I have a database with 3 tables; tblCustomers, tblBookings, tblFlights.
I want to find the customer's last name (LName), from the Customers table where the customers do not appear in the bookings table. It should return just three names, but it returns the names 10 times each. There are 10 records in the bookings table, so I think the command is returning the correct names, but not once...
I have tried:
SELECT tblCustomers.LName
FROM tblCustomers, tblBookings
WHERE tblCustomers.CustID
NOT IN (SELECT CustID FROM tblBookings)
How do I return just one instance of the name, not the name repeated 10 times?
You are doing a CROSS JOIN of the 2 tables.
Use only NOT IN:
SELECT LName
FROM tblCustomers
WHERE CustID NOT IN (SELECT CustID FROM tblBookings)
The (implicit) cross join on The bookings table in the outer query makes no sense - and it multiplies the customer rows.
Also, I would recommend not exists for filtering instead of not in: it usually performs better - with the right index in place, and it is null-safe:
SELECT c.LName
FROM tblCustomers c
WHERE NOT EXISTS (SELECT 1 FROM tblBookings b WHERE b.CustID = c.CustID)
For performance, make sure to have an index on tblBookings(CustID) - if you have a proper foreign key declared, it should already be there.
Related
I have two tables (Table A & Table B) that I have in a database (SpatiaLite database). I would like to join all the entries in Table A with Table B using two foreign keys (TableA.Location & TableB.LSD, TableA.LICENCE_NO & TableB.Licence_No); however, there will be multiple INCIDEN_NO entries in Table A that match up with the joined rows in Table B.
Since there will be many INCIDEN_NO entries associated with the Licence_No in Table B, I would like to evenly distribute the INCIDEN_NO entries among all the LIC_LI_NO entries in Table B that align with the foreign keys. The rows from Table A can be randomly assigned to each LIC_LI_NO in Table B randomly and in no particular order.
I cannot seem to find a SQL expression for this operation, which has really stumped me for weeks.
You could match the rows up randomly with something like this:
with B as (
select row_number() over () as rn, lic_li_no
from B
), A as (
select abs(random()) % cntb + 1 as randnum, a.*
from A cross apply (select count(*) as cntb from B) b
)
select *
from A inner join B on A.randnum = B.rn;
You could also generate the cross product and keep random rows. I tried this query out on SQLite but it didn't seem to work as I expected:
select * from A cross join B where abs(random()) % 20 = 1
That said, I don't understand the purpose behind all this and it's certainly not a common thing to do in a database.
I have two tables, TA and CMI, that contain a person_ID. The ID may exist in TA, it may exist in CMI, or it may exist in both. I want a distinct list of ALL person_ID's regardless whether they are in TA, CMI, or both tables.
I also want to be able to select them where their question_ID's are the same. However, the question_id's have different column names: TA.question and CMI.sco = question_id.
EDIT:
So, if I also wanted to do the select on question as I stated earlier AND a join to the person table, it would look something like:
select ta.person_id, person_key
from ta
left join person on person.person_id = ta.person_id
where question=7033
union -- on purpose to remove duplicates
select cmi.person_id, person_key
from cmi
left join person on person.person_id = cmi.person_id
where sco=7033
You would use union:
select person_id
from ta
union -- on purpose to remove duplicates
select person_id
from cmi;
You can use this as a CTE or subquery in a query.
Let's say there are 2 or more tables.
Table A: aID, name, birthday
Table B: bID, petType, petName
Table C: cID, stackOverFlowUsername
I want to get something like aID, name, birthday, number of cats a person has, stack overflow's username
We can
use joins to join all 3 tables select * from tableA... tableB... tableC...
use multiple select statements, select a.*, (select count(*) from tableB where petType = 'cat') as numberOfCats, (select...) as stackUsername from tableA a
or other ways that I didn't know
My question is when is the right situation to use select, joins or is there even better methods?
