CODEWITHSUNDEEP
sqloracle-databasedynamic
CaptainIK
CaptainIK

Reputation: 23

Query a table and a column name stored in a table

I come here because of a problem that I am not experienced enough to solve, in an Oracle Database. Let me explain:

The tables

I have a table that we'll call Attributes, containing 3 columns : ID, the id of the attribute, -EDIT: Entity_ID as well, the entity it refers to /EDIT-, Table_name, containing the name of the table in which the value of the attribute is stored, and Column_name, containing the name of the column in that table in which is the value is stored.

Every tables referenced in the column Table_name, contains the same column names (such as Value1,Value2, etc..) except for the first one that references another entity (Entity_id), to which the attribute is linked to.

What I am looking for

My goal is to build a query that selects every attribute (based on its id) and its value. But what I don't know is how to query that since the name of table and of the column changes. Is there a way to use variables? But if so, how can I put them in the query so that it automatically changes for each row?

EDIT

EXAMPLE

ATTRIBUTES table
ID         ENTITY_ID      TABLE_NAME   COLUMN_NAME
---------- -------------- ------------ -----------
1          3              Values_A     Value_1  
2          2              Values_B     Value_3
3          2              Values_A     Value_2

VALUES_A table
ENTITY_ID  Value_1        Value_2      Value_3
---------- -------------- ------------ -----------
1          Monday         42           Green
2          Sunday         3000         Blue
3          Wednesday      1            Black

VALUES_B table
ENTITY_ID  Value_1        Value_2      Value_3
---------- -------------- ------------ ------------
1          Tuesday        26           Green
2          Saturday       3            Red
3          Wednesday      15           White

So the result I'm looking for would be:

RESULT:
ID        Value
--------- -----------
1         Wednesday
2         Red
3         3000

Sorry if this is painful to watch, it was as painful to make (didn't find how to format it better)

Upvotes: 2

Views: 1965

Answers (2)

William Robertson
William Robertson

Reputation: 16001

Using Peter M's query to build the SQL text, and then harnessing the dark power of XML:

create table attributes (id, entity_id, table_name, column_name)
as
select 1, 3, 'VALUES_A', 'VALUE_1' from dual union all
select 2, 2, 'VALUES_B', 'VALUE_3' from dual union all
select 3, 2, 'VALUES_A', 'VALUE_2' from dual;

create table values_a (entity_id, value_1, value_2, value_3)
as
select 1, 'Monday', 42, 'Green' from dual union all
select 2, 'Sunday', 3000, 'Blue' from dual union all
select 3, 'Wednesday', 1, 'Black' from dual;

create table values_b (entity_id, value_1, value_2, value_3)
as
select 1, 'Tuesday', 26, 'Green' from dual union all
select 2, 'Saturday', 3, 'Red' from dual union all
select 3, 'Wednesday', 15, 'White' from dual;

Query:

with queries as
     ( select table_name, column_name, entity_id
            , 'select '|| column_name || ' as c from ' || table_name ||
             ' where entity_id = ' || entity_id ||
              case
                  when id = max_id then ''
                  else ' union all '
              end as sqltext
       from 
           ( select a.*, max(a.id) over (order by id) max_id from attributes a ) )
select table_name, column_name, entity_id
     , extractvalue(xmltype(dbms_xmlgen.getxml(sqltext)),'/ROWSET/ROW/C') as sql_result
from   queries;

Results:

TABLE_NAME COLUMN_NAME  ENTITY_ID SQL_RESULT
---------- ----------- ---------- ---------------------------------------------------
VALUES_A   VALUE_1              3 Wednesday
VALUES_B   VALUE_3              2 Red
VALUES_A   VALUE_2              2 3000

Upvotes: 2

Peter M
Peter M

Reputation: 192

this is ugly

but it would generate a sql statement that you could run, maybe it is not what you want

select 'select '|| column_name || ' from ' || table_name || ' where entity_id = ' || entity_id || case when id = max_id then '' else ' union all ' end
from 
  (select a.*, max(a.id) over (order by id) max_id 
   from attributes a)

Upvotes: 2

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