In recent years, many medical studies using healthcare databases have been carried out. And, in most cases, relational databases are used as database management systems. Since it is necessary to handle data by making full use of database manipulation language for analysis purposes, skills different from those for operation purposes are required. What is important here is a deep understanding of the definitions and benefits of the relational data model theory. However, some people do not understand the exact definition of relational data model theory and are confused with relational databases. This paper promotes a deeper understanding of the relational model theory by explaining in detail the relationship between the predicate logic and relational model theory using easy-to-understand examples.