After the class today, I now have an overall concept about what DBMS is. A DBMS is designed to manage the data outside the memory. It gives DBMS 7 aspects:
· Massive—the database can be in terabytes, which is quite a large mount of data that barely can be saved on our PCs.
· Persistent—before and after the operations, the data is always there.
· Safe—the data should be safe from hardware failure, software failure and so on.
· Multi-user—the data can be manipulated by several users at the same time
· Convenient—high level language (easy to interpret in human language) is used in DBMS.
· Efficient—DBMS is using queries. So, every time user wants a data, the system asks from the data base for the single record.
· Reliable—the result produced by DBMS must be 99.99999% correct.
In the vide ‘Relational database’, database, as a set of named relations, is introduced. Also, schema vs variable is introduced. Schema is like the names of the attributes of the data. Variable (instance) is the values of attributes for the specific data. There is a very special value called NULL value. It means the value is undefined. Key refers to the attribute that is unique for each record (similar to our names but without repetition).
I also looked through ‘Data Modeling’ chapter in ‘SQL for Web Nerds’. It tells about how a model is constructed using SQL. SQL is not like most computer languages. It is using three-value logic: true, false and null. Besides assigning attributes to data, triggers should be set to prevent some errors. This chapter also introduces data types and constraints. Constructing a data model in SQL is very similar to establishing variables in other languages. But SQL is more high-level and more rigorous. It limits the values as soon as the variable is announced. The data types are those serve for a database.