db2，oracle, Sql Server 批量更新语句

-- aa(id,qty) ,bb(id,qty,loc_id(货架号))两个表.目的是把aa.qty=bb.qty
db2 update aa set qty=(select qty from bb where bb.id=aa.id and bb.loc_id=9) where exists (select 1 from bb where bb.id=aa.id and bb.loc_id=9)

-- 注：如果不加where exits这个条件,那么会把aa表中aa._id!=bb._id的那部分aa.qty数据给update成null.

 批量更新表时，update一个表的列时，需要依赖另外的表，这种依赖可以是where条件子句，也可以要update的field的值依赖另外的表   通常有两种做法  
 1.使用存储过程   

 2.在程序代码里逐条循环执行这里给出一种更高效、简洁的做法，批量更新SQL ，一句SQL就可以替代麻烦的循环过程，有MS SQLServer、Oracle、DB2下的写法

-- 关键点：t4和t1是同一个table，primary key肯定也是同一个，--并以它进行关联，这样在 select语句里即可引用到要update的表的fields

UPDATE Table1 AS t1
SET (Field1,Field2) = (SELECT Field21, Field22
                       FROM Table2 t2
                            INNER JOIN Table3 t3
                                 ON t3.Field31 = t2.Field23
                            INNER JOIN Table4 t4
                                 ON t3.Field32 = t4.Filed41
                       WHERE t2.Field24 >= ''
                                      AND t1.fId = t4.fId);
----------------------------MS SQLServer --------------------------------------

UPDATE t1
SET Field1 = Field21, Field2 = Field22
FROM Table2 t2
         INNER JOIN Table3 t3
              ON t3.Field31 = t2.Field23
         INNER JOIN Table4 t4
              ON t3.Field32 = t4.Filed41
WHERE ((t2.Field24 >= '')
                  AND t1.fId = t4.fId);

----------------------------oracle--------------------------------------
UPDATE Table1 t1SET (Field1,Field2) = (SELECT Field21, Field22                       
FROM Table2 t2                            
　　　　　　INNER JOIN Table3 t3                                 
　　　　　　　　ON t3.Field31 = t2.Field23                           
　　　　　　INNER JOIN Table4 t4                                 
　　　　　　　　ON t3.Field32 = t4.Filed41                          
WHERE ((t2.Field24 >= '')                                 
　　　　　　　　　　AND t1.fId = t4.fId))
WHERE EXISTS (SELECT Field21, Field22              
FROM Table2 t2                   
　　　　　　INNER JOIN Table3 t3                        
　　　　　　　　ON t3.Field31 = t2.Field23                  
　　　　　　INNER JOIN Table4 t4                        
　　　　　　　　ON t3.Field32 = t4.Filed41              
WHERE ((t2.Field24 >= '')                               
　　　　　　　　　　AND t1.fId = t4.fId));

---------------------------------DB2 ------------------------------------------
UPDATE Table1 AS t1SET (Field1,Field2) = (SELECT Field21, Field22                       
FROM Table2 t2                            
　　　　INNER JOIN Table3 t3                                 
　　　　　　ON t3.Field31 = t2.Field23                            
　　　　INNER JOIN Table4 t4                                
　　　　　　ON t3.Field32 = t4.Filed41                       
WHERE ((t2.Field24 >= '')                                         
　　　　　　　　AND t1.fId = t4.fId))
WHERE EXISTS (SELECT Field21, Field22              
FROM Table2 t2                   
　　　　INNER JOIN Table3 t3                        
　　　　　　ON t3.Field31 = t2.Field23
　　　　INNER JOIN Table4 t4                        
　　　　　　ON t3.Field32 = t4.Filed41              
WHERE ((t2.Field24 >= '')                                
　　　　　　　　AND t1.fId = t4.fId));

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