SQL简单案例声明

Question

这个简单的SQL语句似乎回答了错误的答案。有人能告诉我哪里出错了。我正在使用MySQL Workbench。

我使用以下命令创建并填充表：

drop table if exists TRIANGLES;
create table TRIANGLES(A int, B int, C int);
insert into TRIANGLES values(20,20,23);
insert into TRIANGLES values(20,20,20);
insert into TRIANGLES values(20,21,22);
insert into TRIANGLES values(13,14,30);

我执行我的三角形类型查询：

select (case
when A+B<=C then 'Not a Triangle'
when B+C<=A then 'Not a Triangle'
when A+C<=B then 'Not a Triangle'
when A=B=C then 'Equilateral'
when A=B and B!=C then 'Isoscelus'
when B=C and C!=A then 'Isoscelus'
when A=C and B!=C then 'Isoscelus'
when A!=B!=C then 'Scalene'
end) as typ from TRIANGLES;

但我得到的答案是：

Isoscelus
Scalene -- bad result
Scalene
Not a Triangle

谢谢。

Answer 1

而不是A=B=C使用A=B and B=C：

select *, (case
when A+B<=C then 'Not a Triangle'
when B+C<=A then 'Not a Triangle'
when A+C<=B then 'Not a Triangle'
when A=B and b=C then 'Equilateral'
when A=B and B!=C then 'Isoscelus'
when B=C and C!=A then 'Isoscelus'
when A=C and B!=C then 'Isoscelus'
when A!=B and B!=C and A!=C then 'Scalene' -- or just use: ELSE 'Scalene'
end) as typ 
from TRIANGLES;

结果：

A            B            C            typ             
-------------------------------------------------------
20           20           23           Isoscelus       
20           20           20           Equilateral     
20           21           22           Scalene         
13           14           30           Not a Triangle