feat: update sql solution to lc problem: No.1939 (#1816)

* feat: update sql solution to lc problem: No.1939

* fix: optimize

Author: thinkasany

solution/1900-1999/1939.Users That Actively Request Confirmation Messages/README.md

@@ -88,11 +88,10 @@ Result table
 <!-- 这里可写当前语言的特殊实现逻辑 -->
 
 ```sql
-SELECT DISTINCT
-    c1.user_id AS user_id
+SELECT DISTINCT user_id
 FROM
     Confirmations AS c1
-    INNER JOIN Confirmations AS c2 ON c1.user_id = c2.user_id
+    JOIN Confirmations AS c2 USING (user_id)
 WHERE
     c1.time_stamp < c2.time_stamp
     AND TIMESTAMPDIFF(SECOND, c1.time_stamp, c2.time_stamp) <= 24 * 60 * 60;

solution/1900-1999/1939.Users That Actively Request Confirmation Messages/README_EN.md

@@ -91,11 +91,10 @@ User 7 requested two messages within 24 hours and 1 second of each other, so we
 ### **SQL**
 
 ```sql
-SELECT DISTINCT
-    c1.user_id AS user_id
+SELECT DISTINCT user_id
 FROM
     Confirmations AS c1
-    INNER JOIN Confirmations AS c2 ON c1.user_id = c2.user_id
+    JOIN Confirmations AS c2 USING (user_id)
 WHERE
     c1.time_stamp < c2.time_stamp
     AND TIMESTAMPDIFF(SECOND, c1.time_stamp, c2.time_stamp) <= 24 * 60 * 60;

solution/1900-1999/1939.Users That Actively Request Confirmation Messages/Solution.sql

@@ -1,8 +1,7 @@
-SELECT DISTINCT
-    c1.user_id AS user_id
+SELECT DISTINCT user_id
 FROM
     Confirmations AS c1
-    INNER JOIN Confirmations AS c2 ON c1.user_id = c2.user_id
+    JOIN Confirmations AS c2 USING (user_id)
 WHERE
     c1.time_stamp < c2.time_stamp
     AND TIMESTAMPDIFF(SECOND, c1.time_stamp, c2.time_stamp) <= 24 * 60 * 60;