Question

I would like to hear if anyone can tell me a simple syntax that accomplishes the same as the following (with the same flexibility):

SELECT C.CompanyName,
(SELECT Count(*) FROM Employees WHERE CompanyId = C.Id) as EmployeeCount
FROM Company C

Now, what’s important is that the inner SELECT giving the EmployeeCount is:

1. An independent SELECT statement
   • This means that it should work with any existing SELECT, even if it already contains joins etc.
2. Can use values from the parent SELECT

I know that this scenario can be easily accomplished in other ways, but the above is a simplified example to explain the challenge. My real scenario is a complex SELECT statement where I do not want to complicate it by adding more joins. Performance is no issue.

Answer 1

Using INNER JOIN:

SELECT C.CompanyName, Count(E.*) as EmployeeCount
FROM Company C
INNER JOIN Employees E on E.CompanyId = C.Id 

Using NESTED JOIN:

SELECT C.CompanyName, Count(E.1) as EmployeeCount
FROM Company C, Employess E
WHERE E.CompanyId = C.Id

If you want to use the same syntax, at least put this:

SELECT C.CompanyName,
(SELECT Count(1) FROM Employees WHERE CompanyId = C.Id) as EmployeeCount
FROM Company C

If you need all the data to be shown, even the ones the companies without any Employees, you can use a LEFT OUTER JOIN:

SELECT C.CompanyName, Count(E.*) as EmployeeCount
FROM Company C
LEFT OUTER JOIN Employees E on E.CompanyId = C.Id