在SQL中是否也可以计算不同值的数量？

Question

I am thinking that this statement will and should pick the distinct values from my table and then also have a count of them or at least that is what I would like in a separate column. Is it possible to count the number of distinct values in SQL, from what I am gathering probably not.

Question: Can I count the number of distinct values of last names?

SQL Statement:

select distinct c.LastName as 'Distinct Last Names', COUNT(*)
from Customer as c;

Error:

Msg 8120, Level 16, State 1, Line 1
Column 'Customer.LastName' is invalid in the select list because it is not contained 
in either an aggregate function or the GROUP BY clause.

Output -- Without the count

'Distinct Last Names'

LTest1 LTest2 LTest3

Answer 1

Since you donot have a Group by clause in the query, All your columns in the Select should use some kind of an aggregation or else SQL will not be happy.

Try this

select COUNT(distinct c.LastName) 'Distinct Last Names'
from Customer as c;

Answer 2

If I understand your question correctly, you want the result in two columns.

If so, you can use Group By .

SELECT c.LastName as 'Distinct Last Names', COUNT(*) 
FROM Customer as c
GROUP BY c.LastName

Answer 3

I'd use a subquery for this. The inner query returns a distict list of last names. The outer query counts the distict values. I prefer count(1) over count(*) because there's no need to return metadata for all of the columns in the customer table.

SELECT COUNT(1)
FROM (
    SELECT DISTINCT LASTNAME 
    FROM CUSTOMER
) a