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问题:
I'm writing some migration scripts for an Oracle database, and was hoping Oracle had something similar to MySQL's IF EXISTS construct.
Specifically, whenever I want to drop a table in MySQL, I do something like
DROP TABLE IF EXISTS `table_name`;
This way, if the table doesn't exist, the DROP doesn't produce an error, and the script can continue.
Does Oracle have a similar mechanism? I realize I could use the following query to check if a table exists or not
SELECT * FROM dba_tables where table_name = 'table_name';
but the syntax for tying that together with a DROP is escaping me.
回答1:
The best and most efficient way is to catch the "table not found" exception: this avoids the overhead of checking if the table exists twice; and doesn't suffer from the problem that if the DROP fails for some other reason (that might be important) the exception is still raised to the caller:
BEGIN EXECUTE IMMEDIATE 'DROP TABLE mytable'; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -942 THEN RAISE; END IF; END;
ADDENDUM For reference, here are the equivalent blocks for other object types:
Sequence
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;
Trigger
BEGIN EXECUTE IMMEDIATE 'DROP TRIGGER ' || trigger_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -4080 THEN RAISE; END IF; END;
Index
BEGIN EXECUTE IMMEDIATE 'DROP INDEX ' || index_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -1418 THEN RAISE; END IF; END;
Column
BEGIN EXECUTE IMMEDIATE 'ALTER TABLE ' || table_name || ' DROP COLUMN ' || column_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -904 THEN RAISE; END IF; END;
Database Link
BEGIN EXECUTE IMMEDIATE 'DROP DATABASE LINK ' || dblink_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -2024 THEN RAISE; END IF; END;
Materialized View
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;
Constraint
BEGIN EXECUTE IMMEDIATE 'ALTER TABLE ' || table_name || ' DROP CONSTRAINT ' || constraint_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -2443 THEN RAISE; END IF; END;
Scheduler Job
BEGIN DBMS_SCHEDULER.drop_job(job_name); EXCEPTION WHEN OTHERS THEN IF SQLCODE != -27475 THEN RAISE; END IF; END;
User / Schema
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;
Package
BEGIN EXECUTE IMMEDIATE 'DROP PACKAGE ' || package_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -4043 THEN RAISE; END IF; END;
Procedure
BEGIN EXECUTE IMMEDIATE 'DROP PROCEDURE ' || procedure_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -4043 THEN RAISE; END IF; END;
Function
BEGIN EXECUTE IMMEDIATE 'DROP FUNCTION ' || function_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -4043 THEN RAISE; END IF; END;
Tablespace
BEGIN EXECUTE IMMEDIATE 'DROP TABLESPACE' || tablespace_name; EXCEPTION WHEN OTHERS THEN IF SQLCODE != -959 THEN RAISE; END IF; END;
回答2:
declare c int; begin select count(*) into c from user_tables where table_name = upper('table_name'); if c = 1 then execute immediate 'drop table table_name'; end if; end;
That's for checking whether a table in the current schema exists. For checking whether a given table already exists in a different schema, you'd have to use all_tables instead of user_tables and add the condition all_tables.owner = upper('schema_name')
回答3:
I have been looking for the same but I ended up writing a procedure to help me out:
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;
Hope this helps
回答4:
just wanted to post a full code that will create a table and drop it if it already exists using Jeffrey's code (kudos to him, not me!).
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;
回答5:
With SQL*PLUS you can also use the WHENEVER SQLERROR command:
WHENEVER SQLERROR CONTINUE NONE DROP TABLE TABLE_NAME; WHENEVER SQLERROR EXIT SQL.SQLCODE DROP TABLE TABLE_NAME;
With CONTINUE NONE an error is reported, but the script will continue. With EXIT SQL.SQLCODE the script will be terminated in the case of an error.
see also: WHENEVER SQLERROR Docs
回答6:
There is no 'DROP TABLE IF EXISTS' in oracle, you would have to do the select statement.
try this (i'm not up on oracle syntax, so if my variables are ify, please forgive me):
declare @count int select @count=count(*) from all_tables where table_name='Table_name'; if @count>0 BEGIN DROP TABLE tableName; END
回答7:
Another method is to define an exception and then only catch that exception letting all others propagate.
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;
回答8:
One way is to use DBMS_ASSERT.SQL_OBJECT_NAME :
This function verifies that the input parameter string is a qualified SQL identifier of an existing SQL object.
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 Demo
回答9:
Sadly no, there is no such thing as drop if exists, or CREATE IF NOT EXIST
You could write a plsql script to include the logic there.
http://download.oracle.com/docs/cd/B12037_01/server.101/b10759/statements_9003.htm
I'm not much into Oracle Syntax, but i think @Erich's script would be something like this.
declare cant integer begin select into cant count(*) from dba_tables where table_name='Table_name'; if count>0 then BEGIN DROP TABLE tableName; END IF; END;
回答10:
You could always catch the error yourself.
begin execute immediate 'drop table mytable'; exception when others then null; end;
It is considered bad practice to overuse this, similar to empty catch()'es in other languages.
Regards
K
回答11:
I prefer to specify the table and the schema owner.
Watch out for case sensitivity as well. (see "upper" clause below).
I threw a couple of different objects in to show that is can be used in places besides TABLEs.
.............
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;
And a TABLE example:
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; /
回答12:
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;
// Doing this code, checks if the table exists and later it creates the table max. this simply works in single compilation
回答13:
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.
回答14:
A block like this could be useful to you.
DECLARE table_exist INT; BEGIN SELECT Count(*) INTO table_exist FROM dba_tables WHERE owner = 'SCHEMA_NAME' AND table_name = 'EMPLOYEE_TABLE'; IF table_exist = 1 THEN EXECUTE IMMEDIATE 'drop table EMPLOYEE_TABLE'; END IF; END;
