Question

I have three tables:

CREATE TABLE activities (
activity varchar(20) Primary key
);

with data:

Table_Tennis1
Table_Tennis2
Table_Tennis3

and

CREATE TABLE times (
time varchar(5)
);

with data

09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00

and finally

CREATE TABLE planner (
day varchar(9) foreign key
time varchar(5) foreign key
activity varchar(20) foreign key
member bigint foreign key
);

and Primary Key = (day, time, activity)

with data

friday,09:00,Table_Tennis1,4
friday,10:00,Table_Tennis2,2

I was wondering it was possible to find out all the Table_Tennis rooms that are not being used at a certain time on a certain day, or all rooms that have not yet been booked on all times for one day.

so it should give me a result_set of

09:00, Table_Tennis2, Table_Tennis3
10:00, Table_Tennis1, Table_Tennis3
11:00, Table_Tennis1, Table_Tennis2, Table_Tennis3 ect ect

Answer 1

all the Table_Tennis rooms that are not being used at a certain time
on a certain day,

SELECT activity
FROM   activities a
WHERE  NOT EXISTS (
    SELECT *
    FROM   planner p
    WHERE  p.activity ~~ 'Table_Tennis%' -- may or may not be needed
    AND    p.day  = 'friday'
    AND    p.time = '09:00'
    AND    p.activity = a.activity -- was missing in my 1st draft
    );

---

all rooms that have not yet been booked on all times for one day.

SELECT a.activity
FROM   activities a
LEFT JOIN (
    SELECT activity
    FROM   planner p
    WHERE  day = 'friday'
    GROUP  BY 1
    HAVING count(*) = 12 -- assuming there are exactly 12 slots
    ) p USING (activity)
WHERE p.activity IS NULL; -- excludes all fully booked rooms

Or:

SELECT activity
FROM   activities a
WHERE NOT EXISTS (
    SELECT activity
    FROM   planner p
    WHERE  day = 'friday'
    GROUP  BY 1
    HAVING count(*) = 12 -- assuming there are exactly 12 slots
    );

But not:

SELECT activity
FROM   activities a
JOIN (
    SELECT activity
    FROM   planner p
    WHERE  day = 'friday'
    GROUP BY 1
    HAVING count(*) < 12
    ) p USING (activity);

… because that would drop rooms with no entries for the day at all.

---

You might consider using

slot time

instead of

time varchar(5)

time should not be used as identifier. It is a reserved word in all SQL standards and a type name in PostgreSQL.
Also, the data type time is a better fit for your purpose and occupies less space than varchar(5).

And

 day date foreign key ...
,slot time foreign key ...

instead of

 day varchar(9) foreign key ...
,time varchar(5) foreign key ...

The names of weekdays would let you cover one week. I assume you want more than that.