ER/Studio - Embarcadero Technologies Product Documentation

USING ER/STUDIO > DEVELOPING THE LOGICAL MODEL
Data Movement Rules tab
Displays the data movement rules that have be assigned to the attribute. Allows you to override the default rules
assigned to the table/entity in the Data Movement Rules editor of the Data Dictionary. Here you can apply or
remove data movement rules. The Data Movement rule could be something like “Data for this table is updated
after 30 days.” For more information, see Relating Source and Target Tables and Columns.
Notes
• Choosing the correct datatype is a critical data integrity and storage consideration. Select a datatype that is
efficient for its intended use but properly accommodates the data it stores. Your choice of database platform
dictates your choice of column datatypes because each has its own set of datatypes. For example, some
databases have binary datatypes; others do not. Some provide support for blobs and text, but many do not.
Finally, only a few databases support user-defined datatypes. ER/Studio can convert datatypes between different
databases; however, some conversions can only be approximations.
• Another important consideration in specifying datatypes is determining the appropriate width, precision and
scale, as applicable. You want to store data efficiently, but not at the expense of completeness, accuracy or
precision.
• For information on datatype mapping, see Customizing Datatype Mappings. If you want to enforce datatypes
across common columns, use Data Dictionary Domains. For more information, see Reusing Attribute Definitions
Using Domains.
• When determining attribute and column names, make sure you consider the naming rules of the target database.
Some key considerations are name length, avoiding the use of reserved words, and prohibited characters. Most
databases restrict name lengths to 30 or 18 characters. All have reserved key words that cannot be used in
names unless surrounded by quotes. Certain characters, such as a space, *, +, and % are also prohibited from
being used in names.
• You can specify naming rules, the default datatype and whether NULL values are permitted for the attributes
created with the default datatype on the Logical tab of the Options Editor. Select Tools > Options > Logical.
• You can specify the order in which the attributes appear on the Display tab of the Options Editor. Select Tools >
Options > Display.
• You can specify the level of synchronization between the attribute and column names on the Name Handling tab
of the Options Editor. Select Tools > Options > Name Handling.
• The status bar at the bottom right of the application window displays the number of entities in the selected model.
Creating and Editing Keys
The purpose of keys in a database is to enforce unique values in an entity and provide a means of sorting the tables to
access entity data faster.
In a relational database, there are four types of keys:
• Primary Key: A primary key or unique key is a candidate key to uniquely identify each row in a table. A primary
key comprises a single column or set of columns where no two distinct rows in a table can have the same value
(or combination of values) in those columns. Depending on its design, a table can have many unique keys but at
most one primary key. A unique key must uniquely identify all possible rows that exist in a table and not only the
currently existing rows, such as social security numbers. A primary key is a special case of a unique key in that
for unique keys the implicit NOT NULL constraint is not automatically enforced, while for primary keys it is. The
values in a unique key column may or may not be NULL. Unique keys as well as primary keys can be referenced
by foreign keys.
• Alternate Key: Alternate keys are non-unique keys in a database, such as name and date of birth in an employee
database.
EMBARCADERO TECHNOLOGIES > ER/STUDIO® 8.0.3 USER GUIDE 128