MySQL查询，数据透视表

Question

I'm having trouble building up my Query that will pivot my rows to columns. My Pivot is my DATE. What the User need is to have a view of a full week data.

My Data is constructed like this :

ID - AssemblyDate
----------------------------------
12345 - 2015-01-01
12346 - 2015-01-01
12347 - 2015-01-01
12348 - 2015-01-02
12349 - 2015-01-02
12350 - 2015-01-02
12351 - 2015-01-03
12352 - 2015-01-03
12353 - 2015-01-03

And the Result I'm expecting would be something like this

DAY1 - DAY2 - DAY3
12345  12348  12351
12346  12349  12352
12347  12350  12352

What I have Tried :

SELECT CASE WHEN (AssemblyDate = '2015-01-01') THEN ID ELSE NULL END AS DAY1
       , CASE WHEN (AssemblyDate = '2015-01-02') THEN ID ELSE NULL END AS DAY2
       , CASE WHEN (AssemblyDate = '2015-01-03') THEN ID ELSE NULL END AS DAY3
FROM MyTable
GROUP BY AssemblyDate

This Gives me something like this :

DAY1 - DAY2 - DAY3
12345  NULL   NULL
12346  NULL   NULL
12347  NULL   NULL
NULL   12348  NULL
NULL   12349  NULL
NULL   12350  NULL
NULL   NULL   12351
NULL   NULL   12352
NULL   NULL   12352

but I don't want the nulls ... is there a way I could like GROUP them without the nulls ?

Answer 1

You need to do a bit of fiddling, as you need to further group those results. Since there's nothing to group them on, we need to engineer something to use for grouping - in this case, one variable per column that we increment whenever that column is the one that matches the assembly date:

select if(assemblydate = '2015-01-01', @d1c := @d1c +1, 
          if(assemblydate = '2015-01-02', @d2c := @d2c + 1, @d3c := @d3c +1)) id,  
       case when assemblydate = '2015-01-01' then id end d1,   
       case when assemblydate = '2015-01-02' then id end d2,   
       case when assemblydate = '2015-01-03' then id end d3  
  from d, (select @d1c := 0, @d2c := 0, @d3c := 0) q;

That query just adds an id column, that increments for each day, the results look like this.

+------+-------+-------+-------+
| id   | d1    | d2    | d3    |
+------+-------+-------+-------+
|    1 | 12345 |  NULL |  NULL |
|    2 | 12346 |  NULL |  NULL |
|    3 | 12347 |  NULL |  NULL |
|    1 |  NULL | 12348 |  NULL |
|    2 |  NULL | 12349 |  NULL |
|    3 |  NULL | 12350 |  NULL |
|    1 |  NULL |  NULL | 12351 |
|    2 |  NULL |  NULL | 12352 |
|    3 |  NULL |  NULL | 12352 |
+------+-------+-------+-------+

Once that's done, we just wrap another query around the result, group by the id and pick up the values that aren't null with max()

select max(d1), max(d2), max(d3) from
(
  select if(assemblydate = '2015-01-01', @d1c := @d1c +1, 
          if(assemblydate = '2015-01-02', @d2c := @d2c + 1, @d3c := @d3c +1)) id,  
       case when assemblydate = '2015-01-01' then id end d1,   
       case when assemblydate = '2015-01-02' then id end d2,   
       case when assemblydate = '2015-01-03' then id end d3  
  from d, (select @d1c := 0, @d2c := 0, @d3c := 0) q
) qq
group by id;

fiddle here