A database management system (DBMS) consists of an interrelated collection of data and a set of programs to access said data. The data collection is called database, this contains information relevant to a company. The main objective of the DBMS is to provide a way in which the information found in a database can be stored and retrieved in a way that is practical and efficient.
The databases are widely used, here are some representative applications:
As can be seen in this list, databases are an essential part of almost all companies today. During the last four decades of the twentieth century the use of databases grew significantly in companies. Initially, people did not interact directly with database systems, but without realizing it, they interacted indirectly with printed reports such as credit card statements, or through agents such as bank tellers and airlines reservation agents.
The Structured Query Language, known as SQL is the standard language used to interact with relational database systems. The SQL was developed by IBM in the 1970s with the aim of implementing the management and storage of large data banks developed years before by E. F. Codd.
The relational model proposed by E. F. Codd between 1969 and 1979, introduced important changes in the way in which the databases were conceived. This proposal caused a stir at the time and thirty years later is the pillar of most database managers.
The relational model represents databases as a grouping of relationships, that is, each relationship is similar to a table, or, in the worst case, an independent file. If we visualize a relationship as a table, each row of it represents combinations of data related to each other. This data is defined in the form of columns. In terms of the relational model, a row is called a tuple, a column header is an attribute, and the table is a relation. The type of data that describes the values that a column can store is called a domain. The following describes each of the terms in greater detail:
A key is a combination of one or more attributes that unambiguously identifies the records that are in a table. It is recommended that the keys have the following characteristics:
It is recommended that a key be simple, or in other words, that the length of the attributes of a key be minimal. From the computational point of view, the comparison and storage of numbers are more efficient than that of the characters, this means that the search for records that have a key composed of numerical attributes will be faster than one whose attributes are of alphanumeric type.
The keys in turn can be divided into simple and compound. The simple ones are those that are composed of an attribute (they comply with the condition of minimalism). Compounds are made up of more than one attribute.
The null value is a value supported by the relational model. When it is said that a value is "null" it means that said information is unknown.
The null value is not equal to zero or characters in whites. Comparing any value with a null value brings an undefined response since it is not equal to or different from another value, even to itself.
It is a column or sets of columns of a table that contains values that match the values of the primary key of another table and are used to join two tables. Example:
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