语法：
SUBSTR(string,start_position,[length])，获取子字符串
说明：string:源字符串
start_position:开始位置
length:字符串的长度[可选]
实例1：
一、substr(“ABCDEFG”, 0); //结果：ABCDEFG，截取所有字符
二、substr(“ABCDEFG”, 2); //结果：CDEFG，截取从C开始之后所有字符
实例2：
select a.ID, a.THID, a.NUM, a.CATEGORY, a.WBS_COSTCENTER, a.WBS_COSTCENTER_NAME,a.PROJECT_COMPANY AS DEVICE_DEPART,‘丙烯腈二期项目’ AS PROJECT_COMPANY,
(case SUBSTR(c.CODE,0,10) when ‘2800020201’ then ‘生产装置区’ when ‘2800020202’ then ‘生产辅助设施’ when ‘2800020203’ THEN ‘公共费用’ else ‘工程备料专用WBS’ END ) COSTCOLLECTIONPOINT
from MakeContracts_Extend1 a
LEFT JOIN SYS_TREE c on SUBSTR(a.WBS_COSTCENTER,0,15) =c.EXTEND1
where THID=‘XYHT1800396’ order by Num