Database Tutorial

Oracle

Oracle treats comments as additional properties of tables, and requires that you add them after you’ve created the table. You find the table-level comments in the USER_TAB_COMMENTS, DBA_TAB_COMMENTS, and ALL_TAB_COLUMNS views. You find the column-level comments in the USER_COL_COMMENTS, DBA_COL_COMMENTS, and ALL_COL_COLUMNS views.

The following syntax lets you add a comment on a table:

COMMENT ON TABLE common_lookup IS 'A table of small tables, and it mimics the ENUM or SET data type in Oracle';

The following syntax lets you add a comment on a column of a table. You see an example for mapping the who-audit CREATED_BY column from the COMMON_LOOKUP table back to the SYSTEM_USER table and primary key SYSTEM_USER_ID column that hold the respective primary key. You must qualify the table and column name after the COLUMN keyword. They are separated by a period, which is the component selector (it selects a column from a table, field from a structure, et cetera).

COMMENT ON COLUMN common_lookup.created_by IS 'An unconstrained foreign key references SYSTEM_USER.SYSTEM_USER_ID.';

MySQL

MySQL doesn’t support the COMMENT statement in release 5.1. You can create table and column comments but you must use the CREATE or ALTER statements.

You can create a table with a column and table comment like this:

CREATE TABLE list
( list_id    INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
, list_item  VARCHAR(20) COMMENT 'String with or without leading or trailing white space.')
ENGINE = InnoDB COMMENT = 'Sample table of strings.';

You can see the comments by typing the following SHOW command:

SHOW CREATE TABLE list;

SQL Server

Under development …