docs: add the sql queries we used in this project into an md file

sql.md

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+ # SQL Queries of our project
+ ## Items
+- get all categories
+```sql
+    SELECT *
+    FROM item_category;
+```
+
+- create view that reference all items
+```sql
+    CREATE OR REPLACE VIEW all_items AS
+    SELECT *
+    FROM item_item;
+```
+
+- create view that reference to items filtered by category
+```sql
+   CREATE OR REPLACE VIEW filtered_by_category AS 
+   SELECT *
+   FROM all_items
+   WHERE category_id = %s;
+```
+
+- get all items that match the search query
+```sql
+   SELECT *
+   FROM filtered_by_category
+   WHERE name LIKE '%s' OR description LIKE '%s';
+```
+
+- get all items from 'filtered_by_category' view
+```sql
+   SELECT *
+   FROM filtered_by_category;
+```
+
+- get all items that match the search query
+```sql
+   SELECT *
+   FROM all_items
+   WHERE name LIKE %s OR description LIKE %s;
+```
+
+## detail
+
+- get the item
+```sql
+    SELECT *
+    FROM item_item
+    WHERE id = %s
+```
+
+- get the user who created the item (the seller)
+```sql
+    SELECT users.username
+    FROM auth_user AS users, item_item AS items
+    WHERE items.id = %s AND items.created_by_id = users.id
+```
+
+- get all non sold items that are in the same category with the item
+```sql
+    SELECT *
+    FROM item_item
+    WHERE category_id = %s AND is_sold = False AND id != %s
+```
+
+## new
+
+- insert new item
+```sql
+ INSERT INTO item_item (category_id, name, description, price, image, created_by_id, is_sold, created_at)
+ VALUES (%s, %s, %s, %s, %s, %s, False, CURRENT_TIMESTAMP)
+```
+
+## Update
+- update the item with new information
+```sql
+   UPDATE item_item
+   SET name = %s,
+         description = %s,
+         price = %s,
+         image = %s,
+         is_sold = %s
+   WHERE id = %s;
+```
+
+## Delete
+
+- delete the item
+```sql
+   DELETE FROM item_item
+   WHERE id = %s
+```