feat: add sql solution to lc problem: No.1251 (#749)

No.1251. Average Selling Price

Author: Dhoni77

solution/1200-1299/1251.Average Selling Price/README.md

@@ -83,7 +83,14 @@ Result table:
 ### **SQL**
 
 ```sql
-
+SELECT p.product_id,
+       Round(( Sum(u.units * p.price) + 0.0 ) / ( Sum(units) + 0.0 ), 2)
+       average_price
+FROM   Prices p
+       INNER JOIN UnitsSold u
+               ON p.product_id = u.product_id
+WHERE  u.purchase_date BETWEEN p.start_date AND p.end_date
+GROUP  BY p.product_id; 
 ```
 
 <!-- tabs:end -->

solution/1200-1299/1251.Average Selling Price/README_EN.md

@@ -87,7 +87,14 @@ Average selling price for product 2 = ((200 * 15) + (30 * 30)) / 230 = 16.96
 ### **SQL**
 
 ```sql
-
+SELECT p.product_id,
+       Round(( Sum(u.units * p.price) + 0.0 ) / ( Sum(units) + 0.0 ), 2)
+       average_price
+FROM   Prices p
+       INNER JOIN UnitsSold u
+               ON p.product_id = u.product_id
+WHERE  u.purchase_date BETWEEN p.start_date AND p.end_date
+GROUP  BY p.product_id; 
 ```
 
 <!-- tabs:end -->

solution/1200-1299/1251.Average Selling Price/Solution.sql

@@ -0,0 +1,8 @@
+SELECT p.product_id,
+       Round(( Sum(u.units * p.price) + 0.0 ) / ( Sum(units) + 0.0 ), 2)
+       average_price
+FROM   Prices p
+       INNER JOIN UnitsSold u
+               ON p.product_id = u.product_id
+WHERE  u.purchase_date BETWEEN p.start_date AND p.end_date
+GROUP  BY p.product_id;