What is wrong with this database query? - sql

Question

I have the following tables in a database (i'll only list the important attributes):
Person(ssn,countryofbirth)
Parents(ssn,fatherbirthcountry)
Employment(ssn, companyID)
Company(companyID, name)
My task is this: given fatherbirthcountry as input, output the names of companies where persons work whose countryofbirth match the fatherbirthcountry input.
I pretend that the fatherbirthcountry is Mexico and do this:
SELECT name
FROM Company
WHERE companyid = (SELECT companyid
FROM Employment
WHERE ssn = (SELECT ssn
FROM Person
WHERE countryofbirth = 'Mexico');
but it is giving me an error:
>Scalar subquery is only allowed to return a single row.
am I completely off track? Can anybody please help?

Answer 1

The problem is that your subqueries are returning multiple results, so you have to use where in vs. =.
Change where ssn = to where ssn in, and where companyid = to where companyid in.

Answer 2

try using the IN keyword not '='.
try changing your query to this
SELECT name
FROM Company
WHERE companyid IN (SELECT companyid
FROM Employment
WHERE ssn IN (SELECT ssn
FROM Person
WHERE countryofbirth = 'Mexico');

Answer 3

Use:
SELECT c.name
FROM COMPANY c
JOIN EMPLOYMENT e ON e.companyid = c.companyid
JOIN PERSON p ON p.ssn = e.ssn
AND p.countryofbirth = 'Mexico'

Answer 4

You should use In in the where condition since the (SELECT ssn
FROM Person
WHERE countryofbirth = 'Mexico'); may return multiple ssn values.
SELECT name
FROM Company
WHERE companyid = (SELECT companyid
FROM Employment
WHERE ssn IN (SELECT ssn
FROM Person
WHERE countryofbirth = 'Mexico');

Answer 5

Try using IN instead of =
When you write:
select a from T where a = ( select....)
The sub-query must return a single value. In case if it returns multiple values, you get your error.
To solve this we use the IN operator which allows the sub-query to return a set of value (>=0) and your where condition succeeds if a equals any one of those values.
select a from T where a IN ( select....)

Answer 6

See if this works
SELECT c.Name FROM PERSON p
LEFT JOIN Employment e ON p.ssn=e.ssn
LEFT JOIN Company c ON
e.CompanyID=c.CompanyID WHERE
p.countryofbirth=

Answer 7

The error is due to the fact that the one of the two subqueries are returning multiple rows. I would think it likely that you have multiple people born in Mexico for example.
Select Name
From Companies
Where Exists(
Select 1
From Employment
Join Person
On Person.SSN = Employment.SSN
Join Parents
On Parents.SSN = Person.SSN
Where Parents.FatherBirthCountry = Person.CountryOfBirth
And Parents.FatherBirthCountry = #InputParam
And Employment.CompanyId = Companies.CompanyId
)

Answer 8

Ideally use the answer from OMG Ponies using JOINs.
But if you do not like JOINs for whatever reason, then TOP 1 should do the trick for you:
SELECT name
FROM Company
WHERE companyid =(SELECT TOP 1 companyid
FROM Employment
WHERE ssn = ( SELECT TOP 1 ssn
FROM Person
WHERE countryofbirth = 'Mexico');