在MySql中如何select from一個將要更新的關系目標:
問題陳述:
在《數據庫系統概念第五版》(<DatabaseSystem Concepts> Fifth Edition),第三章,3.10.3講SQL的更新,有個例子是:
+-------------------------+--------------------+------------+
|account_number | branch_name | balance |
+------------------------+---------------------+------------+
|A-101 | Downtown | 500.00 |
|A-102 | Perryridge | 400.00 |
|A-201 | Brighton | 900.00 |
|A-215 | Mianus | 700.00 |
|A-217 | Brighton | 750.00 |
|A-222 | Redwood | 700.00 |
|A-305 | Round Hill | 350.00 |
+------------------------+----------------------+------------+
updateaccount
setbalance = balance * 1.05
wherebalance > (select avg(balance)
fromaccount);
然後就報錯了!有沒有!如下:
Youcan't specify target table 'account' for update in FROM clause。
錯誤就是你不能指向並選擇一個將要修改或是更新的目標關系。
http://dev.mysql.com/doc/refman/5.0/en/update.html寫到:
“Currently,you cannot update a table and select from the same table in asubquery.”
但是很多情況下,我想用一些數據要更新一個關系,而這些數據恰好是通過就指向這個關系的子查詢得到的,例如本例子我需要聚集函數算account的balance均值。
解決方法:
MySQL會將from從句中子查詢的衍生關系(derivedtable)實體化成一個臨時表(temporarytable),所以我們將(selectavg(balance) from account) 再套入一個from從句即可:
updateaccount
setbalance = balance * 1.05
wherebalance >( select avg(tmp.balance)
from (select * from account ) as tmp
)
參考:
http://www.xaprb.com/blog/2006/06/23/how-to-select-from-an-update-target-in-mysql/
http://dev.mysql.com/doc/refman/5.0/en/update.html
http://forge.mysql.com/wiki/MySQL_Internals