customer-who-visited-but-did-not-make-any-transactions.sql

/*

Description
LeetCode Problem 1581.

Table: Visits

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| visit_id    | int     |
| customer_id | int     |
+-------------+---------+
visit_id is the primary key for this table.
This table contains information about the customers who visited the mall.
Table: Transactions

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| transaction_id | int     |
| visit_id       | int     |
| amount         | int     |
+----------------+---------+
transaction_id is the primary key for this table.
This table contains information about the transactions made during the visit_id.
Write an SQL query to find the IDs of the users who visited without making any transactions and the number of times they made these types of visits.

Return the result table sorted in any order.

The query result format is in the following example:

Visits
+----------+-------------+
| visit_id | customer_id |
+----------+-------------+
| 1        | 23          |
| 2        | 9           |
| 4        | 30          |
| 5        | 54          |
| 6        | 96          |
| 7        | 54          |
| 8        | 54          |
+----------+-------------+

Transactions
+----------------+----------+--------+
| transaction_id | visit_id | amount |
+----------------+----------+--------+
| 2              | 5        | 310    |
| 3              | 5        | 300    |
| 9              | 5        | 200    |
| 12             | 1        | 910    |
| 13             | 2        | 970    |
+----------------+----------+--------+

Result table:
+-------------+----------------+
| customer_id | count_no_trans |
+-------------+----------------+
| 54          | 2              |
| 30          | 1              |
| 96          | 1              |
+-------------+----------------+
Customer with id = 23 visited the mall once and made one transaction during the visit with id = 12.
Customer with id = 9 visited the mall once and made one transaction during the visit with id = 13.
Customer with id = 30 visited the mall once and did not make any transactions.
Customer with id = 54 visited the mall three times. During 2 visits they did not make any transactions, and during one visit they made 3 transactions.
Customer with id = 96 visited the mall once and did not make any transactions.
As we can see, users with IDs 30 and 96 visited the mall one time without making any transactions. Also user 54 visited the mall twice and did not make any transactions.

*/

# V0
### NOTE : LEFT JOIN WHERE CONDITION : WHERE transaction_id IS NULL
SELECT DISTINCT customer_id,
                count(customer_id) AS count_no_trans
FROM visits v
LEFT JOIN transactions t ON v.visit_id = t.visit_id
WHERE transaction_id IS NULL
GROUP BY customer_id
ORDER BY NULL;

# V1
# https://circlecoder.com/customer-who-visited-but-did-not-make-any-transactions/
select customer_id, count(visit_id) as count_no_trans
from Visits
where visit_id not in 
    (select visit_id
    from Transactions
    group by visit_id)
group by customer_id

# V2
# Time:  O(n)
# Space: O(n)
SELECT DISTINCT customer_id,
                count(customer_id) AS count_no_trans
FROM visits v
LEFT JOIN transactions t ON v.visit_id = t.visit_id
WHERE transaction_id IS NULL
GROUP BY customer_id
ORDER BY NULL;