I've spent about an hour and a half trying to work this out, and I'm sure its because its not the best function to use, but I need to for my assessment.
The idea is I have 3 tables, and want to return all the information for each 'guestName' along with its related 'hotelName', 'roomNo' and 'dateTo' cells. I can't work out how to make this work with a simple Join.
SELECT guestName, hotelName, roomNo, dateTo
FROM guest
JOIN hotel, booking
ORDER BY guestName
This is just returning every entry from all the tables I have joined with, repeating guest names and hotel names to fit the room number and date to columns.
JOIN error
Each SELECT item has its own table, except 'roomNo' which is within the booking table. All my searching just tells me to use other methods of JOIN, which I'm sure works better, but those are coming up in later assessment pieces so I need to use standard JOIN.
Cheers for any direction on this.
Add ON statement to specify JOIN condition
SELECT guestName, hotelName, roomNo, dateTo
FROM guest g
JOIN booking b ON b.bookingId = g.bookingId
JOIN hotel h ON b.hotelId = h.hotelId
ORDER BY g.guestName
Related
I've never been able to get my head around INNER JOINs (or any other JOIN types for that matter) so I'm struggling to work out how to use it in my specific situation. In fact, I'm not even sure if it's what I need. I've looked at other examples and read tutorials but my brain just doesn't seem to work the way needed to truly get it (or it doesn't function at all).
Here's the scenario:
I have two tables -
phone_numbers - this table has a list of phone numbers that
belong to lots of different customers. A single customer can have
multiple numbers. For simplicity's sake, we'll say the fields are
'number_id', 'customer_id', 'phone_number'.
call_history - this table has a record of every single call that one of these
numbers in the first table could have had. There's a record for
every individual call going back years. Again, for simplicity,
we'll say the relevant fields are customer_id, phone_number,
call_start_time.
What I'm trying to accomplish is to find all of the numbers that belong to a particular customer_id in the phone numbers table and use that information to search through the call_history table and find the number of calls each phone number has received, and group that by the number of calls for each number, preferably also showing zeros where a number hasn't received any calls at all.
The reason the zero calls is important is because that's the data I'm interested in. Otherwise, I could just get all the information out of the call_history table. But what I'm trying to achieve is find the numbers with no activity.
All I've been able to accomplish is run one query to get all of the numbers belonging to one customer:
SELECT customer_id, phone_number FROM phone_numbers WHERE customer_id = Y;
Then run a second query to get all phone calls for that customer_id for a set duration:
SELECT customer_id, phone_number, COUNT(*) FROM call_history WHERE customer_id = Y and call_start_time >= DATE_SUB(SYSDATE(), INTERVAL 30 DAY) GROUP BY phone_number;
I've then had to use the data returned from both queries and use a VLOOKUP function in Excel to match number of calls for each individual number from the second query to the list of all numbers from the first query, thus leaving blanks in my "all numbers" table and identifying those numbers that had no calls for that time period.
I'm hoping there's some way to do all of this with a single query and return a table of results, listing the zero number of calls with it and eliminate the whole manual Excel bit as it's not overly efficient and prone to human error.
Without at least a workable example from you, it's not easy to re-create your situation. Anyway, INNER JOIN might not return the result as how you expected. In my short time with MySQL, I mainly use 2 types of JOIN; one is already mentioned and the other is LEFT JOIN. From what I can understand in your question, what you want to achieve can be done by using LEFT JOIN instead of INNER JOIN. I may not be the best person to explain this to you but this is how I understand it:
INNER JOIN - only return anything that match in ON clause between two (or more) tables.
LEFT JOIN - will return everything from the table on the left side of the join and return NULL if ON get no match in the table on the right side of the join .. unless you specify some WHERE condition from something on the right table.
Now, here is my query suggestion and hopefully it'll be useful for you:
SELECT A.customer_id, A.phone_number,
SUM(CASE WHEN call_start_time >= DATE_SUB(SYSDATE(), INTERVAL 30 DAY)
THEN 1 ELSE 0 END) AS Total
FROM phone_numbers A
LEFT JOIN call_history B
ON A.customer_id=B.customer_id
GROUP BY A.customer_id,A.phone_number;
What I did here is I LEFT JOIN phone_numbers table with call_history on customer_id and I re-position the WHERE call_start_time >= .. condition into a CASE expression in the SELECT since putting it at WHERE will turn this into a normal join or inner join instead.
Here is an example fiddle : https://www.db-fiddle.com/f/hriFWqVy5RGbnsdj8i3aVG/1
For Inner join You should have to do like this way..
SELECT customer_id,phone_number FROM phone_numbers as pn,call_history as ch where pn.customer_id = ch.customer_id and call_start_time >= DATE_SUB(SYSDATE(), INTERVAL 30 DAY) GROUP BY phone_number;
Just add table name whatever you want to join and add condition
I'm trying to write a query which does the below:
For every guest who has the word “Edinburgh” in their address show the total number of nights booked. Be sure to include 0 for those guests who have never had a booking. Show last name, first name, address and number of nights. Order by last name then first name.
