Database Design Guidelines

Database Design Guidelines

Principles

Support popular databases

Name Style

Table Name

Style: Pascal

Example: Employee

It is fine that you may use the singular style or plural style. But singular style is preferred here.

Column Name

Style: Pascal

Example: HouseAddress

Avoid to repeat table name in the column name, for example, For table 'Employee', use 'Name' for the column of employee name.

Primary Key Name

Style: Pascal, PK_${TableName}

Example: PK_Employee

Index Name

Style: Pascal, UK_\({Column1Name}_\){Column2Name} for unique indexes.

Use IK_\({Column1Name}_\){Column2Name} for non-unique indexes.

Example: UK_Code

Foreign Key Name

Style: Pascal, FK_\({Column1Name}_\){Column2Name}

Example: FK_DepartmentId

Columns

Text columns

Do use unicode data type to define text columns.

Do use nvarchar for variable length text columns.

Date/Date Time

If the value of a date column is expected to present same text result among different time zone, use numeric or text data type to define date/date-time columns.

If we define a column with date data type, default, most program language will use date-time type to store values from the column, we should realize the date value will be changed when the data is transited to a different time-zone. For example:

2001/01/15, On the database server, timezone is +8.

2001/01/15, On the application server, timezone is +8.

2001/01/15 01:00:00, On the client, timezone is +9.

So if it is a problem to you, try to use a integer or text data type to store the value.

Nullable

Avoid to define a column as nullable.

Avoid to use null would reduce programing bugs, and bring better performance.

Do Not use null for numeric columns if it has same meaning as zero.

Do Not use null for text columns if it has same meaning as empty.

May use null in a reference column provide that null is acceptable.

Keys

Primary Keys

Do create a primary key for any tables.

Recommend to use one column for the primary key.

Use '${TableName}Id' for the name of the primary key column, the column is denoted as 'Id' columns.

Recommend to use integer data type for the primary key column.

Avoid change the value of the primary key column after created.

It is acceptable to show the the value of the primary key column to customers if need.

Foreign Keys

Do create a foreign key if there is a relation between 2 tables.

It is useful to keep data integrity.

Carefully use delete cascade clause when creating the foreign keys relationship.

Database-specific constraints

Avoid to use reversed words for name of objects of databases



Avoid to use system prefix in your object names

Check length limitation of object names of databases

Tables

Entity Tables

Like departments, employee etc, we store these kind of information into entity tables.

In most case, the table like:

Definition sample

Employee {EmployeeId, Code, Name, DepartmentId, ...}.

If in your system, an employee only exists in one department, you may use above definition.

Tree Tables

Like most organization, departments will be constructed as a tree, there are some top level departments (or is a root department), except these top level departments, departments must have one and only have one parent department.

We define this kind of table as:

Definition sample

Department{DepartmentId, Code, Name, **ParentId**, ...}.

In most case, for the usage convenience, we will define a tree table for each tree relationship, denoted as a tree table.

Definition sample

DepartmentTree{ParentId, ChildId}.

For a parent department, The table stores itself and all its descendants.

From a view of a child department, the table stores itself and all its ancestors.

Hierarchical Tables

Different as tree tables, in the hierarchical tables, a child would have 0 to n parents.

For example, groups and users:

Rules

an user would exists in multiple groups.

an groups would exists in multiple groups.

Definition sample

User(userId, Name, isGroup)
GroupUser(parentId, userId)
GroupUserTree(parentId, userId)

The table GroupUser stores the direct relationships.

Instead, the table GroupUserTRee stores the redundant relationships like tree tables above.