Adding and managing comments with SQL

Adding and managing comments

Adding Comments to Database Objects

1.1 Adding Comments to Tables

Syntax: 

-- Add a comment to a table
COMMENT ON TABLE table_name IS 'Your comment here';

 Example: 

-- Add a comment to the 'employees' table
COMMENT ON TABLE employees IS 'Table containing employee information, including personal details and job-related data.';

Adding Comments to Columns

Syntax: 

-- Add a comment to a column
COMMENT ON COLUMN table_name.column_name IS 'Your comment here';

Example: 

-- Add a comment to the 'salary' column in the 'employees' table
COMMENT ON COLUMN employees.salary IS 'Employee salary in USD';

Adding Comments to Indexes

Syntax: 

-- Add a comment to an index
COMMENT ON INDEX index_name IS 'Your comment here';

Example: 

-- Add a comment to the 'emp_idx' index

COMMENT ON INDEX emp_idx IS 'Index on employee table for optimized search by employee ID';

 Adding Comments to Views

Syntax: 

-- Add a comment to a view
COMMENT ON VIEW view_name IS 'Your comment here';

Example: 

-- Add a comment to the 'employee_view' view
COMMENT ON VIEW employee_view IS 'View showing detailed information of employees for HR analysis';

Adding Comments to Synonyms

Syntax: 

-- Add a comment to a synonym
COMMENT ON SYNONYM synonym_name IS 'Your comment here';

 Example: 

-- Add a comment to the 'emp_synonym' synonym
COMMENT ON SYNONYM emp_synonym IS 'Synonym for the employees table used in reports';

 Reading Comments from Database Objects

To retrieve comments from database objects, Oracle provides several data dictionary views. The most commonly used views for this purpose are ALL_COL_COMMENTS, ALL_TAB_COMMENTS, ALL_IND_COMMENTS, ALL_VIEWS, and similar views.

Reading Table Comments

View: ALL_TAB_COMMENTS

Structure:

• TABLE_NAME: Name of the table.
• TABLE_TYPE: Type of the object (TABLE).
• COMMENTS: Comments associated with the table.

Example Query: 

-- Retrieve comments for tables accessible by the user
SELECT table_name, comments
FROM all_tab_comments
WHERE table_name = 'EMPLOYEES';

Reading Column Comments

View: ALL_COL_COMMENTS

Structure:

• TABLE_NAME: Name of the table.
• COLUMN_NAME: Name of the column.
• COMMENTS: Comments associated with the column.

Example Query: 

-- Retrieve comments for columns in the 'employees' table
SELECT table_name, column_name, comments
FROM all_col_comments
WHERE table_name = 'EMPLOYEES';

Reading Index Comments

View: ALL_IND_COMMENTS

Structure:

• INDEX_NAME: Name of the index.
• COMMENTS: Comments associated with the index.

Example Query: 

-- Retrieve comments for indexes accessible by the user
SELECT index_name, comments
FROM all_ind_comments
WHERE index_name = 'EMP_IDX';

Reading View Comments

View: ALL_VIEWS

Structure:

• VIEW_NAME: Name of the view.
• COMMENTS: Comments associated with the view.

Example Query: 

-- Retrieve comments for views accessible by the user
SELECT view_name, comments
FROM all_views
WHERE view_name = 'EMPLOYEE_VIEW';

Reading Synonym Comments

View: ALL_SYNONYMS

Structure:

• SYNONYM_NAME: Name of the synonym.
• COMMENTS: Comments associated with the synonym.

Example Query: 

-- Retrieve comments for synonyms accessible by the user
SELECT synonym_name, comments
FROM all_synonyms
WHERE synonym_name = 'EMP_SYNONYM';

Benefits of Using Comments

• Documentation: Comments provide context and explanation about the purpose and usage of database objects.
• Maintenance: Helps developers and DBAs understand the intent behind design decisions, making it easier to maintain and modify the schema.
• Collaboration: Facilitates collaboration among team members by providing clear explanations of schema components.

Best Practices

• Be Descriptive: Use clear and concise comments to explain the purpose and constraints of tables, columns, and other database objects.
• Keep Comments Updated: Ensure comments are updated as the schema evolves to maintain accuracy.
• Avoid Redundancy: Do not repeat information that is already evident from the object’s name or structure.

Conclusion

Adding and reading comments in Oracle Database is an important practice for ensuring that your database schema is well-documented and easy to understand. Comments enhance clarity, aid in maintenance, and improve collaboration within development and DBA teams. By using the provided SQL syntax and querying the appropriate data dictionary views, you can effectively manage and retrieve comments for various database objects.