Test case added for the percentile disc function

mysql-test/t/percentile.test

+CREATE TABLE student (name CHAR(10), test double, score TINYINT); 
+
+INSERT INTO student VALUES 
+('Chun', 0, null), ('Chun', 0, 4), 
+('Esben', 1, null), ('Esben', 1, null), 
+('Kaolin', 0.5, 56), ('Kaolin', 0.5, 88), 
+('Tatiana', 0.8, 2), ('Tatiana', 0.8, 1);
+
+
+
+select name, percentile_disc(0.6)  within group(order by score) over (partition by name) from student;
+select name, percentile_disc(test)  within group(order by score) over (partition by name) from student;
+select name, percentile_disc(0.4)  within group(order by score) over (partition by name) from student;
+
+
+#select name, percentile_cont(null)  within group(order by score) over (partition by name) from student;
+#select name, cume_dist()  over (partition by name order by score) from student;
+
+
+#normal parsing
+#select percentile_cont(0.5) within group(order by score)  over w1  from student
+#window w1 AS (partition by name);
+
+# no partition clause
+#select percentile_cont(0.5) within group(order by score)  over ()  from student;
+
+
+# only one sort allowed
+#select percentile_cont(0.5) within group(order by score) over (partition by name);
+
+#parameter value should be in the range of 0 to 1
+#select percentile_cont(1.5) within group(order by score) over (partition by name);
+
+
+#
+#select rank() over (partition by name order by score  ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) from student;
+
+
+
+drop table student;
+