Problem

Given a table of information about customer orders, write a SQL query to find the total number of orders for each customer.
Table: 
customer_id | order_id | order_date
1 | 1001 | 2019-01-01
1 | 1002 | 2019-01-02
2 | 1003 | 2019-01-03
3 | 1004 | 2019-01-04
3 | 1005 | 2019-01-05
Example input: 
customer_id | order_id | order_date
1 | 1001 | 2019-01-01
1 | 1002 | 2019-01-02
2 | 1003 | 2019-01-03
3 | 1004 | 2019-01-04
3 | 1005 | 2019-01-05
Example output: 
customer_id | num_orders
1 | 2
2 | 1
3 | 2

Solution

SELECT customer_id, COUNT(order_id) AS num_orders
FROM orders
GROUP BY customer_id;