查询不会在 iif ms-access sql 中返回真值

Question

I want to get RollNo if it is present when certain conditions meet otherwise it should return -1 . But in my case it is returning RollNo when its present, otherwise it returns nothing .

SQL is under:-

SELECT iif(isnull(RollNo),-1,RollNo) from students where SName = "sf" and FName= "da" and 
DOB=#7/16/2020# and ClassID = 0

Answer 1

Hmm. . . If you always want to return exactly one row, then I would suggest aggregation:

SELECT NZ(MAX(RollNo), -1)
FROM students 
WHERE SName = "sf" and FName= "da" and DOB=#7/16/2020# and ClassID = 0;

If nothing matches the WHERE clause, then MAX(RollNo) returns NULL -- which is converted to a -1 by NZ() .

Answer 2

This returns exactly one row with field RollNo .

SELECT iif(isnull(max(RollNo)),-1,max(RollNo)) from students where SName = "sf" and
 FName= "da" and DOB=#7/16/2020# and ClassID = 0

if the conditions in where clause meet it return RollNo otherwise return -1