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What is a Database By: Cristian Dubon.

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1 What is a Database By: Cristian Dubon

2 What is a Database? The definition of a database is an ordered compilation of records or information that is accumulated in a computer system. Databases are intended to offer a structured system for filing, sorting, managing, and retrieving information electronically.

3 Advancements In Databases
The term “data base” started being used around 1964 by worker in military information systems. The idea of shared applications and data by multiple users at once was great for efficiency. Early database data structure classes worked off of punch cards. When magnetic tape was introduced it advanced the technology but it wasn’t till the RAM-based data structures such as arrays, tables and lists that database truly showed the effect it would have on information storage and distribution. Most databases today are large hard drives, or servers, run by a program called a database management system, that dictates how information is filed, managed, sorted, and retrieved.

4 Types of Databases The hierarchical model database is a tree like structure that branches into other folders, each child data has only one parent data, the way most computer operating system display folders to users. Each link is nested to keep data organized on the same level. The defining feature of a network model is that a record is stored with a link to other records allowing the branches to overlap, here a child data can have more than one parent data, working like a shortcut on you desktop. These two models are not as flexible or fast in the retrieval of data because the search follows a trickle down pattern.

5 Types of Databases The relational model is the most commonly used database model and an exceptionally potent tool. The relational database model is based on a table that is organized by rows, or records, and columns, or fields, with a special field reserved for the key. The key field serves as the identifying marker for data, with the possibility that a unique field can act as a primary key. A table can have many records and each record can have many fields. This model is designed to allow data to be processed with eight mathematical operations, that fall into two categories, set operations and relational operations, as well as combining the operations. The set operations are union, difference, intersection and Cartesian product, leaving the operations join, projection, selection and division, as the relational operations.

6 Relational Databases The set operations act on sets of rows to produce a new set of rows, determining which rows from the input appear in the output. The union and intersection operations do just as they would in math when you say A union B or A intersection B. Union takes to two or more tables with related fields and combines the tables together to get a new table that consist of all rows with no duplicates entries. Intersection takes two or more related tables and extracts only the rows that the tables have in common and outputs a new table. The different operation can extract all of the rows from the first table that are not included in the second table producing a new table of either A-B or B-A, depending on which rows are to be excluded from the new table, remember the tables have to consist of the same fields. The Cartesian product operation is a process that merges all rows in the two tables, for example two tables of three rows resulted in a table of nine rows. However unlike the other operation this operation requires the fields are not related.

7 Relational Databases The relational operations work differently from the set operations. The projection operation can extract columns from one or more tables. The selection operation can extract rows from one or more tables. So, one operation act vertical on tables while the other acts horizontally on tables. The join operation literally takes two tables that have at least one field in common and merges the tables around that one common field, outputting a well organized table that gives more data than either table alone. The division operation extracts the rows from a table that match values in the columns of the second table, but returns only the columns that do not exist in the second table. Remember that the new tables themselves can have operations used on them.1

8 Before Building A Database
Databases are built based on an entity-relationship model in order to understand the conditions that govern the data you want to place in the database and how it should be arranged. An entity is a real-world object or thing. The relation must be found between them its either one-to-one, one-to-many, or many-to-many. In a many-to-many relation, M objects have a relation with N objects; the number of associations between entities is called cardinality. From this an unnormalized form is obtained, a table in which items that appear more than once have not been removed.

9 Before Building A Database
This table is then divided into smaller, simpler tables and primary and foreign key are decided on to allow interaction between tables. In a relational database, a value is called functionally dependent if that value determines values in other columns. In the second normal form, the table is divided so that values in other columns are functionally dependent on the primary key. In the third normal form, a table is divided so that a value is not determined by any non-primary key. In a relational database, a value is called transitively dependent if that value determines values in other columns indirectly, which is part of functionally dependent operation. In the third normal form, the table is divided so that transitively dependent values are removed.

10 Structured Query Language (SQL)
This is done using a Structured Query Language (SQL) also called sequel SQL’s has three distinct types of commands, data definition language (DDL), data manipulation language (DML), and data control Language (DCL). The DDL commands that SQL creates the framework of a database, creates a table within a database, as well as changing and deleting tables. The DML commands allow for manipulation of data, like inserting, deleting, and updating data in tables. It also has commands that allow you to search for data. The DCL commands offers the ability to control a database so that no data conflicts do not happen as multiply user enter data.

11 The SQL statement Syntax
CREATE TABLE. The syntax inside a table depends on the kind of database being used. When a table is created the column names must be given, a primary key or foreign key is given to each column. Constraints such as amount or types of character to be in a certain row may have to be given to prevent data conflict later on. The INSERT, UPDATE, and DELETE statement do as they describe to data from tables made by the CREATE TABLE statement, considering that any of the database’s constraints are not violated. Next is the view table that is viewed by a user of the database. It was created by extracting data from the selected tables. This table is derived from these selected tables that are called base tables. Structured Query Language use basic data search functions in the form of query-type statements as input. To search for a certain set of tables, commands like select, from, where, group by, having, and so on are used. In these statement comparison operators like less than (<), greater than (>), equal to (=), not equal to (<>), less than or equal to (<=), greater than or equal to (>=), and between x and y. Logical operators such as like, and, or and not are used to create more complex comparison statements. There are also conditions that let you search for pattern matching using wildcard characters. Such as * to search all, _ plus a letter before or after to retrieve matches with two characters that start or end with a specific letter, % plus a letter before or after to retrieve matches with any number of characters that start or end with a specific letter, and is null if you are looking for empty records. Aggregate functions or set functions like maximum and minimum values, number of items, and sum, are used to collect and output new tables.



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