update关联其他表批量更新数据
为了做分析，需要整合一些数据到一个表中，涉及到通过主键关联多个表，获取其中的某些字段的值，通过update可以简单实现，找了一篇文章，挺不错：
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 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))
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 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))
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));
