在数据库中创建表以后，有时需要添加表或者列的描述信息，也就是注释。这样当后来者查看表结构时，就可以清楚的知道表或者字段的含义。

1. 语法

COMMENT ON

{ TABLE [ schema. ]

{ table | view }

| COLUMN [ schema. ]

{ table. | view. | materialized_view. } column

| OPERATOR [ schema. ] operator

| INDEXTYPE [ schema. ] indextype

| MATERIALIZED VIEW materialized_view

}

IS 'text' ;

2. 添加注释

(1)添加表级注释

SQL> COMMENT ON TABLE EMP IS 'EMPLOYEES';

(2)添加列级注释

SQL> COMMENT ON COLUMN EMP.ENAME IS 'EMPLOYEES NAME';

3. 查看已添加注释

(1)查看表级注释

SQL> SELECT * FROM USER_TAB_COMMENTS WHERE TABLE_NAME='EMP';

(2)查看列级注释

SQL> SELECT * FROM USER_COL_COMMENTS WHERE COLUMN_NAME='ENAME' AND COMMENTS IS NOT NULL;

4. 删除注释(即，添加空注释)

SQL> COMMENT ON TABLE EMP IS '';

SQL> COMMENT ON COLUMN EMP.ENAME IS '';

SQL> SELECT * FROM USER_TAB_COMMENTS WHERE TABLE_NAME='EMP';

SQL> SELECT * FROM USER_COL_COMMENTS WHERE COLUMN_NAME='ENAME' AND TABLE_NAME='EMP';