Commenting SQL database objects

Did you know that you can annotate database object like tables, views and columns with comments in many SQL database systems? By that I don’t mean comments in SQL scripts, indicated by double dashes (--), but comments attached to the objects themselves, stored in the database. These may be helpful to the database admin by providing context via a description text on what is stored in these objects.

For PostgreSQL and Oracle databases the syntax is as follows:

COMMENT ON TABLE [schema_name.]table_name IS '...';
COMMENT ON COLUMN [schema_name.]table_name.column_name IS '...';

For example:

COMMENT ON COLUMN books.author IS 'The main author''s last name';
COMMENT ON TABLE books IS 'Contains only the best books';

These comments can be viewed in database tools like SQL Developer:

Comments on columns
Comments on tables

You can also view the comments in psql:

db=# \d+ books
 Column |  Type   |          Description
--------+---------+------------------------------
id      | integer |
author  | text    | The main author''s last name
title   | text    |

And for a table:

db=# \dt+ books
                    List of relations
 Schema | Name  | Type  |     |        Description
--------+-------+-------+ ... +------------------------------
public  | books | table |     | Contains only the best books

In Oracle you can query the comments from the data dictionary views ALL_TAB_COMMENTS and ALL_COL_COMMENTS:

> SELECT * FROM all_col_comments WHERE table_name='BOOKS';
OWNER    TABLE_NAME  COLUMN_NAME  COMMENTS
--------------------------------------------------------------
LIBRARY	 BOOKS	     ID           (null)
LIBRARY	 BOOKS	     AUTHOR       The main author's last name
LIBRARY	 BOOKS	     TITLE        (null)

> SELECT * FROM all_tab_comments WHERE table_name='BOOKS';
OWNER    TABLE_NAME  TABLE_TYPE  COMMENTS
--------------------------------------------------------------
LIBRARY	 BOOKS	     TABLE       Contains only the best books

In Oracle comments are limited to tables, views, materialized views, columns, operators and indextypes, but in PostgreSQL you can attach comments to nearly everything. Another good use case for this are documentation comments on database functions:

COMMENT ON FUNCTION my_function IS $$
This function does something important.

Parameters:
...
Example usage:
...
$$;

Note: the $$ delimits multi-line strings (called dollar quoted string constants).

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