Does anyone know the difference between UNION operator and the "OR" in
where clause ?

Thanks ahead.

"Mike" <gongweigang (AT) gmail (DOT) com> wrote

Yes. What makes you think there is any confusion between them?

On Jul 10, 12:49 am, Mike <gongweig... (AT) gmail (DOT) com> wrote:
If all tables in UNIONed SELECT statements are same, both would
logically produced same result.(UNION ALL may be resulted redundant
rows. So, it might not be possible to replace with OR.)
But, performance may be same or different depending on DBMS which you
are using.
Generally speaking, OR may be not neccesary sort to eliminate
redundant rows which are sometimes neccesary at the time using UNION.

For example, in RMDBs, if we have the following tables:

T_Person
ID, Name, Age
1, john, 32
2, mary, 25

T_Car
ID, Name, Age
1, Accord, 2
2, Impala, 3

Then, we can ...
SELECT * FROM T_Person
UNION
SELECT * FROM T_Car;

SELECT * FROM T_Person WHERE age=32 OR age=25;

In dbd, all things are represented by nodes, thus people, cars and
ages can be "OR"ed. The following two expressions are equivalent to
above SELECT statements:

(or (get person instance *)
(get car instance *))

(and (get person instance *)
(get * age (or 32 25)))

For example, in RMDBs, if we have the following tables:

T_Person
ID, Name, Age
1, john, 32
2, mary, 25

T_Car
ID, Name, Age
1, Accord, 2
2, Impala, 3

Then, we can ...
SELECT * FROM T_Person
UNION
SELECT * FROM T_Car;

and also ...
SELECT * FROM T_Person WHERE age=32 OR age=25;

In dbd, all things are represented by nodes, thus people, cars and
ages can be "OR"ed. The following two expressions are equivalent to
above SELECT statements:

(or (get person instance *)
(get car instance *))

(and (get person instance *)
(get * age (or 32 25)))

On Jul 24, 6:22 pm, Neo <neo55... (AT) hotmail (DOT) com> wrote:

Actually Neo you point out a difference that the original poster might
not be aware of. It would be very difficult to write that union as a
join with a WHERE OR clause. In some DBMS SQL implementations, it
would be impossible.

Ed