MySQL select count only new id's for each year - mysql

Question

I have a MySQL table that looks like this
id | client_id | date
--------------------------------------
1 | 12 | 02/02/2008
2 | 15 | 12/06/2008
3 | 23 | 11/12/2008
4 | 12 | 18/01/2009
5 | 12 | 03/03/2009
6 | 18 | 02/07/2009
7 | 23 | 08/09/2010
8 | 18 | 02/10/2010
9 | 21 | 30/11/2010
What I am trying to do is get the number of new clients for each year. 2008 has 3 new clients(12,15,23), 2009 has 1 new client(18) and 2010 has 1 new client(21).
So far I have this query that gives me the distinct clients for each year, that is 3 for 2008, 2 for 2009 and 3 for 2010.
SELECT COUNT(DISTINCT client_id) FROM table GROUP BY YEAR(date)
Any help would be appreciated..

Answer 1

You could use a subquery to get the first year of every client_id grouped by client_id, and then count the occurrence of client_id grouped by year, so:
SELECT COUNT(client_id), YEAR_MIN FROM (
SELECT client_id, MIN(YEAR(date)) AS YEAR_MIN
FROM table
GROUP BY client_id) AS T
GROUP BY YEAR_MIN
SQL Fiddle here

Answer 2

So you want to count the first date a client appears in the table. In other words, the row for which no other row exists with an earlier date and the same client. You can do this with an exclusion join.
Then you can count them per year as you're doing now.
SELECT YEAR(t.date) AS yr, COUNT(t.client_id) AS client_count
FROM (
SELECT t1.client_id, t1.date
FROM mytable AS t1
LEFT JOIN mytable AS t2 ON (t1.client_id=t2.client_id AND t1.date > t2.date)
WHERE t2.client_id IS NULL) AS t
GROUP BY yr
You should store dates using the DATE data type, which uses YYYY-MM-DD format. You won't be able to do > comparisons if your dates are stored as strings in DD-MM-YYYY format.

Answer 3

DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(id SERIAL PRIMARY KEY
,client_id INT NOT NULL
,date INT NOT NULL
);
INSERT INTO my_table VALUES
(1,12,2008),
(2,15,2008),
(3,23,2008),
(4,12,2009),
(5,12,2009),
(6,18,2009),
(7,23,2010),
(8,18,2010),
(9,21,2010);
SELECT year
, COUNT(*) total
FROM
( SELECT client_id, MIN(date) year FROM my_table GROUP BY client_id ) x
GROUP
BY year;
+------+-------+
| year | total |
+------+-------+
| 2008 | 3 |
| 2009 | 1 |
| 2010 | 1 |
+------+-------+