oracle comment on的用法

古城微笑少年丶 2021-06-26 16:06 379阅读 0赞

`oracle`中用`comment on`命令给表或字段加以说明，语法如下：

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' ;

用法如下：

1.对表的说明

comment on table table_name is 'comments_on_tab_information';

2.对表中列的说明

comment on column table.column_name is 'comments_on_col_information';

3.查看表的说明

SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';
    
    TABLE_NAME                     TABLE_TYPE  COMMENTS
    ------------------------------ ----------- ----------
    EMPLOYEES                      TABLE       员工表
    
    SQL> select * from user_tab_comments where comments is not null;
    
    TABLE_NAME                     TABLE_TYPE  COMMENTS
    ------------------------------ ----------- --------------------------
    EMPLOYEES                      TABLE       员工表

4.查看表中列的说明

SQL> select * from user_col_comments where TABLE_NAME='EMPLOYEES';
    
    TABLE_NAME                     COLUMN_NAME                    COMMENTS
    ------------------------------ ------------------------------ ------------
    EMPLOYEES                      EMPLOYEE_ID                    
    EMPLOYEES                      MANAGER_ID                     
    EMPLOYEES                      FIRST_NAME                     
    EMPLOYEES                      LAST_NAME                      
    EMPLOYEES                      TITLE                          
    EMPLOYEES                      SALARY                         员工薪水
    
    SQL> select * from user_col_comments where comments is not null;
    
    TABLE_NAME                     COLUMN_NAME                    COMMENTS
    ------------------------------ ------------------------------ -------------
    EMPLOYEES                      SALARY                         员工薪水

5.我们也可以从下面这些视图中查看表级和列级说明：

ALL_COL_COMMENTS
    USER_COL_COMMENTS
    ALL_TAB_COMMENTS
    USER_TAB_COMMENTS

6.删除表级说明，也就是将其置为空

SQL> comment on table employees is '';
    Comment added
    
    SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';
    
    TABLE_NAME                     TABLE_TYPE  COMMENTS
    ------------------------------ ----------- -------------
    EMPLOYEES                      TABLE

7.删除列级说明，也是将其置为空

SQL> comment on column employees.salary is '';
    Comment added
    
    SQL> select * from user_col_comments where TABLE_NAME='EMPLOYEES';
    
    TABLE_NAME                     COLUMN_NAME                    COMMENTS
    ------------------------------ ------------------------------ -------------
    EMPLOYEES                      EMPLOYEE_ID                    
    EMPLOYEES                      MANAGER_ID                     
    EMPLOYEES                      FIRST_NAME                     
    EMPLOYEES                      LAST_NAME                      
    EMPLOYEES                      TITLE                          
    EMPLOYEES                      SALARY