Update one table select values from other table

desc test
Name       Null     Type        
---------- -------- -------------
LEVEL_ID            NUMBER      
LEVEL_NAME          VARCHAR2(10)
LEVEL_DESC          VARCHAR2(255)
CAST_NUM   NOT NULL NUMBER(7)    


desc test_bk
Name       Null     Type        
---------- -------- -------------
LEVEL_ID            NUMBER      
LEVEL_NAME          VARCHAR2(10)
LEVEL_DESC          VARCHAR2(255)
CAST_NUM   NOT NULL NUMBER(7)    



Update cast_num from test with values of cast_num from test_bk

UPDATE  SET CAST_NUM= (SELECT CAST_NUM FROM TEST_BK WHERE TEST_BK.LEVEL_ID=TEST.LEVEL_ID)
where exists (select 1 FROM TEST_BK WHERE TEST_BK.LEVEL_ID=TEST.LEVEL_ID);