Question

I have a SQL Server database, I need to store an unknown number of columns, so I have 2 tables like this:

1. Table Master, with these columns: id, data1, data2 (fixed columns)
2. Table unknown_fields with these columns: id_master_record, id_field, value_field

I do the query like this:

SELECT id, data1, data2, 
  (SELECT value_field FROM unknown_fields
    WHERE id_master_record = Master.id AND id_field = 1) AS data3
  (SELECT value_field FROM unknown_fields 
    WHERE id_master_record = Master.id AND id_field = 2) AS data4
  (SELECT value_field FROM unknown_fields 
    WHERE id_master_record = Master.id AND id_field = 3) AS data5
 ... SO for each 
FROM Master

I want to know whether there is a better way to do this query.

Answer 1

You could try this, it may be slightly faster. Best to test both. Clustering the second table on (id_master_record, id_field) will probably help both queries.

Select
  m.id,
  m.data1,
  m.data2,
  Max(Case u.id_field When 1 Then u.value_field End) As data3,
  Max(Case u.id_field When 2 Then u.value_field End) As data4,
  Max(Case u.id_field When 3 Then u.value_field End) As data5
From
  xmaster m
    Left Outer Join
  unknown_fields u
    On m.id = u.id_master_record
Group By
  m.id,
  m.data1,
  m.data2;

You could also try using Pivot

Select
  id,
  data1,
  data2,
  [1] as data3,
  [2] as data4,
  [3] as data5
From (
  Select
    m.id,
    m.data1,
    m.data2,
    id_field,
    value_field
  From
    Master m
      left outer join
    unknown_fields u
      On m.id = u.id_master_record
    ) as s
Pivot (
  max(value_field)
For 
    id_field In ([1], [2], [3])
) As p;

http://sqlfiddle.com/#!6/f9bca/11