Bookings system - mysql

Question

This is the database schema:
CREATE TABLE `bookings` (
`id` int(2) NOT NULL,
`start` time NOT NULL,
`end` time NOT NULL
)
INSERT INTO `bookings` VALUES(1, '13:00:00', '14:30:00');
INSERT INTO `bookings` VALUES(2, '15:00:00', '16:00:00');
I tried to run the following query, to find the free times:
SELECT free_from, free_until
FROM (SELECT a.end AS free_from,
(SELECT MIN(c.start)
FROM bookings c
WHERE c.start>a.end
) as free_until
FROM bookings a
WHERE NOT EXISTS (SELECT 1
FROM bookings b
WHERE b.start BETWEEN a.end AND a.end + INTERVAL your_duration HOURS
) AND
a.end BETWEEN '10:00:00' AND '18:00:00'
) t
The output generated is:
free_from free_until
14:30:00 15:00:00
16:00:00 NULL
Why is there a NULL in the end? Please help me solve this problem. Expected output should be:
free_from free_until
10:00:00 13:00:00
14:30:00 15:00:00
16:00:00 18:00:00

Answer 1

try this:
first create table and insert records
CREATE TABLE bookings (
id int NOT NULL,
start time NOT NULL,
endtime time NOT NULL
)
INSERT INTO bookings VALUES(1, '13:00:00', '14:30:00');
INSERT INTO bookings VALUES(2, '15:00:00', '16:00:00');
then run this select query
SELECT free_from, free_until
FROM (SELECT a.endtime AS free_from,
(SELECT MIN(c.start)
FROM bookings c
WHERE c.start>a.endtime
) as free_until
FROM bookings a
WHERE a.endtime BETWEEN '10:00:00' AND '18:00:00'
) t where free_until IS NOT NULL;
Note: I changed your column end to endtime

Answer 2

The first row of your output corresponds to the following data set:
a:
1, '13:00:00', '14:30:00'
c:
1, '13:00:00', '14:30:00'
2, '15:00:00', '16:00:00'
for "free_until", c.start must be higher than a.end, which is satisfied for this data set.(for 2nd row of C)
The second row of your output corresponds to the following data set:
a:
2, '15:00:00', '16:00:00'
c:
1, '13:00:00', '14:30:00'
2, '15:00:00', '16:00:00'
for "free_until", c.start>a.end but as you can see, a.end is 16:00 here, and no c.start is higher than this.
that's why null
Modify a.end of the second row to 14:00 and you will not get null value. :D
modify the data in the table:
Delete everything from bookings, then
INSERT INTO `bookings` VALUES(1, '13:00:00', '14:30:00');
INSERT INTO `bookings` VALUES(2, '15:00:00', '14:00:00');