What is a database and a Database Management System (DBMS)?       

What are the applications of a database system?  

• Banking: For customer information, accounts, loans, bank transfers, etc.  
• Airlines: They are used for reservations and the management of schedules. These were the first to use the databases in a geographically distributed way.  
• Universities: They are used to manage student information such as: enrollment in subjects, courses, etc.  
• Credit card transactions: They are used for everything that has to do with the purchase by credit cards and also for the generation of monthly statements.  
• Telecommunications: Helps to keep a record of calls made, generate monthly bills, and maintain the balance on prepaid phone cards and to store information about communication networks.  
• Finance: Used to store information of holding companies, purchases and sales of financial products such as stocks and bonds; and also to store market data in real time to allow customers to buy and sell online.  
• Sales: To store the information of customers, products and purchases.  

SQL language      

Relational model     

• Domain: The range of valuesto which an object is limited. Domain example: characters, numbers with floating point, integers or logical values.  
• Attributes: Are those objects that can store the values restricted by the domain, that is, it belongs exclusively to a particular domain. Example of attributes: date, name, surname. The attributes are also known as columns. 
• Relation: it is a correspondence between groups of attributes. For example, the following relationship is defined as a user (name, surname, date of birth) and identifies a correspondence between these attributes. When there are many relationships this is called a set of relationships, which must have unique names that identify them.  
• Degrees of a relationship: The degree of the relationship refers to the number of attributes that exist in a relationship.  
• Tuples: Series of records that are stored in relationships.  

Keys    

1. Minimalism: This occurs when the key has the minimum number of attributes. 
2. Stability: The values of the attributes that make up the keys should not be constantly changing. 
3. Familiarity: Keys must be composed of attributes that make sense to users.  

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.  

Types of keys      

• Candidate keys: Set of attributes that unambiguously identify the records of a table. Example: if we put the identification of an automobile as an example we can notice that we have three candidate keys: license plate number, engine serial and body serial. By defining which of the three will identify the records, this becomes the primary key and the others would be secondary keys.  
• Primary key: It is the one that is selected by the database designer to uniquely identify the records of a table.  
• Secondary key: Attribute sets that uniquely identify records in a table but were not selected as the primary key.  

  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. 

  
  

  Null Value     

  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. 

  
  

  Foreign key  

  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:

  

  Figure 1. Primary and Foreign Keys of Department and Employee Relations. ^{Own source}  
  
  I hope that this information has been to your liking, very soon I will be publishing more content about SQL and its functionalities. Thank you very much for reading and sharing.
  Source of Images:  A B C D  

  Bibliographic references: 

  1. Website: https://es.wikipedia.org/wiki/Base_de_datos 
  2. Website: https://es.wikipedia.org/wiki/SQL 
  3. Website: https://www.ibm.com/support/knowledgecenter/en/SSGU8G_11.50.0/com.ibm.ddi.doc/ids_ddi_183.htm 
  4. Book: Basics of database. Fifth edition. ABRAHAM SILBERSCHATZ. 
  5. Book: Introduction to SQL Theory and Practice. Olinto Rodríguez Atencio.