaded area depicts a row.
·
Figure 1.2. A row
(highlighted).
You may also see the terms record or tuple? these
are equivalent to a row.
·
View
A view is an alternative way to
present a table (or tables). You might think of a view as a "virtual"
table. A view is (usually) defined in terms of one or more tables. When you
create a view, you are not storing more data, you are instead creating a different
way of looking at existing data. A view is a useful way to give a name to a
complex query that you may have to use repeatedly.
·
Client/server
PostgreSQL is built around a client/server architecture.
In a client/server product, there are at least two programs involved. One is a
client and the other is a server. These programs may exist on the same host or
on different hosts that are connected by some sort of network. The server
offers a service; in the case of PostgreSQL, the server offers to store,
retrieve, and change data. The client asks a server to perform work; a
PostgreSQL client asks a PostgreSQL server to serve up relational data.
·
Client
A client is an application that
makes requests of the PostgreSQL server. Before a client application can talk
to a server, it must connect to a postmaster (see postmaster)
and establish its identity. Client applications provide a user interface and
can be written in many languages. Chapters 8 through 17 will show you how to
write a client application.
·
Server
The PostgreSQL server is
a program that services commands coming from client applications. The
PostgreSQL server has no user interface?you can't talk to the server directly,
you must use a client application.
·
Postmaster
Because PostgreSQL is a client/server
database, something has to listen for connection requests coming from a client
application. That's what the postmaster does.
When a connection request arrives, the postmaster creates
a new server process in the host operating system.
·
Transaction
A transaction is a collection
of database operations that are treated as a unit. PostgreSQL guarantees that
all the operations within a transaction complete or that none of them complete.
This is an important property?it ensures that if something goes wrong in the
middle of a transaction, changes made before the point of failure will not be
reflected in the database. A transaction usually starts with a BEGIN command
and ends with a COMMIT or ROLLBACK (see
the next entries).
·
Commit
A commit marks the successful
end of a transaction. When you perform a commit, you are telling PostgreSQL
that you have completed a unit of operation and that all the changes that you
made to the database should become permanent.
·
Rollback
A rollback marks the unsuccessful
end of a transaction. When you roll back a transaction, you are telling
PostgreSQL to discard any changes that you have made to the database (since the
beginning of the transaction).
·
Index
An index is a data structure
that a database uses to reduce the amount of time it takes to perform certain
operations. An index can also be used to ensure that duplicate values don't
appear where they aren't wanted. I'll talk about indexes in Chapter 4,
"Query Optimization."
·
Result set
When you issue a query to a database,
you get back a result
set. The result set contains all the rows that satisfy your
query. A result set may be empty.
Forms of Database
Flat File Database
A flat file database
is a database that stores data in a plain text file. Each line of the
text file holds one record, with fields separated by delimiters, such as commas
or tabs. While it uses a simple structure, a flat file database cannot contain
multiple tables like a relational database can. Fortunately, most database
programs such as Microsoft Access and FileMaker Pro can import flat file
databases and use them in a larger relational database.
Hierarchical Database
A hierarchical database model is a data model in which the
data is organized into a tree-like structure. The data is stored as records which are
connected to one another through links. A record is a collection of fields, with each field
containing only one value. The entity type of a record defines which fields the record contains. A
record in the hierarchical database model corresponds to a row (or tuple) in
the relational database
model and an entity type corresponds to a table (or relation).
Relational Database
A relational database is a collection of data items organized as a set of formally-described tables from which data can be accessed or reassembled in many
different ways without having to reorganize the database tables. The relational
database was invented by E. F. Codd at IBM in 1970.
Relational database can also be viewed as a set of
tables containing data fitted into predefined categories. Each table (which is
sometimes called a relation) contains one or more data categories in columns. Each row contains a
unique instance of data for the categories defined by the columns. For example,
a typical business order entry database would include a table that described a
customer with columns for name, address, phone number, and so forth. Another
table would describe an order: product, customer, date, sales price, and so
forth.
Examples of Database
·
ADABAS.
·
IBM DB2.
·
Microsoft Access.
·
Microsoft SQL Server.
·
MySQL.
·
Oracle RDBMS.
·
QuickBase.
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