solve problem 4

Author: a97410985

MySol/Create Session bar chart.md

@@ -0,0 +1,30 @@
+把資料庫建起來的SQL
+```SQL
+CREATE TABLE session_2(
+    session_id int,
+    duration   int,
+    PRIMARY KEY (session_id)
+);
+
+INSERT INTO session_2 (session_id, duration) VALUES
+(1, 30),
+(2, 199),
+(3, 299),
+(4, 580),
+(5, 1000);
+```
+
+查詢的SQL
+```SQL
+SELECT 
+CASE
+	WHEN duration < 300 THEN "[0-5>"
+    WHEN duration < 600 THEN "[5-10>"
+    WHEN duration < 900 THEN "[10-15>"
+    ELSE "15 or more"
+END AS bin, count(*) as count
+FROM myjdbc.session_2 group by bin;
+```
+![alt text](image-4.png)
+上面這種做法的問題點就是有出現count為0的狀況，那種`bin`不會被列出來。
+另一種做法就是分開做然後再全部UNION起來，如原作者的解答那樣