Update:
Here is another question. If I have 3 stackoverflow accounts, Tom has 1 and Peter has 2,
using
A left join B left join C
will return a total of 6 rows
select a.*, select count(*) from tableB where..., select top 1 stackOverFlowUsername from tableC
returns 3 rows because there are 3 person
Can I use joins to achieve something similar if I only want one row of data for each person in tableA regardless how many stackoverflow accounts he/she has?
Thanks
A selected sub-select (case 2) might be scanned for every result row, while joined tables/views/subselects are calculates only once: saving memory and joining time (with pre-built indices). Once you are used to talking SQL, you will find that the JOIN syntax is many times easier to read.
I have 2 database tables:
customers and customers_1
I have 100 customers in the customers table but only 99 customers in the customers_1 table. I would like to write a query that will compare the 2 tables and will result in the missing row.
I have tried this following SQL:
select * from customers c where in (select * from customers_1)
But this will only check for the one table.
Your query shouldn't work this way. You have to compare one column to another and use NOT IN instead of IN:
select *
from customers c
where customerid not in (select customerid from customers_1)
However, Since you are on SQL Server 2008, you can use EXCEPT:
SELECT * FROM customers
EXCEPT
SELECT * FROM customers_1;
This will give you the rows which are in the customers table that are not in customers_1 table:
EXCEPT returns any distinct values from the left query that are not
also found on the right query.
This is easy. Just join them with a left outer join and check for NULL in the table which has the 99 rows. It will look something like this.
SELECT * FROM customers c
LEFT JOIN customers1 c1 ON c.some_key = c1.some_key
WHERE c1.some_key IS NULL
Instead of NOT IN clause consider using NOT EXISTS. NOT EXISTS clause performs better in this particular scenario. Your query would look like:
SELECT * FROM Customer c WHERE NOT EXISTS (SELECT 1 FROM Customer_1 c1 WHERE c.Customer_Id = c1.Customer_Id)
SELECT 1 is just for readability so everyone will know that I don't care about the actual data.
I have a statement like this:
select lastname,firstname,email,floorid
from employee
where locationid=1
and (statusid=1 or statusid=3)
order by floorid,lastname,firstname,email
The problem is the column floorid. The result of this query is showing the id of the floors.
There is this table called floor (has like 30 rows), which has columns id and floornumber. The floorid (in above statement) values match the id of the table floor.
I want the above query to switch the floorid values into the associated values of the floornumber column in the floor table.
Can anyone show me how to do this please?
I am using Microsoft sql server 2008 r2.
I am new to sql and I need a clear and understandable method if possible.
select lastname,
firstname,
email,
floor.floornumber
from employee
inner join floor on floor.id = employee.floorid
where locationid = 1
and (statusid = 1 or statusid = 3)
order by floorid, lastname, firstname, email
You have to do a simple join where you check, if the floorid matches the id of your floor table. Then you use the floornumber of the table floor.
select a.lastname,a.firstname,a.email,b.floornumber
from employee a
join floor b on a.floorid = b.id
where a.locationid=1 and (a.statusid=1 or a.statusid=3)
order by a.floorid,a.lastname,a.firstname,a.email
You need to use a join.
This will join the two tables on a certain field.
This way you can SELECTcolumns from more than one table at the time.
When you join two tables you have to specify on which column you want to join them.
In your example, you'd have to do this:
from employee join floor on employee.floorid = floor.id
Since you are new to SQL you must know a few things. With the other enaswers you have on this question, people use aliases instead of repeating the table name.
from employee a join floor b
means that from now on the table employee will be known as a and the table floor as b. This is really usefull when you have a lot of joins to do.
Now let's say both table have a column name. In your select you have to say from which table you want to pick the column name. If you only write this
SELECT name from Employee a join floor b on a.id = b.id
the compiler won't understand from which table you want to get the column name. You would have to specify it like this :
SELECT Employee.name from Employee a join floor b on a.id = b.id or if you prefer with aliases :
SELECT a.name from Employee a join floor b on a.id = b.id
Finally there are many type of joins.
Inner join ( what you are using because simply typing Join will refer to an inner join.
Left outer join
Right outer join
Self join
...
To should refer to this article about joins to know how to use them correctly.
Hope this helps.