Understanding relational databases and SQL

Now that you have learned how to describe a real-world entity/relationship using a document comprised of key-value pairs, it is time to consider the downside of it (i.e., the inherited complexity of this modeling approach).First of all, key-value pairs within a document do not require any order. For example, the First Name key-value pair can show up either before or after the Last Name key-value pair. Furthermore, documents describing the same type of observation units (such as customers) can have different keys in different documents. A customer may have key-value pairs such as pets or children, or both, but also may not have either. These two characteristics, the lack of order and the flexibility of definition, reflect the real-world scenario and provide flexibility to the usage. The downside of them is that the applications utilizing these databases must carry the burden of checking key existence and searching for key-value pairs in an unsorted...