Problem

Given two tables, write a SQL query to find all the customers who have at least one order in each table.
Input:
Table 1:
customer_id | order_id
1 | 100
2 | 101
3 | 102
4 | 103
5 | 104
6 | 105
7 | 106
8 | 107
9 | 108
10 | 109
Table 2:
customer_id | order_id
1 | 200
2 | 201
3 | 202
4 | 203
5 | 204
6 | 205
7 | 206
8 | 207
9 | 208
10 | 209
Output:
customer_id
1
2
3
4
5
6
7
8
9
10

Solution

SELECT customer_id
FROM table1
WHERE customer_id IN (SELECT customer_id FROM table2)

/*

The solution above is optimal because it uses the IN operator to check if the customer_id from table1 is in table2.

*/