Need help on writing sql query with dynamic columns - sql

Question

I have to write a query which does below. I tried but couldn't write. Please help me.
I have table which returns below result set.
select *
from table1; --(rowid and ColumnName are columns of the table)
Output:
rowid ColumnName
------------------------------
1 Segment1
2 Segment2
I have another table which has below structure : (Segment1 and Segment2 are columns here)
select *
from table2;
Output:
appId Segment1 Segment2 Segment3
---------------------------------------------
a1 fld1 fld2 per
a2 cmp1 hcd4 klp
I need to write a query, which reads the "ColumnName" values from first table and retrieves column values in the second table.
That means, from the table1, I will know what are the available columns I the table2 and from table2, I will know what is the data stored against those columns.
Please let me know if I am not clear.
This query is in Oracle SQL

Answer 1

As mentioned in the comment you need a PLSQL block with dynamic sql. See below an example:
Tables:
create table table1 (row_id number,
ColumnName varchar2(100))
create table table2 (appId number,
Segment1 varchar2(100),
Segment2 varchar2(100),
Segment3 varchar2(100));
Insert all
into TABLE1 (ROW_ID, COLUMNNAME) Values (1, 'Segment1')
into TABLE1 (ROW_ID, COLUMNNAME) Values (2, 'Segment2')
into TABLE2 (APPID, SEGMENT1, SEGMENT2, SEGMENT3) Values (1, 'RRR', 'KKK', 'MMM')
into TABLE2 (APPID, SEGMENT1, SEGMENT2, SEGMENT3) Values (2, 'ZZZ', 'PPP', 'QQQ')
into TABLE2 (APPID, SEGMENT1, SEGMENT2, SEGMENT3) Values (3, 'LLL', 'NNN', 'DDD')
select * from dual;
Code:
DECLARE
var VARCHAR2 (1000);
v_sql VARCHAR2 (2000);
TYPE x_var IS TABLE OF VARCHAR2(1000);
z_var x_var;
num number:=0;
BEGIN
FOR rec IN ( SELECT DISTINCT columnname
FROM table1
ORDER BY 1)
LOOP
num := num +1;
if num = 1 then
var:= rec.columnname;
else
var := var || ' || '' , ''||' || rec.columnname;
end if;
END LOOP;
var := RTRIM (LTRIM (var, ','), ',');
v_sql := 'select '|| var ||' from table2';
EXECUTE IMMEDIATE v_sql BULK COLLECT INTO z_var;
FOR i IN 1 .. z_var.COUNT
LOOP
DBMS_OUTPUT.put_line (z_var(i));
END LOOP;
END;
Output:
SQL> /
RRR , KKK
ZZZ , PPP
LLL , NNN

Answer 2

Dynamic columns in a SQL statement are almost always a bad idea. There's usually a way to avoid these kind of problems and build a simpler solution.
But if this is one of those rare times when you really need to run dynamic SQL in SQL then you'll need to install and run something like my open source project Method4.
For example:
create table table1 as
select 1 id, 'Segment1' columnName from dual union all
select 2 id, 'Segment2' columnName from dual;
create table table2 as
select 'a1' appId, 'fld1' Segment1, 'fld2' Segment2, 'per' Segment3 from dual union all
select 'a2' appId, 'cmp1' Segment1, 'hcd4' Segment2, 'klp' Segment3 from dual;
select * from table(method4.dynamic_query(
q'[
select
'select appID, '
||listagg(columnName, ',') within group (order by id)
||' from table2'
sql_statement
from table1
]'
));
APPID SEGMENT1 SEGMENT2
----- -------- --------
a1 fld1 fld2
a2 cmp1 hcd4
There are a lot of downsides to running this way. The code is complicated, slow, and has some odd behavior. For an explanation of how this works, see this article
by Adrian Billington.

Answer 3

Will the below PL SQL block help your requirement.
BEGIN
FOR iter IN (
SELECT column_name
FROM all_tab_columns
WHERE upper(table_name) = 'table1'
AND UPPER(column_name) LIKE 'SEGMENT%'
)
LOOP
SELECT iter.column_name INTO temp_table FROM table1
dbms_output.put_line(temp_table.column_name);
END LOOP;
END;
/

Answer 4

Say you have tables like the following:
SQL> select * from someTable;
COLUMN1 COLUMN2 COLUMN3
---------- ---------- ----------
1 2 3
2 4 6
3 6 9
SQL> select * from tableOfColumns;
COLUMNN
-------
column1
column3
You may need something like the following:
SQL> declare
2 type tListOfResults is table of varchar2(1000);
3 vSQL varchar2(1000);
4 vResult tListOfResults ;
5 begin
6 select 'select ' || listagg (columnName, ' || '', '' || ') within group (order by columnName) || ' from someTable'
7 into vSQL
8 from tableOfColumns;
9 --
10 execute immediate vSQL bulk collect into vResult;
11 if vResult.count() > 0 then
12 for i in vResult.first .. vResult.last loop
13 dbms_output.put_line(vResult(i));
14 end loop;
15 end if;
16 end;
17 /
1, 3
2, 6
3, 9
PL/SQL procedure successfully completed.