如何对重复的行进行分组并对其进行计数？

Question

my table:

id | item_id
1  | 5
2  | 5
3  | 7
4  | 2

sql:

$countWeek = $conn->query("SELECT count(item_id) FROM `myTable` GROUP BY `item_id`")->fetchColumn();

As you can see i have 2 duplicated rows with item_id = 5 i want to group these duplicated rows and output 3 rows on the count, but when i do echo $countWeek it output 1 , why?

When i change the above sql to:

$countWeek = $conn->query("SELECT item_id FROM `myTable` GROUP BY `item_id`")->rowCount();

It returns the correct value, but i don't want to use rowCount() because i only need to count the rows and fetchColumn() with count() is far better in terms of speed.

Answer 1

You could use counct(distinct item_id)

    SELECT count(distinct item_id) 
    FROM `myTable`