生成3位的序列号_仅仅CASE WHEN的简单应用

 SELECT TOP 200
     PNo, IDENTITY( INT,1,1 ) AS sn
 INTO
     #temp1
 FROM
     tbl_test 

 SELECT
     CASE WHEN sn < 10 THEN '' + CONVERT(CHAR(1), sn)
          WHEN sn >= 10
               AND sn <= 99 THEN '' + CONVERT(CHAR(2), sn)
          WHEN sn >= 100 THEN CONVERT(CHAR(3), sn)
     END AS sn, PNo
 FROM
     #temp1