Relation Mapping in Databases

We have been working with a single table and altering it, as well as reading back the data. However, the real power of an RDBMS comes from the handling of relationships among different objects (tables). In this section, we are going to create a new table called comments and link it with the user table in a 1: N relationship. This means that one user can have multiple comments. The way we are going to do this is by adding the user table's primary key as a foreign key in the comments table. This will create a 1: N relationship.

When we link two tables, we need to specify to the database engine what should be done if the parent row is deleted, which has many children in the other table. As we can see in the following diagram, we are asking what happens at the place of the question marks when we delete row1 of the user table:

Figure 8.6: Illustration of relations

In a non-RDBMS situation, this situation can quickly become difficult...