USE tempdb
GO
CREATE TABLE test
(
    id int ,
    count1    decimal ,
    count2    decimal
)

INSERT INTO test
SELECT 1, 2 ,3
UNION ALL 
SELECT 2 ,NULL, 2
UNION ALL 
SELECT 3, 4, 3
UNION ALL 
SELECT 4 ,2 ,null


SELECT id,count1,count2,CONVERT(decimal(4,2),(ISNULL(count1,0)+ISNULL(count2,0))/ISNULL(count1,1))
 FROM test
 /*
 id          count1                                  count2                                  
----------- --------------------------------------- --------------------------------------- ---------------------------------------
1           2                                       3                                       2.50
2           NULL                                    2                                       2.00
3           4                                       3                                       1.75
4           2                                       NULL                                    1.00

(4 行受影响)

 
 */

------解决方案--------------------

SQL code

SELECT id,count1,count2,CONVERT(decimal(4,2),(ISNULL(count2,0)-ISNULL(count1,0))/(case when ISNULL(count1,0)=0 then 1 else count1 end))
 FROM test

------解决方案--------------------

select count1,count2,(convert(float,count2)-convert(float,count1))/convert(float,count1) increase
  from table