MySQL在同一行中选择数据

Question

I'm really confused about this query. I would like to get all the data for one context in one row. I really don't know how to do that. Here's the tables's sample.

data (table)                               io(table)
IDData    IDIO   ReadVal   No              IDIO   IDContext
1          io1     12       1              io1       c1     
2          io2     12.5     2              io2       c1 
3          io3     11       3              io3       c1 
4          io4     12.2     1              io4       c2  
5          io5     10       2              io5       c2  
6          io6     10.9     3              io6       c2

I would like to get this result

IDContext    IO-1   IO-2   IO-3
   c1         12    12.5    11
   c2        12.2    10    10.9

I've tried to do this in a loop to print in a table and it works, but I would like to do it in a query so it will be faster.

Actually, my query looks like that :

SELECT IDContext, IDInput, ReadVal
FROM data
LEFT JOIN io ON io.IDIO = data.IDIO
ORDER BY IDContext, No

Is this possible to get that result in one single query?

Answer 1

You can do this with conditional aggregation:

SELECT IDContext,
       MAX(CASE WHEN no = 1 THEN data.ReadVal END) as io1,
       MAX(CASE WHEN no = 2 THEN data.ReadVal END) as io2,
       MAX(CASE WHEN no = 3 THEN data.ReadVal END) as io3
FROM data LEFT JOIN
     io
     ON io.IDIO = data.IDIO
GROUP BY IDContext
ORDER BY IDContext;