feat: update sql solution to lc problem: No.1142

Author: thinkasany

solution/1100-1199/1142.User Activity for the Past 30 Days II/README.md

@@ -75,13 +75,23 @@ Activity 表：
 WITH
     T AS (
         SELECT
-            count(DISTINCT session_id) AS sessions
+            COUNT(DISTINCT session_id) AS sessions
         FROM Activity
-        WHERE activity_date <= '2019-07-27' AND datediff('2019-07-27', activity_date) < 30
+        WHERE activity_date <= '2019-07-27' AND DATEDIFF('2019-07-27', activity_date) < 30
         GROUP BY user_id
     )
-SELECT ifnull(round(avg(sessions), 2), 0) AS average_sessions_per_user
+SELECT IFNULL(ROUND(AVG(sessions), 2), 0) AS average_sessions_per_user
 FROM T;
 ```
 
+```sql
+SELECT
+    IFNULL(
+        ROUND(COUNT(DISTINCT session_id) / COUNT(DISTINCT user_id), 2),
+        0
+    ) AS average_sessions_per_user
+FROM Activity
+WHERE DATEDIFF('2019-07-27', activity_date) < 30;
+```
+
 <!-- tabs:end -->

solution/1100-1199/1142.User Activity for the Past 30 Days II/README_EN.md

@@ -71,13 +71,23 @@ Activity table:
 WITH
     T AS (
         SELECT
-            count(DISTINCT session_id) AS sessions
+            COUNT(DISTINCT session_id) AS sessions
         FROM Activity
-        WHERE activity_date <= '2019-07-27' AND datediff('2019-07-27', activity_date) < 30
+        WHERE activity_date <= '2019-07-27' AND DATEDIFF('2019-07-27', activity_date) < 30
         GROUP BY user_id
     )
-SELECT ifnull(round(avg(sessions), 2), 0) AS average_sessions_per_user
+SELECT IFNULL(ROUND(AVG(sessions), 2), 0) AS average_sessions_per_user
 FROM T;
 ```
 
+```sql
+SELECT
+    IFNULL(
+        ROUND(COUNT(DISTINCT session_id) / COUNT(DISTINCT user_id), 2),
+        0
+    ) AS average_sessions_per_user
+FROM Activity
+WHERE DATEDIFF('2019-07-27', activity_date) < 30;
+```
+
 <!-- tabs:end -->

solution/1100-1199/1142.User Activity for the Past 30 Days II/Solution.sql

@@ -2,10 +2,10 @@
 WITH
     T AS (
         SELECT
-            count(DISTINCT session_id) AS sessions
+            COUNT(DISTINCT session_id) AS sessions
         FROM Activity
-        WHERE activity_date <= '2019-07-27' AND datediff('2019-07-27', activity_date) < 30
+        WHERE activity_date <= '2019-07-27' AND DATEDIFF('2019-07-27', activity_date) < 30
         GROUP BY user_id
     )
-SELECT ifnull(round(avg(sessions), 2), 0) AS average_sessions_per_user
+SELECT IFNULL(ROUND(AVG(sessions), 2), 0) AS average_sessions_per_user
 FROM T;