Question

Customers            Holidays

id | name            customer_id | start  | end
---+------           ------------+--------+------
 1 | Peter           1           |   5    | 10
 2 | Simon           1           |  15    | 20
 3 | Mary            2           |   5    | 20

What’s a working SQL query that gives me all customers without holidays on a specific date? E.g. date=12

• Peter
• Mary

Is this even manageable with a simple SQL join, or do I need to use sub-queries?

Answer 1

First create a query that finds the opposite of what you want: the customers who do have a holiday on that specific date:

SELECT DISTINCT name
FROM Customers
JOIN Holidays
ON id = customer_id
WHERE start <= 12 AND end >= 12

Result:

Simon

Then join this result back to the customer table and select the results where the join fails:

SELECT name
FROM Customers
LEFT JOIN (
    SELECT DISTINCT id
    FROM Customers
    JOIN Holidays
    ON id = customer_id
    WHERE start <= 12 AND end >= 12
) AS T1
ON Customers.id = T1.id
WHERE T1.id IS NULL

Result:

Peter
Mary

Note that a JOIN isn’t the only alternative here. You could also use NOT EXISTS, NOT IN or EXCEPT. Since you didn’t specify which database, I chose JOIN because it is a portable and efficient way to do it in all the major relational databases.

The test data I used (taken from the question):

CREATE TABLE Holidays (customer_id INT NOT NULL, start INT NOT NULL, end INT NOT NULL);
INSERT INTO Holidays (customer_id, start, end) VALUES
(1, 5, 10),
(1, 15, 20),
(2, 5, 20);

CREATE TABLE Customers (id INT NOT NULL, name NVARCHAR(100) NOT NULL);
INSERT INTO Customers (id, name) VALUES
(1, 'Peter'),
(2, 'Simon'),
(3, 'Mary');