查询返回NULL输出时的MYSQL Select情况

Question

Running a nested case when query to classify entries in a table based on the parameters below. When I run the code, the output for Isosceles returns Null . I'm not sure why. I'm comparing integers a,b, and c, which represent columns for each of those distinct integers for a triangle, for instance.

Any suggestions?

select case when a+b > c and b+c > a and c+a > b then 
case 
    when a=b|a=c|b=c then "Isosceles"
    when a=b and a=c and b=c then "Equilateral"
    when a<>b and b<>c and a<>c then "Scalene"
end
else "Not A Triangle"
end 
from triangles

Answer 1

First problem: Instead of single pipe, use double pipe || for OR .

Second problem: Use Equilateral condition before Isosceles So if your sides are 2 , 2 and 2 then as per your logic, Isosceles condition satisfies first. So it will never equate it to Equilateral

New Answer: In fact, you don't need 2 case statements. Use this instead

http://rextester.com/PTDI48631

select t.*,
case when not (a+b > c and b+c > a and c+a > b)  
        then "Not A Triangle"
     when (a=b and a=c and b=c) 
         then "Equilateral"
     when (a=b or a=c or b=c) 
         then "Isosceles"
     else "Scalene"
end as Triangle_type
from triangle t;

Previous Answer: http://rextester.com/ELLB75736

select t.*,
case  when a+b > c and b+c > a and c+a > b 
then
(case 
    when a=b and a=c and b=c then "Equilateral"
    when a=b or a=c or b=c then "Isosceles"
    when a<>b and b<>c and a<>c then "Scalene"
    end
)
else 
"Not A Triangle"
end as Triangle_type
from triangle234 t

Answer 2

select case when a+b > c and b+c > a and c+a > b then 
case 
    when (a=b & a=c)|(a=b & b=c) | (a=c & b=c) then "Isosceles"
    when a=b and a=c and b=c then "Equilateral"
    when a<>b and b<>c and a<>c then "Scalene"
    Else "Triangle"
end
else "Not A Triangle"
end 
from triangles

Just try above code.I think you missed else for internal WHEN . Hope this will helps.