Question

I have four tables:

days

CREATE TABLE days (
day text priamry key
);

These days run monday to saturday.

times

CREATE TABLE times (
time time Primary Key,
peak text 
);

The time run from 8:00 am to 9:00 pm in hour slots E.g. 8:00, 9:00, 10:00, 11:00

activities

CREATE TABLE activities (
activity text primary key
);

and planner

CREATE TABLE planner (
day text foriegn key references days (day)
time time foriegn key references times (time)
activity text foriegn key references activities (activity)
member bigint
primary key (day, time, member)
);

The planner table will have data like:

friday, 09:00, squash_court1 , 2
friday, 09:00, squash_court2 , 3
friday, 09:00, squash_court3 , 1

What I am wanting to do i make a list of all the none booked times for these three courts
So I would have a list like

time  activity
08:00 squash_court1
10:00 squash_court1
...rest of times...
08:00 squash_court2
10:00 squash_court2
...rest of times...
08:00 squash_court3
10:00 squash_court3
...rest of times...

an the reason these is no 9:00 between 8:00 and 10:00 is because it has been booked

EDIT

At the moment I have the basic join of:

SELECT time , activity FROM times, activities;

All I need now is the WHERE clause to remove the ones that are booked in the planner table.

Thank you for any advice on this matter.

Answer 1

All I need now is the WHERE clause to remove the ones that are booked in the planner table.

This can be done in various ways. LEFT JOIN / WHERE .. IS NULL often produces the fastest plan in PostgreSQL:

SELECT t.time, a.activity
FROM   (days d CROSS JOIN  times t CROSS JOIN activities a)
LEFT   JOIN planner p ON (p.day, p.time, p.activity)
                       = (d.day, t.time, a.activity)
WHERE  p.activity IS NULL;

I use parenthesis to make clear you want to CROSS JOIN (same as a comma between the tables) daysand times and activities first. These parenthesis are redundant, because tables are joined from left to right by default.

The JOIN condition is just a shorter form of:

 ON p.day = d.day
AND p.time = t.time
AND p.activity = a.activity

---

Another way would be a NOT EXISTS semi-join:

SELECT t.time, a.activity
FROM   (days d CROSS JOIN  times t CROSS JOIN activities a)
WHERE  NOT EXISTS (
   SELECT *
   FROM   planner 
   WHERE (p.day, p.time, p.activity)
       = (d.day, t.time, a.activity)
   );