Relationship between two tables

linux1880 · #1 (**permalink**) 07-31-10, 01:40

Hi all I am new to database and this db forums,

Wondering, If there are two tables in a database with primary key are they in relationship without needing to add foreign keys ?

I did

PHP Code:


			
select col1, col2 from table1,table2

it fetch two columns from two different tables. There was no fk.

Is that possible without foreign key ?

Pat Phelan · #2 (**permalink**) 07-31-10, 03:18

It is possible (that is called a Cartesian Join), but it is extremely unlikely in the real world.

-PatP

r937 · #3 (**permalink**) 07-31-10, 04:33

Quote:

Originally Posted by linux1880

Wondering, If there are two tables in a database with primary key are they in relationship without needing to add foreign keys ?

they can be, if the database designer did a less than comprehensive job

is it possible to join them? sure, you can join two tables in a query on any columns you want, or, as pat has said, on no columns at all

you can also UNION them --

Code:

SELECT col1
  FROM table1
UNION
SELECT col2
  FROM table2

if you could be a bit more specific than the obfuscatory "col1,col2" and "table1,table2" then maybe we could offer more than just general comments

rein01 · #4 (**permalink**) 08-19-10, 18:39

yes it is possible....but could you be more specific on your example? What is the logical relationship between the two tables? Usually you can join them if one of the attributes in both tables have the same value, in this case this attribute can act as your foreign key even though it's not declared as one. But if you don't have attributes in both tables that does not hold the same value, you can still perform a table wide merge.

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