In database design, table columns play a crucial role in organizing and storing data efficiently. The following rules are essential for creating and maintaining table columns in a database, ensuring data integrity, consistency, and usability:
Naming conventions: Use clear, concise, and meaningful names for table columns. Avoid using spaces, special characters, or reserved keywords. Use underscores (_) to separate words, and follow a consistent naming pattern, such as CamelCase or snake_case.
Data types: Assign appropriate data types to each column based on the nature of the data it will store. Common data types include INTEGER, FLOAT, DECIMAL, VARCHAR, CHAR, DATE, and TIMESTAMP. Choose data types that minimize storage requirements and ensure data integrity.
Null values: Determine whether a column can contain NULL values or not. If a column should always have a value, set it as NOT NULL. This constraint ensures that the database will not accept records missing a value for that particular column.
Primary keys: Designate a unique identifier for each row in the table by assigning one or more columns as primary keys. Primary keys must be unique, non-null, and rarely updated. They are essential for maintaining data consistency and supporting efficient retrieval and modification of records.
Foreign keys: If a column contains data that references another table, define it as a foreign key. Foreign keys establish relationships between tables, allowing for more efficient querying and maintaining referential integrity within the database.
Default values: Assign default values to columns where appropriate. Default values are automatically inserted when a new record is added without specifying a value for that column. This can help maintain data consistency and reduce the likelihood of NULL values in columns where they are not allowed.
Column constraints: Use constraints such as UNIQUE, CHECK, and REFERENCES to enforce rules on the data stored in a column. Constraints help maintain data integrity and prevent errors due to invalid or duplicate data entries.
Indexing: Create indexes on columns that are frequently used in search queries or join operations. Indexes can significantly improve query performance, but they should be used judiciously as they can also increase storage requirements and impact data modification operations.
Comments and documentation: Provide descriptive comments for each column, explaining its purpose and any constraints or relationships that apply to it. This documentation can be invaluable for future database maintenance, troubleshooting, and development.
By adhering to these rules for table columns, you can create a well-structured and efficient database that supports data integrity, consistency, and usability across different applications and use cases.
Columns store Attributes of the Entity
When you create a table, you use the columns to store attributes of the entity represented by the table.
For example, a computer might have attributes of manufacturer, processor speed, amount of random access memory, hard-disk space, floppy-disk drive, speed of a CD-ROM drive, and monitor size. Each of those attributes is represented as a column in the Computer table.
Table Column obeys Two Rules
Each column in a table must adhere to two rules:
The column must have a unique name within the table
The values for the column must come from a single domain.  (You will learn about column domains in a later lesson.)
The first rule seems obvious: every column must represent a different attribute of the entity represented by the table.
If the same attribute occurs in more than one table, a relationship exists between those two tables. Imagine that your database contains two tables: one listing computer manufacturers, the other listing computers owned by your company.
These two tables have the ManufId column in common.
Link Tables in Query
Establishing relationships between tables enables you to link tables in a query. If you have a problem with a computer and need to contact the manufacturer, you can look up the computer's record in the Computer table and, with the ManufId value in the record, find the manufacturer's contact information. Another benefit to establishing relationships between tables is that, if two tables are in a one-to-many relationship , you can ensure that every record in the table on the many side has a related record in the table on the one side. In the graphic above, it would make no sense to enter a manufacturer into a record describing a computer unless that manufacturer was listed in the Manufacturers table.
If you enforce referential integrity between two tables, your (RDBMS) Relational Database Management System will prevent you from making that mistake. The next lesson lists the rules for table rows.
Domain: Determines the type of data values that are permitted for that attribute.
relationship: If the same attribute occurs in more than one table, a relationship exists between those two tables.
one-to-many relationship: In relational database design, a one-to-many (1:N) relationship exists when, for one instance of entity A,
there exists zero, one, or many instances of entity B; but for one instance of entity B, there exists zero or one instance of entity A.
Referential integrity: The means of maintaining the integrity of data between one or more tables that relate to each other.
In other words, a column of data in a table has a null or matching value in a corresponding table. Referential integrity is usually enforced with foreign keys.
(RDBMS)Relational Database Management System: A software package that manages and provides access to a database. These packages follow Codd’s 12 rules of relational databases and normally use SQL to access data.