joined table rows into columns with column titles from original rows - mysql

Question

I have three MYSQL tables, simplified, with following columns:
ModuleEntries (MDE_ID)
ModuleFields (MDF_ID, MDF_Label)
ModuleEntryFieldValues (MFV_ID, MFV_MDE_ID, MFV_MDF_ID, MFV_Value)
As you can see from the simplified column list, ModuleEntryFieldValues is the table which states that "for an entry, this field has this value".
Now, I would like to retrieve this information in one "flat" recordset. I have managed to get things working, but not entirely to what i want.
With the help of a different SO article, I managed to get the dynamic, variable amount of columns to work, by using a cursor. declare finish int default 0;
declare fldid int;
declare str varchar(10000) default 'SELECT *,';
declare curs cursor for select MDF_ID from ModuleFields group by MDF_ID;
declare continue handler for not found set finish = 1;
open curs;
my_loop:loop
fetch curs into fldid;
if finish = 1 then
leave my_loop;
end if;
set str = concat(str, 'max(case when MFV_MDF_ID = ',fldid,' then MFV_Value else null end) as field_',fldid,',');
end loop;
close curs;
set str = substr(str,1,char_length(str)-1);
set #str = concat(str, ' from ModuleEntries LEFT join ModuleEntryFieldValues ON MDE_ID = MDF_MDE_ID GROUP BY MDE_ID');
prepare stmt from #str;
execute stmt;
deallocate prepare stmt;
What this code doesn't do, is allow me to put the column values of MDF_Label as the actual column headers of the rows. Now, the above code gives me "field_1, field_2, ..."
Is it possible to join these three tables, and have the MDF_Label as column header for the rows that are now columns in the joined table ?
I want this...
ModuleEntries | ModuleFields | ModuleEntryFieldValues
------------- | ------------------ | -----------------------------------
MDE_ID | MDF_ID - MDF_Label | MFV_MDE_ID - MFV_MDF_ID - MFV_Value
1 | 1 - Height | 1 - 1 - 120cms
| 2 - Width | 1 - 2 - 30cms
| 3 - Weight |
into this
Recordset
---------
MDE_ID - Height - Width - Weight
1 - 120cms - 30cms - null
I hope my question was clear enough. If not please comment and I will give more information where needed, if I can.

Answer 1

I'd just use GROUP_CONCAT() instead of cursors to generate the prepared statement:
SELECT CONCAT(
' SELECT MDE_ID,'
, GROUP_CONCAT(
't', MDF_ID, '.MFV_Value AS `', REPLACE(MDF_Label, '`', '``'), '`'
)
,' FROM ModuleEntries '
, GROUP_CONCAT(
'LEFT JOIN ModuleEntryFieldValues AS t', MDF_ID, '
ON t', MDF_ID, '.MFV_MDE_ID = ModuleEntries.MDE_ID
AND t', MDF_ID, '.MFV_MDF_ID = ', MDF_ID
SEPARATOR ' ')
) INTO #qry FROM ModuleFields;
Save for whitespace editing to make it more readable, with your sample data #qry would then contain:
SELECT MDE_ID,
t1.MFV_Value AS `Height`,
t2.MFV_Value AS `Width`,
t3.MFV_Value AS `Weight`
FROM ModuleEntries
LEFT JOIN ModuleEntryFieldValues AS t1
ON t1.MFV_MDE_ID = ModuleEntries.MDE_ID AND t1.MFV_MDF_ID = 1
LEFT JOIN ModuleEntryFieldValues AS t2
ON t2.MFV_MDE_ID = ModuleEntries.MDE_ID AND t2.MFV_MDF_ID = 2
LEFT JOIN ModuleEntryFieldValues AS t3
ON t3.MFV_MDE_ID = ModuleEntries.MDE_ID AND t3.MFV_MDF_ID = 3
One can then prepare and execute that statement:
PREPARE stmt FROM #qry;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET #qry = NULL;
Which yields the following results:
MDE_ID Height Width Weight
1 120cms 30cms (null)
See it on sqlfiddle.