Add content for DISTINCT

src/data/courses/sql/chapters/sql-basics/lessons/selecting-distinct-values.md

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+---
+title: Selecting DISTINCT Values
+description: Learn the basics of SQL, the language for querying databases.
+order: 120
+type: lesson-challenge
+---
+
+We can use the `DISTINCT` keyword in our `SELECT` statement to select the unique values of a column.
+
+Continuing the same example of an online bookstore, let's say we have a table called `orders` that contains the information about the orders placed by customers.
+
+Let's say we have the following data in the `orders` table:
+
+| id | customer_id | order_date | status |
+|----|-------------|------------|--------|
+| 1  | 101         | 2024-01-01 | completed |
+| 2  | 102         | 2024-01-02 | pending |
+| 3  | 101         | 2024-01-03 | completed |
+| 4  | 103         | 2024-01-04 | cancelled |
+| 5  | 102         | 2024-01-05 | completed |
+
+The `status` column contains the status of the orders. We want to select the distinct values of the `status` column.
+
+The SQL query to do this would be:
+
+```sql
+SELECT DISTINCT status 
+FROM orders;
+```
+
+This will return the following result:
+
+| status |
+|--------|
+| completed |
+| pending |
+| cancelled |
+
+---
+
+## Challenge
+
+Given the same `orders` table, i.e.
+
+| id | customer_id | order_date | status |
+|----|-------------|------------|--------|
+| 1  | 101         | 2024-01-01 | completed |
+| 2  | 102         | 2024-01-02 | pending |
+| 3  | 101         | 2024-01-03 | completed |
+| 4  | 103         | 2024-01-04 | cancelled |
+| 5  | 102         | 2024-01-05 | completed |
+
+Write an SQL query to select the distinct values of the `order_date` for us to see the unique dates on which orders were placed.