在 SQL 中计算百分比

Question

I am trying to return the % of a record's value in a field, where the field is created by using GROUP BY and COUNT like this:

SELECT col_1, COUNT(col_2)
FROM table_name
GROUP BY col_1
ORDER BY 2;

So, this code gives us something like:

col_1.  |.  col_2
A       |  5
B       |  10
C       |  5
...

What I'd like is to write a query that would then give me:

col_1.  |.  col_2
A       |  .25
B       |  .50
C       |  .25
...

How can I use what I originally created in order to get this sort of output? I've tried to use something like this but it doesn't work:

SELECT col_1, COUNT(col_2) / SUM(col_2)
FROM table_name
GROUP BY col_1
ORDER BY 2;

Thank you for any help you can offer!

Answer 1

Just use a window function:

SELECT col_1, COUNT(col_2),
       COUNT(col_2) * 1.0 / SUM(COUNT(col_2)) OVER ()
FROM table_name
GROUP BY col_1
ORDER BY 2;

Note: I added the * 1.0 . You don't specify the database you are using and some do integer division (so 1 / 2 is 0 rather than 0.5 ).

Answer 2

You can always do:

SELECT
  col_1, 
  1.0 * COUNT(col_2) / (select sum(col_2) from table_name)
FROM table_name
GROUP BY col_1
ORDER BY 2;

Answer 3

The issue is that in SQL, when you want to count total, its an aggregate function and the individual rows will be "lost".

One way is to use "window function" if you are on MS SQL newer versions.

Another way build the total as another table (example below totalTable), and then cross apply to join the original table with this new table. Then you are use the "total" column. Something like below.

SELECT col_1, COUNT(col_2) / total as percentage
FROM table_name
    CROSS APPLY 
        ( SELECT COUNT(*) as total from table_name ) totalTable
        )
GROUP BY col_1, total