Query tables and columns based on table data - sql

Question

Without knowing the name of a table and columns, I want to query the database retrieve the table and column names and then query the given tables.
I have an Oracle database schema that is like the following:
Item table:
Item_id, Item_type,
=================
1 box
2 book
3 box
Book table:
Item_id, title, author
===========================
2 'C# Programer', 'Joe'
Box table:
Item_id, Size
=====================
1, 'Large'
3, 'X Large'
Column_mapping table
Item_type, column_name, display_order
=====================================
box, Size, 1
book, title, 1
book, author 2
Table_mapping table:
Item_type, Table_name
========================
box, Box
book, Book
I would like a SQL statement that would give something like the following results:
Item_id, Item_type column1 column2
====================================
1, box, 'Large', <null>
2, book, 'C# Programer', 'Joe'
3, box, 'X Large', <null>
When I tried the simplified query
select *
from
(select Table_name
from Table_mapping
where Item_type = 'box')
where
Item_id = 1;
I get an error that Item_id is invalid identifier
and if I try
select *
from
(select Table_name
from Table_mapping
where Item_type = 'box');
I just get
Table_name
===========
Box
I am not sure how to proceed.

Answer 1

One way is to join both table and then use a coalesce on the column that can contain data from either table
SELECT
i.Item_id,
i.Item_type,
COALESCE(b.title, bx.size) column1,
b.author column2
FROM
Item i
LEFT JOIN Book b
ON i.item_id = b.item_id
LEFT JOIN Box bx
ON i.item_id = bx.item_id
Depending on how large your datasets are you may want to add a filter on the join e.g.
LEFT JOIN Book b
ON i.item_id = b.item_id
and i.item_type = 'book'
LEFT JOIN Box bx
ON i.item_id = bx.item_id
and i.item_type = 'box'
See it work at this SQLFiddle
If you wanted to do something based on the data in table_mapping or column_mapping you'd need to use dynamic SQL

Answer 2

Basically it is two separate queries. One for boxes and one for books. You can use union to merge the result sets together.
select i.Item_id, i.Item_type, b.size, null
from Item i inner join Box b on i.Item_id=b.Item_id
where i.Item_type = "box"
UNION
select i.Item_id, i.Item_type, b.title, b.author
from Item i inner join Book b on i.Item_id=b.Item_id
where i.Item_type = "book"

Answer 3

ORACLE actually stores the table- and column names in its data dictionary, so there is no need for you to maintain those data separately. Try this to get the table names:
SELECT table_name FROM user_tables;
Then do this to get the columns for each table:
SELECT column_name FROM user_tab_columns WHERE table_name = 'MYTABLE';
Once you do that, you will need to create a stored procedure in order to execute dynamic SQL. I don't think you can do this in a plain-vanilla query.