如何使用servlet和jsp获取Null值查询表达式

Question

I need to calculate and retrieve data from SQL server database table in my table having the NoOfErrors and NoOfAudited columns.

I need to calculate the Accuracy based on these columns.

I have a query like this

select 
   a.id, 
   (100-((a.NoOfErrors*100)/ NULLIF(a.NoOfAudited,0))) as Accuracy 
from Table1 a 
 join Table2 pd 
   on a.batchid=pd.id 
where 
  a.charge='"+Poster+"' and status=1 

The above query is executed only when the NoOfErrors and NoOfAudited columns contain some values.

I have all the values but if a value in any one of the NoOfErrors and NoOfAudited columns contains Null value it doesn't retrieve any data, and it shows No data found .

ResultSet data is not accepting null values

How can I make the query accept null values and retrieve data by replacing null to empty in jsp table?

Answer 1

This line is problematic and should be corrected.

   (100-((a.NoOfErrors*100)/ NULLIF(a.NoOfAudited,0))) as Accuracy 

problem #1 :

NULLIF(a.NoOfAudited,0)

if NoOfAudited is NULL then the expression will return 0 ; basically you are dividing by zero. This should be replaced with 1 like below

NULLIF(a.NoOfAudited,1)

problem #2

No checks for NULL value for NoOfErrors column. In SQL server most NULL arithmetic results in NULL . For example all below queries will yield NULL

select 100-NULL*100/0
select NULL*100
select NULL/0
select 100-NULL

Therefore you'll get NULL value as Accuracy whenever NoOfErrors is NULL

To correct this part, you should write your query expression like

   (100-((COALESCE(a.NoOfErrors,0)*100)/ COALESCE(a.NoOfAudited,1))) as Accuracy 

Note that I've replaced NULLIF with COALESCE which is ANSI compliant and will run on both SQL server and MySQL

TLDR;

your query should be like

select 
   a.id, 
  case 
   when COALESCE(a.NoOfErrors,a.NoOfAudited,'') ='' 
   then''
   else 
    CAST((100-((COALESCE(a.NoOfErrors,0)*100)/ COALESCE(a.NoOfAudited,1))) as VARCHAR(10))as Accuracy
 end
from Table1 a 
 join Table2 pd 
   on a.batchid=pd.id 
where 
  a.charge='"+Poster+"' and status=1