SQL命令按C＃WinForms计数后的行数

Question

I have a table called Order(Id,Number,Date). "Id" and "Number" are main keys.

table Order

Id Number Date
0   1     1995   
0   2     1995        
0   3     1995   
1   1     1999   
1   2     1999   
2   1     2001

I need the number of rows but without duplicates (in this example: 3 ) I know that the "group by Id" will give me: Id(0,1,2) but I cant count it after the group by.

Answer 1

Check this SQL Fiddle working code , it returns what you are looking for. It groups by Number and then counts the total of rows, on this case, 3 rows.

EDIT

Sorry, I see you are counting by ID, modified the code. Nevertheless is the same logic just different column.

SELECT COUNT(*) Rows_count
FROM (
      SELECT COUNT([Id]) count_rows
        FROM tOrder
       GROUP BY [Id]) AS T1

Result:

ROWS_COUNT
   3

EDIT

This code works too indeed and is shorter. My previous one helps if you need to add a HAVING clause after the GROUP BY in case you want some particular value to be counted or not.

SELECT COUNT(DISTINCT id) FROM [Order]

Answer 2

select count(distinct id) from order

Answer 3

Try this :

select DISTINCT COUNT(Id) from Order