query from tables where table name is in the current query - sql

Question

I have this query:
select main_value, subtable, subtable_column from table1
I want to know can I query values from subtable at the same time that this query is running?
EDIT
----
table1 structure
main_value varchar
subtable varchar
subtable_column varchar
branches structure(subtable in the query)
id number
branch_name varchar
when I query from table1 and if got subtable's value as branches, and subtable_column's value as branch_name then query branch_name from branches.(becuase subtable column's value is branches retrieved from the query and column names vise versa).
EDIT for tables and samples
---------------------------
table1
+-----------------+--------------------+
| Field | Type |
+-----------------+--------------------+
| ID | number(20,0) |
| subtable | varchar2(50 BYTE) |
| subtable_column | varchar2(100 BYTE) |
+-----------------+--------------------+
branches
+-------------+-------------------+
| Field | Type |
+-------------+-------------------+
| ID | number(20,0) |
| branch_name | varchar2(50 BYTE) |
+-------------+-------------------+
NB: these are just examples, so please dont suggest other ways rather than select from tables where table names and column names are in the current query at the same time.
i have records in table1 as below:
ID | subtable | subtable_column
-------------------------------
1 | branches | branch_nmae
2 | null | null
and branches table as below:
ID | branch_name |
------------------
1 | new york
2 | colombo
output:
+-------------+-------------------+---------------------------------+
| subtable | subtable_column | selected values from branches |
+-------------+-------------------+---------------------------------+
| branches | branch_name | new york, colombo |
| null | null | |
+-------------+-------------------+---------------------------------+

Answer 1

If you know all possible tables and all possible columns which table1 may contain then you could build query with case when containing all possibilities. Something like this:
select id, subtable, subtable_column,
case
when subtable = 'branches' and subtable_column = 'branch_name' then
(select listagg(branch_name, ', ') within group (order by branch_name)
from branches)
when subtable = 'branches' and subtable_column = 'id' then
(select listagg(id, ', ') within group (order by id)
from branches)
end as subtable_values
from table1;
But more common solution for such problems is to use dynamic SQL. So write a function like here:
create or replace function get_subtable_values(i_table in varchar2, i_column in varchar2)
return varchar2 is
v_sql varchar2(32767);
v_ret varchar2(32767);
begin
if i_table is null then
return null;
end if;
v_sql := 'select listagg('||i_column||', '', '') within group (order by null) from '||i_table;
execute immediate v_sql into v_ret;
return v_ret;
end;
... and use it in query:
select id, subtable, subtable_column,
get_subtable_values(subtable, subtable_column) as subtable_values
from table1;
My test data and output for both queries:
create table table1(id, subtable, subtable_column) as (
select 1, 'branches', 'branch_name' from dual union all
select 2, 'branches', 'id' from dual union all
select 3, null, null from dual);
create table branches(id, branch_name) as (
select 1, 'New York' from dual union all
select 2, 'Colombo' from dual );
Result:
ID SUBTABLE SUBTABLE_COLUMN SUBTABLE_VALUES
------ -------- --------------- --------------------
1 branches branch_name Colombo, New York
2 branches id 1, 2
3 null null null
I used function listagg() which glues all values into one string, comma separated. It's also possible to return table of varchars or user defined types. Of course there may be many issues like strings too long, data types other than varchar, exceptions should be handled, etc. But you see that's possible.

Answer 2

SELECT DISTINCT
table1.id,
table1.subtable,
table1.subtable_column,
t.*
FROM
table1
JOIN branches
LEFT JOIN ( SELECT group_concat( branch_name ) AS NAMES FROM branches ) AS t ON 1 = 1
this sql is ok，but it is less efficient