feat: add sql solution to lc problem: No.1321 (#2197)

Author: thinkasany

solution/1300-1399/1321.Restaurant Growth/README.md

@@ -104,4 +104,18 @@ FROM t
 WHERE rk > 6;
 ```
 
+```sql
+# Write your MySQL query statement below
+SELECT
+    a.visited_on,
+    SUM(b.amount) AS amount,
+    ROUND(SUM(b.amount) / 7, 2) AS average_amount
+FROM
+    (SELECT DISTINCT visited_on FROM customer) AS a
+    JOIN customer AS b ON DATEDIFF(a.visited_on, b.visited_on) BETWEEN 0 AND 6
+WHERE a.visited_on >= (SELECT MIN(visited_on) FROM customer) + 6
+GROUP BY 1
+ORDER BY 1;
+```
+
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solution/1300-1399/1321.Restaurant Growth/README_EN.md

@@ -100,4 +100,18 @@ FROM t
 WHERE rk > 6;
 ```
 
+```sql
+# Write your MySQL query statement below
+SELECT
+    a.visited_on,
+    SUM(b.amount) AS amount,
+    ROUND(SUM(b.amount) / 7, 2) AS average_amount
+FROM
+    (SELECT DISTINCT visited_on FROM customer) AS a
+    JOIN customer AS b ON DATEDIFF(a.visited_on, b.visited_on) BETWEEN 0 AND 6
+WHERE a.visited_on >= (SELECT MIN(visited_on) FROM customer) + 6
+GROUP BY 1
+ORDER BY 1;
+```
+
 <!-- tabs:end -->