I am having problems with making the join work properly,
ER Diagram Snippet:
Here is my current (broken) solution:
SELECT last_name, first_name, address, nights
FROM booking
RIGHT JOIN guest ON (booking.booking_id = guest.id)
WHERE address LIKE '%Edinburgh%';
Here is the results from that query:
The query is partially complete, hoping someone can help me out and create a working version. I'm currently in the process of learning SQL so apologies if its a rather basic or dumb question!
Your query seems almost correct. You were joining the booking id with guets id which gave you some results because of overlapping (matching) ids, but this most likely doesn't correspond to the foreign keys. You should join on guest_id from booking to id from guest.
I'd add grouping to sum all booked nights for a particular guest (assuming that nights is an integer):
SELECT g.last_name, g.first_name, g.address, SUM(b.nights) AS nights
FROM guest AS g
LEFT JOIN booking AS b ON b.guest_id = g.id
WHERE g.address LIKE '%Edinburgh%'
GROUP BY g.last_name, g.first_name, g.address;
Are you sure that nights spent should be calculated using nights field? Why can it be null? If you'd like to show zero for null values just wrap it up with a coalesce function like that:
COALESCE(SUM(b.nights), 0)
Notes:
Rewriten RIGHT JOIN into LEFT JOIN, but that doesn't affect results - it's just cleaner for me
Using aliases eg. AS g makes the code shorter when specifying joining columns
Reference every column with their table alias to avoid ambiguity
SELECT g.first_name,
g.last_name,
g.address,
COALESCE(Sum(b.nights), 0)
FROM booking b
RIGHT JOIN guest g
ON ( b.guest_id = g.id )
WHERE address LIKE 'edinburgh%'
GROUP BY g.last_name,
g.first_name,
g.address;
This post answers your questions about how to make the query.
MySQL SUM with same ID
You can simply use COALESCE as referenced here to avoid the NULL Values
How do I get SUM function in MySQL to return '0' if no values are found?
I have 3 tables "Employees", "EmployeeLeaveDays" and "EmployeeLeaves".
I'm looking to create a view that displays the date of the leave and the employee name. So in order for my calendar to work I have split everyones leave into individual days(EmployeeLeaveDays) which has an FK that links each day back to (EmployeeLeaves) which has other details around the leave, in EmployeeLeaves I have a column "employee" which is an FK back to employees which contains the name.
So In my view I want to return the name as you can see is 2 tables away, I've wrote this MySQL query but it doesn't work (returns no data), I'm wondering if there is anyway to do what I need to do?
SELECT
EmployeeLeaveDays.id,
EmployeeLeaveDays.employee_leave,
EmployeeLeaveDays.leave_date,
EmployeeLeaveDays.leave_type
FROM EmployeeLeaveDays
INNER JOIN EmployeeLeaves
ON EmployeeLeaveDays.employee_leave=EmployeeLeaves.employee
INNER JOIN Employees
ON EmployeeLeaves.employee=Employees.employee_id;
Hopefully from that you're able to see what I'm trying to achieve, how ever I've attached some screenshots of the table structure.
Thanks
After some thinking I got there in the end. Here's the final query.
SELECT
EmployeeLeaveDays.id,
EmployeeLeaveDays.employee_leave,
EmployeeLeaveDays.leave_date,
EmployeeLeaveDays.leave_type,
EmployeeLeaves.employee,
Employees.employee_id,
Employees.first_name,
Employees.last_name
FROM EmployeeLeaveDays
LEFT JOIN EmployeeLeaves ON EmployeeLeaveDays.employee_leave = EmployeeLeaves.id
LEFT JOIN Employees ON EmployeeLeaves.employee = Employees.id;
I'm aware of the INSERT INTO table_name QUERY; however, I'm unsure how to go about achieving the desired result in this case.
Here's a slightly contrived example to explain what I'm looking for, but I'm afraid I cannot put it more succiently.
I have two tables in a database designed for a hotel.
BOOKING and CUSTOMER_BOOKING
Where BOOKING contains PK_room_number, room_type, etc. and CUSTOMER_BOOKING contains FK_room_number, FK_cusomer_id
CUSTOMER_BOOKING is a linking table (many customers can make many bookings, and many bookings can consist of many customers).
Ultimately, in the application back-end I want to be able to list all rooms that have less than 3 customers associated with them. I could execute this a separate query and save the result in the server-side scripting.
However, a more elegant solution (from my point of view) is to store this within the BOOKING table itself. That is to add a column no_of_bookings that counts the number of times the current PK_room_number appears as the foreign key FK_room_number within the CUSTOMER_BOOKING table. And why do this instead? Because it would be impossible for me to write a single complicated query which will both include the information from all ROOMS, among other tables, and also count the occurrences of bookings, without excluding ROOMS that don't have any bookings. A very bad thing for a hotel website attempting to show free rooms!
So it would look like this
BOOKING: PK_room_number (104B) room_type (double) room_price (high), no_of_bookings (3)
BOOKING: PK_room_number (108C) room_type (single) room_price (low), no_of_bookings (1)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (4312)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (6372)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (1112)
CUSTOMER_BOOKING: FK_room_number (108C) FK_customer_id (9181)
How would I go about creating this?
