>识别要修改的行和
>计算新值.

.

这是一个例子：

UPDATE TABLE A
SET A.FLD_SUPV = ( SELECT B.FLD_SUPV
FROM TABLEA A,TABLEB B,TABLEC C,TABLED D
WHERE A.FLD1= B.FLD1
AND A.FLD_DT >= B.FLD_FM_DT
AND A.FLD_DT <= B.FLD_THRU_DT
AND A.FLD_DT > D.FLD_THRU_DT
AND A.FLD_DT < C.FLD_EFF_DT )
WHERE EXISTS ( SELECT B.FLD_SUPV
FROM TABLEA A,TABLED D
WHERE A.FLD1= B.FLD1
AND A.FLD_DT >= B.FLD_FM_DT
AND A.FLD_DT <= B.FLD_THRU_DT
AND A.FLD_DT > D.FLD_THRU_DT
AND A.FLD_DT < C.FLD_EFF_DT )

要更新两个字段,您可以使用如下示例：

UPDATE table1 t1 
 SET (col1,col2) = (
  SELECT col3,col4 
  FROM  table2 t2 
  WHERE t1.col8=t2.col9
 )

优化器将看到SET和FROM子句中的子查询是相同的,它应该在内部执行计划中合并它们.