Dwight - SQL: Learn SQL in 24 Hours or Less - A Beginners Guide To Learning SQL Programming Now (SQL, SQL Programming, SQL Course)

This chapter will teach you the basics of the SQL language and the relational databases. By reading this material, you'll know how the language works and how to use a relational database.

The Databases

SQL (i.e. Structured Query Language) allows you to create and maintain relational databases. It also allows the management of information inside those databases. Before discussing SQL further, let's define what the term database means. People use this term when referring to a collection of information (e.g. names, phone numbers, addresses, etc.) that is organized according to a definite structure.

Here are the most common structures used in modern databases:

• Hierarchical This structure uses parent-child relationships. It organizes information through nodes, which are the counterpart of tables in relational databases. Parent nodes can have multiple child nodes. Child nodes, however, can have just one parent. The hierarchical structure is extremely popular, although it is inflexible and doesn't support complicated relationships.
• Network The network structure solves the problems found in the hierarchical structure. Here, you will organize information into record types. This structure arranges information into a system that links pairs of records: members and owners. A record type can work with other record types. Thus, a network database supports complicated searches and relationships. The main disadvantage of this structure is its complexity: you need to know how the structure works and how information is stored.
• Relational This is the most popular structure these days. A relational database addresses the issues linked with network and hierarchical databases. In a network or hierarchical system, you'll rely on a particular database implementation, which you will hard code onto the database application you are using. You need to adjust the database application whenever you add items to your database. In a relational database, however, you won't have to worry about your application. You may alter your database without affecting the application itself.

Since you will use SQL with relational databases, we need to discuss this kind of structure in detail. The main element of a relational database is the relation, which are essentially groups of rows and columns placed in a table that represents one object. An object (also known as entity), on the other hand, consists of related information.

Entities are things, events, places, persons, or concepts about which information is gathered. Each relation consists of different columns (which represent attributes). Attributes are units that define or characterize entities. For instance, in the table given below, the entity is a book with size, color, and weight as attributes.

In this table, all of the attributes have a domain (i.e. the name that specifies which type of information you can store in the attribute). You shouldn't confuse attributes with data types. Data types are particular kinds of constraints (i.e. tools used to secure information integrity) linked with columns, while domains describe what information you can include in the attributes associated with them. For instance, the attribute named Product_Weight is linked with the Weight domain.

Important Note: It would be best if you will use a domain that describes the attribute it will hold. Although this approach is optional, it can help you improve the usability and readability of your database entries.