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
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;
/
只是想发布一个完整的代码，将创建一个表，并删除它，如果它已经存在使用Jeffrey的代码(赞美他，不是我!)
BEGIN
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE tablename';
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -942 THEN
RAISE;
END IF;
END;
EXECUTE IMMEDIATE 'CREATE TABLE tablename AS SELECT * FROM sourcetable WHERE 1=0';
END;
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
And if you want to make it re-enterable and minimize drop/create cycles, you could cache the DDL using dbms_metadata.get_ddl and re-create everything using a construct like this: declare v_ddl varchar2(4000); begin select dbms_metadata.get_ddl('TABLE','DEPT','SCOTT') into v_ddl from dual; [COMPARE CACHED DDL AND EXECUTE IF NO MATCH] exception when others then if sqlcode = -31603 then [GET AND EXECUTE CACHED DDL] else raise; end if; end; This is just a sample, there should be a loop inside with DDL type, name and owner being variables.
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE "IMS"."MAX" ';
EXCEPTION
WHEN OTHERS THEN
IF SQLCODE != -942 THEN
RAISE;
END IF;
EXECUTE IMMEDIATE '
CREATE TABLE "IMS"."MAX"
( "ID" NUMBER NOT NULL ENABLE,
"NAME" VARCHAR2(20 BYTE),
CONSTRAINT "MAX_PK" PRIMARY KEY ("ID")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "SYSAUX" ENABLE
) SEGMENT CREATION IMMEDIATE
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "SYSAUX" ';
END;
//执行这段代码，检查表是否存在，然后创建表max。这只适用于单次编译
