SQL计数地址总数

Question

Here is my data

Vehicle    | Street       | Number | Postcode
Car          Fakestreet      3       0010NK
Motorbike    Fakestreet      3       0010NK
Car          Fakestreet      4       0010NK
Car          Fakestreet      1       0010NK
Companycar   Fakestreet      1       0010NK

I want to count unique adresses per postcode so this is want I want to see:

Postcode | Total_adresses
0010NK   | 3

I thought this must be a simple query so I have this:

SELECT Postcode, COUNT(*) AS total_adresses
FROM my_database
WHERE Postcode = '0010NK'
GROUP BY Postcode, Street, Number

But I get this result:

Postcode | total_adresses
0010NK   | 2
0010NK   | 2
0010NK   | 1

I can't get my head around why it doesn't give me the desired result. Any ideas why?

Answer 1

Just aggregate by the one column:

SELECT Postcode, COUNT(*) AS total_adresses
FROM my_database
WHERE Postcode = '0010NK'
GROUP BY Postcode;

The combination of keys in the GROUP BY defines each row in the aggregated result set. You want only one row per Postcode , so that is the only key you should have in the GROUP BY .

EDIT:

To count the number of distinct addresses, use COUNT(DISTINCT) . Here is one way:

SELECT Postcode, COUNT(DISTINCT CONCAT(STREET, ' ', NUMBER, ' ', POSTALCODE) AS total_adresses
FROM my_database
WHERE Postcode = '0010NK'
GROUP BY Postcode;

Some databases support:

SELECT Postcode, COUNT(DISTINCT STREET, NUMBER, POSTALCODE) AS total_adresses

Answer 2

Have a derived table to return the unique addresses. Then GROUP BY its result:

select Postcode, count(*)
from
(
    SELECT DISTINCT Street, Number, Postcode
    FROM my_database
) dt
group by Postcode