Tony 
Write a query identifying the type of each record in the TRIANGLES table using its three side lengths. Output one of the following statements for each record in the table:
- Equilateral: It’s a triangle with 3 sides of equal length.
- Isosceles: It’s a triangle with 2 sides of equal length.
- Scalene: It’s a triangle with 3 sides of differing lengths.
- Not A Triangle: The given values of A, B, and C don’t form a triangle.
Input Format
The TRIANGLES table is described as follows: |Column | Type | |—|—| |A| Integer | B| Integer | C| Integer | Each row in the table denotes the lengths of each of a triangle’s three sides.
Sample Input
|A | B |C| |—|—|—| |20| 20 |23| 20| 20 |20 20| 21 |22 13|14|30
Sample Output
Isosceles
Equilateral
Scalene
Not A Triangle
Explanation
Values in the tuple (20,20,23) form an Isosceles triangle, because A==B.
Values in the tuple (20,20,20) form an Equilateral triangle, because A==B==C. Values in the tuple (20,21,22) form a Scalene triangle, because A!=B!=C.
Values in the tuple (13,14,30) cannot form a triangle because the combined value of sides A and B is not larger than that of side C.
Analysis
此題需要用到IF或SWITCH CASE去判斷三邊長的關係，構成三角形的條件是任兩邊和必須大於第三邊，先判斷出ABC是否能構成三角形，接替再判斷是否符合正三角形或等腰三角形的條件。
IF()返回一個數字或字串，當x為True時返回y，否則返回z，類似三元運算子。
SELECT IF(x, y, z)
SWITCH CASE裡每一個區間都要搭配WHEN…THEN…關鍵字，都不符合的情況用ELSE處理，最後要以END關鍵字結尾。
SELECT CASE col
WHEN 100 THEN '1'
WHEN 50 THEN '2'
ELSE '3'
END
SOLUTION
IF
SELECT IF(A+B>C AND A+C>B AND B+C>A,
IF(A=B AND A=C AND B=C,'Equilateral',
IF(A=B OR B=C OR A=C,'Isosceles','Scalene')),
'Not A Triangle')
FROM TRIANGLES
CASE
SELECT
CASE
WHEN A+B>C AND A+C>B AND B+C>A THEN
CASE
WHEN A=B AND B=C THEN 'Equilateral'
WHEN A=B OR B=C OR A=C THEN 'Isosceles'
ELSE 'Scalene'
END
ELSE 'Not A Triangle'
END
FROM TRIANGLES
Expected Output
Equilateral
Equilateral
Isosceles
Equilateral
Isosceles
Equilateral
Scalene
Not A Triangle
Scalene
Scalene
Scalene
Not A Triangle
Not A Triangle
Scalene
Equilateral