Select common columns from two tables - sql

Question

I have two tables
temp_data_holder
temp_data_holder1
These two tables will have some common columns but they can also have some extra columns which might be present in only one table. I need a query which will select the common columns from each table.
I can get the common column names using the following query
Select column_name
from all_tab_columns
where table_name like 'temp_data_holder'
intersect
Select column_name
from all_tab_columns
where table_name like 'temp_data_holder1';
Is there a way to use this query to get the resultant columns from each table?
I'm asking for something like this
Select columns=(Select column_name from all_tab_columns where table_name like 'temp_data_holder' intersect Select column_name from all_tab_columns where table_name like 'temp_data_holder1') from temp_data_holder;
I.e. the tables will be inside a for loop and the structure will change after each iteration so i cannot simply hard-code the column names

Answer 1

In general you can get the common columns (evaluated by name) from two tables using the statement below.
select listagg(column_name,',') within group (order by column_name)
from (
select column_name
from user_tab_columns
where table_name in (t1,t2)
group by column_name having count(*) = 2
);
the resulting string could be used to generate useful statements for comparing the two tables t1 and t2

Answer 2

SQL and PL/SQL are strongly-typed and rigorous languages. Doing things on the fly is hard. That makes applications reliable but developers schooled in more reflexive languages butt up against such inflexibility.
Which is to say there is no easy way to achieve what you want. You need to assemble the query as a string and use Dynamic SQL to execute the finished statement. You also will have problems reading the result set, because SQL doesn't handle ad hoc projections. Usually this means code has to be called from say Java using a Ref Cursor to map to a JDBC ResultSet.
create or replace function get_data_for_common_cols return sys_refcursor is
stmt varchar2(32767) := 'select ';
rc sys_refcursor;
begin
for recs in ( select t.column_name
, row_number() over (partition by t.table_name order by t.column_id) as rn
from user_tab_columns t
join user_tab_columns t1 on t1.column_name = t.column_name
where t.table_name = 'TEMP_DATA_HOLDER'
and t1.table_name = 'TEMP_DATA_HOLDER1'
order by t.column_id)
loop
if recs.rn != 1 then
stmt := stmt || ',';
end if;
stmt := stmt || recs.column_name;
end loop;
stmt := stmt || ' from TEMP_DATA_HOLDER';
open rc for stmt;
return rc;
end;
/
This function can be parameterised to take a table name - or two table names - for use in the loop query and the assembled statement. Doing so is left as an exercise for the reader :)

Answer 3

SELECT COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME IN ('articles', 'articles')
GROUP BY column_name HAVING COUNT(*) = 2;