我正在为Oracle数据库写一些迁移脚本，并希望Oracle有类似于MySQL的IF EXISTS构造的东西。
具体来说，每当我想在MySQL中删除一个表时，我就会做这样的事情
DROP TABLE IF EXISTS `table_name`;
这样，如果表不存在，DROP也不会产生错误，脚本可以继续。
Oracle有类似的机制吗?我意识到我可以使用下面的查询来检查表是否存在
SELECT * FROM dba_tables where table_name = 'table_name';
但是把它和DROP绑定在一起的语法让我很困惑。
当前回答
一种方法是使用DBMS_ASSERT。SQL_OBJECT_NAME:
此函数验证输入参数字符串是否是现有SQL对象的合格SQL标识符。
DECLARE
V_OBJECT_NAME VARCHAR2(30);
BEGIN
BEGIN
V_OBJECT_NAME := DBMS_ASSERT.SQL_OBJECT_NAME('tab1');
EXECUTE IMMEDIATE 'DROP TABLE tab1';
EXCEPTION WHEN OTHERS THEN NULL;
END;
END;
/
DBFiddle演示
其他回答
最好和最有效的方法是捕获“table not found”异常:这避免了检查表是否存在两次的开销;并且不会遇到DROP因为其他原因(可能很重要)失败时仍然会向调用者抛出异常的问题:
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE ' || table_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -942 THEN
RAISE;
END IF;
END;
齿顶高 作为参考，下面是其他对象类型的等效块:
序列
BEGIN
EXECUTE IMMEDIATE 'DROP SEQUENCE ' || sequence_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -2289 THEN
RAISE;
END IF;
END;
View
BEGIN
EXECUTE IMMEDIATE 'DROP VIEW ' || view_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -942 THEN
RAISE;
END IF;
END;
触发
BEGIN
EXECUTE IMMEDIATE 'DROP TRIGGER ' || trigger_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -4080 THEN
RAISE;
END IF;
END;
指数
BEGIN
EXECUTE IMMEDIATE 'DROP INDEX ' || index_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -1418 THEN
RAISE;
END IF;
END;
列
BEGIN
EXECUTE IMMEDIATE 'ALTER TABLE ' || table_name
|| ' DROP COLUMN ' || column_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -904 AND SQLCODE != -942 THEN
RAISE;
END IF;
END;
数据库链接
BEGIN
EXECUTE IMMEDIATE 'DROP DATABASE LINK ' || dblink_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -2024 THEN
RAISE;
END IF;
END;
物化视图
BEGIN
EXECUTE IMMEDIATE 'DROP MATERIALIZED VIEW ' || mview_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -12003 THEN
RAISE;
END IF;
END;
Type
BEGIN
EXECUTE IMMEDIATE 'DROP TYPE ' || type_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -4043 THEN
RAISE;
END IF;
END;
约束
BEGIN
EXECUTE IMMEDIATE 'ALTER TABLE ' || table_name
|| ' DROP CONSTRAINT ' || constraint_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -2443 AND SQLCODE != -942 THEN
RAISE;
END IF;
END;
调度程序工作
BEGIN
DBMS_SCHEDULER.drop_job(job_name);
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -27475 THEN
RAISE;
END IF;
END;
用户/架构
BEGIN
EXECUTE IMMEDIATE 'DROP USER ' || user_name;
/* you may or may not want to add CASCADE */
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -1918 THEN
RAISE;
END IF;
END;
包
BEGIN
EXECUTE IMMEDIATE 'DROP PACKAGE ' || package_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -4043 THEN
RAISE;
END IF;
END;
过程
BEGIN
EXECUTE IMMEDIATE 'DROP PROCEDURE ' || procedure_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -4043 THEN
RAISE;
END IF;
END;
函数
BEGIN
EXECUTE IMMEDIATE 'DROP FUNCTION ' || function_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -4043 THEN
RAISE;
END IF;
END;
表空间
BEGIN
EXECUTE IMMEDIATE 'DROP TABLESPACE ' || tablespace_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -959 THEN
RAISE;
END IF;
END;
同义词
BEGIN
EXECUTE IMMEDIATE 'DROP SYNONYM ' || synonym_name;
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -1434 THEN
RAISE;
END IF;
END;
我一直在寻找同样的东西，但我最终写了一个程序来帮助我:
CREATE OR REPLACE PROCEDURE DelObject(ObjName varchar2,ObjType varchar2)
IS
v_counter number := 0;
begin
if ObjType = 'TABLE' then
select count(*) into v_counter from user_tables where table_name = upper(ObjName);
if v_counter > 0 then
execute immediate 'drop table ' || ObjName || ' cascade constraints';
end if;
end if;
if ObjType = 'PROCEDURE' then
select count(*) into v_counter from User_Objects where object_type = 'PROCEDURE' and OBJECT_NAME = upper(ObjName);
if v_counter > 0 then
execute immediate 'DROP PROCEDURE ' || ObjName;
end if;
end if;
if ObjType = 'FUNCTION' then
select count(*) into v_counter from User_Objects where object_type = 'FUNCTION' and OBJECT_NAME = upper(ObjName);
if v_counter > 0 then
execute immediate 'DROP FUNCTION ' || ObjName;
end if;
end if;
if ObjType = 'TRIGGER' then
select count(*) into v_counter from User_Triggers where TRIGGER_NAME = upper(ObjName);
if v_counter > 0 then
execute immediate 'DROP TRIGGER ' || ObjName;
end if;
end if;
if ObjType = 'VIEW' then
select count(*) into v_counter from User_Views where VIEW_NAME = upper(ObjName);
if v_counter > 0 then
execute immediate 'DROP VIEW ' || ObjName;
end if;
end if;
if ObjType = 'SEQUENCE' then
select count(*) into v_counter from user_sequences where sequence_name = upper(ObjName);
if v_counter > 0 then
execute immediate 'DROP SEQUENCE ' || ObjName;
end if;
end if;
end;
希望这能有所帮助
另一种方法是定义一个异常，然后只捕获该异常，让所有其他异常传播。
Declare
eTableDoesNotExist Exception;
PRAGMA EXCEPTION_INIT(eTableDoesNotExist, -942);
Begin
EXECUTE IMMEDIATE ('DROP TABLE myschema.mytable');
Exception
When eTableDoesNotExist Then
DBMS_Output.Put_Line('Table already does not exist.');
End;
您总是可以自己发现错误。
begin
execute immediate 'drop table mytable';
exception when others then null;
end;
过度使用它被认为是不好的实践，类似于其他语言中的空catch()。
问候 K
我更倾向于指定表和模式所有者。
还要注意大小写敏感性。(见下文“上”条款)。
我扔了几个不同的对象，以表明它可以用在桌子之外的地方。
.............
declare
v_counter int;
begin
select count(*) into v_counter from dba_users where upper(username)=upper('UserSchema01');
if v_counter > 0 then
execute immediate 'DROP USER UserSchema01 CASCADE';
end if;
end;
/
CREATE USER UserSchema01 IDENTIFIED BY pa$$word
DEFAULT TABLESPACE users
TEMPORARY TABLESPACE temp
QUOTA UNLIMITED ON users;
grant create session to UserSchema01;
还有一个TABLE的例子:
declare
v_counter int;
begin
select count(*) into v_counter from all_tables where upper(TABLE_NAME)=upper('ORDERS') and upper(OWNER)=upper('UserSchema01');
if v_counter > 0 then
execute immediate 'DROP TABLE UserSchema01.ORDERS';
end if;
end;
/
