sql从特定字段计算唯一值

Question

|from| to | 
|  7 |  9 |
|  7 |  9 |
|  3 |  9 |
|  3 |  2 |

Lets say I'm user 9 and want to find out how many user have wrote to me. With the query:

SELECT COUNT(*) FROM tab WHERE to = 9;

I get 3 which means I got 3 new messages, but how to find out ho many users wrote me which would be 2 in this case?

Answer 1

You want count(distinct) :

SELECT COUNT(DISTINCT `from`)
FROM tab
WHERE `to` = 9;

Answer 2

If you want a more detailed result, you can use GROUP BY .

This way make it possible to see how many times the from user have sent messages to to user.

Eg

SELECT from, to, count(*)
FROM tab
GROUP BY from, to

This will return:

|from  |to  |count(*) |
|------|----|---------|
|7     |9   |2        |
|3     |9   |1        |
|3     |2   |1        |

You can even insert your restriction, if you want:

SELECT from, to, count(*)
FROM tab
GROUP BY from, to
HAVING to = 9

That would result in:

|from  |to  |count(*) |
|------|----|---------|
|7     |9   |2        |

Just an extra way to do it.