Because it would be impossible for me to write a single complicated
query which will both include the information from all ROOMS, among
other tables, and also count the occurrences of bookings, without
excluding ROOMS that don't have any bookings.
I wouldn't say it's impossible and unless you're running into performance issues, it's easier to implement than adding a new summary column:
select b.*, count(cb.room_number)
from bookings b
left join customer_booking cb on b.room_number = cb.room_number
group by b.room_number
Depending on your query may need to use a derived table containing the booking counts for each room instead instead
select b.*, coalesce(t1.number_of_bookings,0) number_of_bookings
from bookings b
left join (
select room_number, count(*) number_of_bookings
from customer_booking
group by room_number
) t1 on t1.room_number = b.room_number
You have to left join the derived table and select coalesce(t1.number_of_bookings,0) in case a room does not have any entries in the derived table (i.e. 0 bookings).
A summary column is a good idea when you're running into performance issues with counting the # of bookings each time. In that case I recommend creating insert and delete triggers on the customer_booking table that either increment or decrement the number_of_bookings column.
You could do it in a single straight select like this:
select DISTINCT
b1.room_pk,
c1.no_of_bookings
from cust_bookings b1,
(select room_pk, count(1) as no_of_bookings
from cust_bookings
group by room_pk) c1
where b1.room_pk = c1.room_pk
having c1.no_of_bookings < 3
Sorry i used my own table names to test it but you should figure it out easily enough. Also, the "having" line is only there to limit the rows returned to rooms with less than 3 bookings. If you remove that line you will get everything and could use the same sql to update a column on the bookings table if you still want to go that route.
Consider below solutions.
A simple aggregate query to count the customers per each booking:
SELECT b.PK_room_number, Count(c.FK_customer_id)
FROM Booking b
INNER JOIN Customer_Booking c ON b.PK_room_number = c.FK_room_number
GROUP BY b.PK_room_number
HAVING Count(c.FK_customer_id) < 3; # ADD 3 ROOM MAX FILTER
And if you intend to use a new column no_of_booking, here is an update query (using aggregate subquery) to run right after inserting new value from web frontend:
UPDATE Booking b
INNER JOIN
(SELECT b.PK_room_number, Count(c.FK_customer_id) As customercount
FROM Booking b
INNER JOIN Customer_Booking c ON b.PK_room_number = c.FK_room_number
GROUP BY b.PK_room_number) As r
ON b.PK_room_number = r.PK_room_number
SET b.no_of_booking = r.customercount;
the following generates a list showing all of the bookings and a flag of 0 or 1 if the the room has a customer for each of the rooms. it will display some rooms multiple times if there are multiple customers.
select BOOKING.*,
case CUSTOMER_BOOKING.FK_ROOM_NUMBER is null THEN 0 ELSE 1 END AS BOOKING_FLAG
from BOOKING LEFT OUTER JOIN CUSTOMER_BOOKING
ON BOOKING.PK_room_numer = CUSTOMER_BOOKING.FK_room_number
summing and grouping we arrive at:
select BOOKING.*,
SUM(case when CUSTOMER_BOOKING.FK_ROOM_NUMBER is null THEN 0 ELSE 1 END) AS BOOKING_COUNT
from BOOKING LEFT OUTER JOIN CUSTOMER_BOOKING
ON BOOKING.PK_room_number = CUSTOMER_BOOKING.FK_room_number
GROUP BY BOOKING.PK_room_number
there are at least two other solutions I can think of off the top of my head...
I have 2 simple tables - Firm and Groups. I also have a table FirmGroupsLink for making connections between them (connection is one to many).
Table Firm has attributes - FirmID, FirmName, City
Table Groups has attributes - GroupID, GroupName
Table FirmGroupsLink has attributes - FrmID, GrpID
Now I want to make a query, which will return all those firms, that have less groups then #num, so I write
SELECT FirmID, FirmName, City
FROM (Firm INNER JOIN FirmGroupsLink ON Firm.FirmID =
FirmGroupsLink.FrmID)
HAVING COUNT(FrmID)<#num
But it doesn't run, I try this in Microsoft Access, but it eventually should work for Sybase. Please show me, what I'm doing wrong.
Thank you in advance.
In order to count properly, you need to provide by which group you are couting.
The having clause, and moreover the count can't work if you are not grouping.
Here you are counting by Firm. In fact, because you need to retrieve information about the Firm, you are grouping by FirmId, FirmName and City, so the query should look like this:
SELECT Firm.FirmID, Firm.FirmName, Firm.City
FROM Firm
LEFT OUTER JOIN FirmGroupsLink
ON Firm.FirmID = FirmGroupsLink.FrmID
GROUP BY Firm.FirmID, Firm.FirmName, Firm.City
HAVING COUNT(FrmID) < #num
Note that I replace the INNER JOIN by a LEFT OUTER JOIN, because you might want Firm which doesn't belongs to any groups too.