trunc是截断；round是四舍五入；下面通过一个例子具体看一下trunc和round的不同
SQL> select trunc(45.923,2),trunc(45.923),trunc(45.923,-1)
2 from dual;
TRUNC(45.923,2) TRUNC(45.923) TRUNC(45.923,-1)
--------------- ------------- ----------------
45.92 45 40
SQL> select round(45.923,2),round(45.923),round(45.923,-1)
2 from dual;
ROUND(45.923,2) ROUND(45.923) ROUND(45.923,-1)
--------------- ------------- ----------------
45.92 46 50
原文：http://www.cnblogs.com/tomatoes-/p/6065140.html