Question

I would like to create a view from a table like this:

Configuration |   Size  | Runtime    
0             |    10   |  32.5    
1             |    10   |  30.8   
2             |    10   |  40.1    
0             |    20   |  61.0   
1             |    20   |  65.3   
2             |    20   |  56.8

Where configuration number 0 is special (it is the default configuration) and it is
present in every group of “Size” values.
My goal is to create a view like this:

Configuration |   Size  | Speedup
0             |    10   |  1.0
1             |    10   |  1.05
2             |    10   |  0.81
0             |    20   |  1.0
1             |    20   |  0.93
2             |    20   |  1.07

For each group of rows with the same size I would like to find the default configuration (identified by number 0) and then compute the ratio between the runtime of the considered row and the default runtime.

This can be splitted in two queries:

1. look for the default runtime for the given “Size”
2. perform the ratio between the runtime and the default.

The problem is that in query number 1 (that should be a subquery in the select statement of query 2) I don’t know which “Size” I am currently considering.
I hope you understand what I mean.

I there a way around this problem ?

Answer 1

Another approach you could use is a subselect:

CREATE VIEW yourview AS
SELECT
    Configuration,
    Size,
    (SELECT Runtime
     FROM yourtable AS T2
     WHERE T1.Size = T2.Size
     AND T2.Configuration = 0) / Runtime AS SpeedUp
FROM yourtable AS T1

See it working online: sqlfiddle