文档课题：oracle表字段设置为unused的相关知识.
数据库：oracle 19.3
表emp_test相关信息.
SQL> select table_name,column_name,data_type,data_length,data_precision,data_scale,to_char(last_analyzed,'yyyy-mm-dd hh24:mi:ss') from user_tab_columns where table_name='EMP_TEST'

TABLE_NAME      COLUMN_NAME          DATA_TYPE       DATA_LENGTH DATA_PRECISION DATA_SCALE TO_CHAR(LAST_ANALYZ
--------------- -------------------- --------------- ----------- -------------- ---------- -------------------
EMP_TEST        EMPLOYEE_ID          NUMBER                   22              6          0 2022-09-04 12:10:17
EMP_TEST        FIRST_NAME           VARCHAR2                  9                           2022-09-04 12:10:17
EMP_TEST        LAST_NAME            VARCHAR2                 25                           2022-09-04 12:10:17
EMP_TEST        EMAIL                VARCHAR2                 25                           2022-09-04 12:10:17
EMP_TEST        PHONE_NUMBER         VARCHAR2                 20                           2022-09-04 12:10:17
EMP_TEST        HIRE_DATE            TIMESTAMP(6)             11                         6 2022-09-04 12:10:17
EMP_TEST        JOB_ID               VARCHAR2                 12                           2022-09-04 12:10:17
EMP_TEST        SALARY               NUMBER                   22              8          2 2022-09-04 12:10:17
EMP_TEST        COMMISSION_PCT       NUMBER                   22              2          2 2022-09-04 12:10:17
EMP_TEST        MANAGER_ID           NUMBER                   22              6          0 2022-09-04 12:10:17
EMP_TEST        DEPARTMENT_ID        NUMBER                   22              4          0 2022-09-04 12:10:17

11 rows selected.
SQL> desc emp_test
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 EMPLOYEE_ID                                                    NUMBER(6)
 FIRST_NAME                                                     VARCHAR2(9)
 LAST_NAME                                             NOT NULL VARCHAR2(25)
 EMAIL                                                 NOT NULL VARCHAR2(25)
 PHONE_NUMBER                                                   VARCHAR2(20)
 HIRE_DATE                                             NOT NULL TIMESTAMP(6)
 JOB_ID                                                NOT NULL VARCHAR2(12)
 SALARY                                                         NUMBER(8,2)
 COMMISSION_PCT                                                 NUMBER(2,2)
 MANAGER_ID                                                     NUMBER(6)
 DEPARTMENT_ID                                                  NUMBER(4)
设置email字段为unused.
SQL> alter table emp_test set unused (email);

Table altered.
再次查询，email字段不再显示.
SQL> select table_name,column_name,data_type,data_length,data_precision,data_scale,to_char(last_analyzed,'yyyy-mm-dd hh24:mi:ss') from user_tab_columns where table_name='EMP_TEST'
  2*

TABLE_NAME      COLUMN_NAME          DATA_TYPE       DATA_LENGTH DATA_PRECISION DATA_SCALE TO_CHAR(LAST_ANALYZ
--------------- -------------------- --------------- ----------- -------------- ---------- -------------------
EMP_TEST        EMPLOYEE_ID          NUMBER                   22              6          0 2022-09-04 12:10:17
EMP_TEST        FIRST_NAME           VARCHAR2                  9                           2022-09-04 12:10:17
EMP_TEST        LAST_NAME            VARCHAR2                 25                           2022-09-04 12:10:17
EMP_TEST        PHONE_NUMBER         VARCHAR2                 20                           2022-09-04 12:10:17
EMP_TEST        HIRE_DATE            TIMESTAMP(6)             11                         6 2022-09-04 12:10:17
EMP_TEST        JOB_ID               VARCHAR2                 12                           2022-09-04 12:10:17
EMP_TEST        SALARY               NUMBER                   22              8          2 2022-09-04 12:10:17
EMP_TEST        COMMISSION_PCT       NUMBER                   22              2          2 2022-09-04 12:10:17
EMP_TEST        MANAGER_ID           NUMBER                   22              6          0 2022-09-04 12:10:17
EMP_TEST        DEPARTMENT_ID        NUMBER                   22              4          0 2022-09-04 12:10:17

10 rows selected.
通过数据字典user_unused_col_tabs查出某表有几个字段被设置为unused.
SQL> desc user_unused_col_tabs;
 Name                    Null?    Type
 ----------------------- -------- ----------------
 TABLE_NAME              NOT NULL VARCHAR2(128)
 COUNT                            NUMBER
SQL> select * from user_unused_col_tabs;

TABLE_NAME           COUNT
--------------- ----------
EMP_TEST                 1

说明：如上所示，表emp_test有1个字段被设置为unused，但目前未找到查询具体哪个字段被设置为unused的视图.
删除unused字段.
SQL> alter table emp_test drop unused column;

Table altered.

SQL> select * from user_unused_col_tabs;

no rows selected