Question

I just need some direction on this. I have the following tables:

Table: entity
 - ID (INT)

Table: attributes
 - ID (INT)
 - name (VARCHAR)
 - internalname (VARCHAR)

Table: values
 - ID (INT)
 - entity (ID)
 - attributes (INT)
 - value (Text)

What I want is to make a Select statment that will return something like the following:

 - ID = entity.ID
 - {attributes.internalname ID 1} = values.{attribe internalname 1}.value
 - {attributes.internalname ID 2} = values.{attribe internalname 2}.value
 - {attributes.internalname ID 3} = values.{attribe internalname 3}.value
 - {attributes.internalname ID n} = values.{attribe internalname n}.value
 - etc...

It would be like combining:

SELECT entity.id FROM entity;

and

SELECT (SELECT values.value FROM values WHERE values.entity = entity.ID AND values.attributes = attributes.ID) FROM attributes;

It is a difficult thing to explain, however if you need me to explain further, please let me know.

I effectively want to rotate all values in attributes to columns and turn all the values into its corresponding attribute’s value with the ID as the selector.

I give the query an ID (The element ID), and in one result row it returns all the data.

Thanks in Advance!

Answer 1

You can’t create columns dynamically, so you need to know beforehand what you want as columns.

If attributes (1,2,3,4) represent (firstname, lastname, extraname, additionalname) you can query it like this:

select e.id
      ,v1.value as firstname
      ,v2.value as lastname
      ,v3.value as extraname
      ,v4.value as additionalname
  from entity e
  left join values v1 on(e.id = v1.entity and v1.attributes = 1)
  left join values v2 on(e.id = v2.entity and v2.attributes = 2)
  left join values v3 on(e.id = v3.entity and v3.attributes = 3)
  left join values v4 on(e.id = v4.entity and v4.attributes = 4)
 where e.id = ?

or

select e.id
      ,max(case when v.attributes = 1 then value) as firstname
      ,max(case when v.attributes = 2 then value) as lastname
      ,max(case when v.attributes = 3 then value) as extraname
      ,max(case when v.attributes = 4 then value) as additionalname
  from entity e
  left join values v on(e.id = v.entity)
 where v.attributes in(1,2,3,4)
   and e.id = ?
 group by e.id;

You can also use the GROUP_CONCAT to return the values in a comma separated list in one column.

select e.id
      ,group_concat(v.value)
  from entity e
  left join values v on(e.id = v.entity)
 group 
    by e.id;

Oh, and values is a reserved word. Don’t use it as table name.

Oh 2, don’t use this model unless you really really have to. You will pay a big juicy price in terms of performance and data consistency.