5.4. Data Logical Model (DLM)
The DLM is the normal result of MERISE process. Its purpose
is to bring us closer to the physical model. For this, we start from the
Conceptual Data Model and the relationships we remove it within certain
pre-established rules.
5.4.1. Definition of concepts
The LDM provides technical vocabulary again that it is
important to understand well.
5.4.1.1. Tables, columns and rows
So that was an entity Data Conceptual Model is a table in the
DLM. Thus this table is organized in line or column and each column represents
the fields of the table and each line represents the values for each field.
5.4.1.2. Primary and foreign keys of each table
The identifier that had the DCM, DLM is the primary key which
can be recognized by its underlining. Also a table can contain the primary key
of another table that at that moment becomes a foreign key. Which foreign key
is symbolized by a sharp before his name. Thus:
- A table has only one primary key, but can have multiple
foreign keys;
- The foreign key can be made;
- The column of the primary key cannot be empty;
- The foreign key can also be primary key in the table.
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