What are the Data Models in SQL?

Hello friends, I wanted to know about the data model available in SQL. I have studied little about SQL but I am not aware about the data model available in SQL. I have heard that there are many data models available in SQL but I am not sure whether it is true or not? Can anyone let me know the answer about this query then I will be very thankful to you. Can anybody also tell me what is the role of hierarchical model in SQL?

Before the introduction of database management systems, all data permanently stored on a computer system, such as payroll and accounting records, was stored in individual files. A file management system, usually provided by the computer manufacturer as part of the computer's operating system, kept track of the names and locations of the files. The file management system basically had no data model; it knew nothing about the internal contents of files. To the file management system, a file containing a word processing document and a file containing payroll data appeared the same.

Knowledge about the contents of a file-which data it contained and how the data was organized -was embedded in the application programs that used the file. In this payroll application, each of the COBOL programs that processed the employee master file contained a file description (FD) that described the layout of the data in the file. If the structure of the data changed-for example, if an additional item of data was to be stored for each employee-every program that accessed the file had to be modified. As the number of files and programs grew over time, more and more of a data-processing department's effort went into maintaining existing applications rather than developing new ones.

The bill of materials for products has a natural hierarchical structure. To store this kind of data, the hierarchical data model, was developed. In this model, each record in the database represented a specific part. The records had parent/child relationships, linking each part to its subpart, and so on. You should be now aware of it that hierarchical data model was specially designed to store the kind of data which is been used.

To deal with applications such as order processing, a new network data model was developed. The network data model extended the hierarchical model by allowing a record to participate in multiple parent/child relationships. These relationships were known as in the network model. This is the most important model in the SQL where it is been using the data models. In network data model it uses an hierarchical model which many of them think that it is a simple model but it is not in real.