Visible and Invisible Columns in SQL
Definition of Visible and Invisible Columns
- Visible Columns: These are the columns that are shown by default when querying a table or view. They are the standard columns that users interact with in their queries.
- Invisible Columns: These columns are present in the table or view but are not displayed by default in query results. They are used for purposes such as maintaining historical data, for internal calculations, or for compatibility reasons without exposing them to end users.
Creating and Modifying Invisible Columns
Creating a Table with Visible and Invisible Columns
When creating a table, you can specify whether a column is visible or invisible.
Syntax:
CREATE TABLE table_name ( visible_column1 datatype, visible_column2 datatype, invisible_column1 datatype INVISIBLE, invisible_column2 datatype INVISIBLE );
Example:
CREATE TABLE employees ( employee_id NUMBER PRIMARY KEY, first_name VARCHAR2(50), last_name VARCHAR2(50), salary NUMBER, hire_date DATE INVISIBLE, internal_notes VARCHAR2(100) INVISIBLE );
In this example, hire_date and internal_notes are invisible columns, while employee_id, first_name, last_name, and salary are visible columns.
Adding an Invisible Column to an Existing Table
You can add invisible columns to an existing table using the ALTER TABLE statement.
Syntax:
ALTER TABLE table_name ADD column_name datatype INVISIBLE;
Example:
ALTER TABLE employees ADD last_review_date DATE INVISIBLE;
Making a Visible Column Invisible
You can change a column from visible to invisible using the ALTER TABLE statement.
Syntax:
ALTER TABLE table_name MODIFY column_name INVISIBLE;
Example:
ALTER TABLE employees MODIFY salary INVISIBLE;
Making an Invisible Column Visible
You can also change a column from invisible to visible.
Syntax:
ALTER TABLE table_name MODIFY column_name VISIBLE;
Example:
ALTER TABLE employees MODIFY last_review_date VISIBLE;
Querying Tables with Invisible Columns
Invisible columns are not displayed in standard SELECT * queries. However, you can explicitly include invisible columns in your queries.
Syntax:
SELECT visible_column1, visible_column2, invisible_column1 FROM table_name;
Example:
SELECT employee_id, first_name, hire_date FROM employees;
In this example, hire_date is an invisible column, so it needs to be explicitly specified in the query to be retrieved.
Use Cases and Benefits
- Data Security: Invisible columns can be used to store sensitive or internal information that should not be exposed to general users but still needs to be maintained in the database.
- Compatibility: Invisible columns can be used for backward compatibility or for maintaining historical data without affecting existing queries or applications.
- Performance Optimization: By hiding certain columns, you can streamline query performance and reduce the amount of data processed or transmitted.
- Clean Data Exposure: You can ensure that only relevant columns are visible to users, simplifying the interaction with the database and reducing the risk of accidental misuse of data.
Best Practices
- Document Invisible Columns: Clearly document the purpose of invisible columns and their usage to avoid confusion for developers and database administrators.
- Access Control: Use invisible columns judiciously, especially when dealing with sensitive information. Ensure proper access control measures are in place.
- Testing: Thoroughly test any changes involving invisible columns to ensure they do not disrupt existing queries or application functionality.
- Backup and Recovery: Ensure that invisible columns are included in database backup and recovery plans, as they might contain critical information.
Limitations
- Compatibility: Not all SQL databases support invisible columns. This feature is particularly available in Oracle Database and might not be present in other SQL systems.
- Complexity: Managing invisible columns can add complexity to database design and querying. Ensure that the benefits outweigh the additional complexity.
Conclusion
Invisible columns provide a flexible way to manage how data is exposed in a SQL database. They are useful for maintaining internal data, enhancing security, and improving data management. By understanding how to create, modify, and query invisible columns, you can effectively use this feature to meet your database requirements while keeping the interface clean and focused.