在SQL查询中计算不同行的条件

Question

I have a table in a MySQL database with an ID column. This is not a key of the table and several rows can have the same ID.

I don't really know SQL but I already figured out how to obtain the number of distinct IDs:

SELECT COUNT(DISTINCT ID) FROM mytable;

Now I want to count only those IDs which appear more than 2 times in the table.

So if the ID column contains the values

 3 4 4 5 5 5 6 7 7 7

the query should return 2.

I have no idea how to do this. I hope someone can help me!

Btw, my table contains a huge number of rows. So if there are several possibilities I would also be happy to know which solution is the most efficient.

Answer 1

Try this:

SELECT COUNT(ID) FROM (
    SELECT ID FROM mytable
    GROUP BY ID
    HAVING COUNT(ID) > 2) p

Answer 2

SELECT 
      COUNT(ids)
FROM 
      (SELECT 
           COUNT(ID)AS ids 
       FROM 
           mytable 
       GROUP BY 
           ID
       HAVING 
           ids>2
       )AS tbl1

Answer 3

select count(*) from 
    (select count(id) as cnt,id from mytable group by id) da 
where da.cnt>2

The inner query will give you how many elements does each id have. And the outer query will filter this.

Answer 4

Updated :

SELECT count(ID) 
FROM (
SELECT ID FROM mytable
GROUP BY ID
HAVING count(ID) > 2
) p

should do what you need