SQL - JOIN using UNION ?? UNION using JOIN ?? with example!

Question

I was asked this question during one of my interviews. Can you do JOIN using UNION keyword? Can you do UNION using JOIN keyword?
That is -
1. I should get same output as JOIN without using JOIN keyword, but using UNION Keyword?
2. I should get same output as UNION without using UNION keyword, but using JOIN Keyword?
Can you give me an example of how to do this if possible?

Answer 1

Hi,
Welcome to the forum!
user13067794 wrote:
I was asked this question during one of my interviews. Can you do JOIN using UNION keyword? Can you do UNION using JOIN keyword?The correct answer to those questions is: Why would you want to? All versions of Oracle (and probably any other database product) provide JOIN to do certain things and UNION to do other things. Why not use those features the way they were designed to be used? Even if it is possible to do what you ask, it's going to be more complicated and less efficient.
If you really must:
That is -
1. I should get same output as JOIN without using JOIN keyword, but using UNION Keyword? You can select the relevant columns from each table, and NULLs for all the columns from other tables, in a UNION query. Then you can use GROUP BY or analytic functions to combine data from different rows. For example, this JOIN:
SELECT     d.dname
,     e.mgr
FROM     scott.dept     d
JOIN     scott.emp     e  ON     d.deptno  = e.deptno
;could be written using UNION, but no JOIN, like this:
WITH     union_data     AS
     SELECT     deptno
     ,     dname
     ,     NULL     AS empno
     ,     NULL     AS mgr
     FROM     scott.dept
    UNION ALL
     SELECT     deptno
     ,     NULL     AS dname
     ,     empno
     ,     mgr
     FROM     scott.emp
,     quasi_join     AS
     SELECT     MAX (dname) OVER (PARTITION BY deptno)     AS dname
     ,     mgr
     ,     empno
     FROM     union_data
SELECT     dname
,     mgr
FROM     quasi_join
WHERE     empno     IS NOT NULL
;Depending on your tables and your requirements, you might be able to do something a little simpler.
2. I should get same output as UNION without using UNION keyword, but using JOIN Keyword?A FULL OUTER JOIN is similar to UNION.
This UNION query:
SELECT     dname          AS txt
FROM     scott.dept
UNION
SELECT     TO_CHAR (mgr)     AS txt
FROM     scott.emp
;Can be written like this, using JOIN but no UNION:
SELECT DISTINCT
     NVL2 ( e.empno
          , TO_CHAR (e.mgr)
          , d.dname
          )          AS txt
FROM          scott.dept     d
FULL OUTER JOIN     scott.emp     e  ON       1 = 2
user13067794 wrote:I too don't any example as such, but I am thinking on this line -
Select a.x, b.y
from a,b
where a.key=b.key and sal<5000
UNION
Select a.x, b.y
From a,b
Where a.key=b.key and sal>7000
can we get same result using JOIN?That's a very special case. You can get the same results without using UNION like this:
Select distinct
     a.x
,      b.y
from      a
,     b
where      a.key     = b.key
and      (        sal < 5000
     OR     sal > 7000
Can we do something similar using UNION without using JOIN keyword??What you posted does not use the JOIN keyword.
To get the same results without using a join (either with or without the JOIN keyword), you can use UNION together with aggregate or analytic functions, as I showed earlier.
Edited by: Frank Kulash on Jul 5, 2011 9:01 PM