SQL - CASE

 SELECT

x , y, z, CASE  

WHEN x + y > z AND y + z > x AND x + z > y

THEN 'Yes'

ELSE 'No'

END AS 'Triangle'

FROM

Triangle;

Input: 
Triangle table:
+----+----+----+
| x  | y  | z  |
+----+----+----+
| 13 | 15 | 30 |
| 10 | 20 | 15 |
+----+----+----+
Output: 
+----+----+----+----------+
| x  | y  | z  | triangle |
+----+----+----+----------+
| 13 | 15 | 30 | No       |
| 10 | 20 | 15 | Yes      |
+----+----+----+----------+