Division with Aggregate Functions in SQL Not Behaving as Expected

问题

I'm trying to do some crosstabs in SQL Server 2008 R2. That part is alright, however, if I try to get percentages for each cell, I run into a problem.

Here is a distilled use case: A survey where people give their favorite color and their favorite fruit. I'd like to know how many like a given fruit AND a given color:

with survey as (
    select 'banana' fav_fruit, 'yellow' fav_color
     union select 'banana', 'red'
     union select 'apple', 'yellow'
     union select 'grape', 'red'
     union select 'apple', 'blue'
     union select 'orange', 'purple'
     union select 'pomegranate', 'green'
)
select
    s.fav_color, 
    sum(case 
          when s.fav_fruit = 'banana' then 1
          else 0
        end) as banana, 
    sum(case 
           when s.fav_fruit = 'banana' then 1
           else 0
         end) / sum(1)   -- why does division always yield 0? "+", "-", and "*" all behave as expected.
         * 100 as banana_pct,
     sum(1) as total
from 
    survey s
group by
    s.fav_color;

Results:

fav_color   banana banana_pct  total
------------------------------------
blue        0      0            1
green       0      0            1
purple      0      0            1
red         1      0            2
yellow      1      0            2

What I was expecting:

fav_color   banana banana_pct  total
------------------------------------
blue        0      0           1
green       0      0           1
purple      0      0           1
red         1      50          2
yellow      1      50          2

Please help me to get what I was expecting?

回答1:

You are using SQL Server. Here is a much simpler example that replicates the issue:

select 1/2

SQL Server does integer division.

Replace the denominator with something like sum(1.0) or sum(cast 1 as float) or sum(1e0) instead of sum(1).

Contrary to my expectation at least, SQL Server stores numbers with decimal points as numeric/decimal type (see here) rather than float. The fixed number of decimal spaces might affect subsequent operations.

回答2:

Query:

SQLFIddleexample

SELECT s.fav_color,
       sum( CASE WHEN s.fav_fruit = 'banana' THEN 1 ELSE 0 END ) AS banana,
       sum( CASE WHEN s.fav_fruit = 'banana' THEN 1 ELSE 0 END) / sum(1.00) -- why does division always yield 0? "+", "-", and "*" all behave as expected.
 * 100 AS banana_pct,
   sum(1) AS total
FROM survey s
GROUP BY s.fav_color

Result:

| FAV_COLOR | BANANA | BANANA_PCT | TOTAL |
-------------------------------------------
|      blue |      0 |          0 |     1 |
|     green |      0 |          0 |     1 |
|    purple |      0 |          0 |     1 |
|       red |      1 |         50 |     2 |
|    yellow |      1 |         50 |     2 |

来源：https://stackoverflow.com/questions/14326435/division-with-aggregate-functions-in-sql-not-behaving-as-expected

标签

sql

sql-server

aggregate-functions

division

crosstab