Say I have two tables, Table1 and Table2. Table1 has a primary key field "ID", and Table2 has a foreign key of "ID". Table1 has a one-to-many relationship with Table2, ie. there is only one entry for an ID in Table1, but there are many entries in Table2 that have that ID.

If I want to join Table1 with Table2 where Table1.ID = Table2.ID, what does the result look like?

I'm using SQL Server2000 if that makes a difference.

the result of the join looks like this

stooge curly
stooge larry
stooge moe
reindeer dasher
reindeer dancer
reindeer prancer
reindeer vixen
mercury NULL
venus NULL
earth luna
mars deimos
mars phobos
jupiter ... (i'm too tired to list them all, i think there's 61 of them)


note that the rows with NULL for table2.id are only seen in a left outer join

i just thought i'd mention that, because there are several different kinds of join

rudy
http://r937.com/

So if I have tables like the following:

Table_Planet
Planet_ID (pk)
Planet_Name

Table_Moon
Planet_ID (fk)
Moon_Name

and I join on Planet_ID, then the output will look like:

001, earth, luna
002, mars, deimos
002, mars, phobos
003, jupiter, moon1
003, jupiter, moon2
003, jupiter, moon3 (etc.)

is that correct?

yes, that's right

what you show is the result of an inner join -- mercury and venus have no moon, so they "drop out" of the inner join

rudy

Thanks for your help Rudy.

One more question. If I want to join (inner) three tables instead of two, and the third table is related to the first in the same way as the second table, what kind of output does that generate, and how do you construct that join statement?

Table_Planet
Planet_ID (pk)
Planet_Name

Table_Moon
Planet_ID (fk)
Moon_Name

Table_Inhabitants
Planet_ID (fk)
Inhabitant (for our example, lets pretend there are aliens)

ah, then you're in trouble

you will get a "cross join" effect

let's say all three stooges live on mars

mars curly deimos
mars curly phobos
mars larry deimos
mars larry phobos
mars moe deimos
mars moe phobos

basically, if two unrelated tables (planet inhabitants and planet moons) are joined to the same table, you will get this cross-join effect

not much you can do about it except "don't do that then"
( http://www.jargon.net/jargonfile/d/Dontdothatthen.html )

I guess I "won't do that then", although it would be nice. If we were somehow able to relate Table_Moon and Table_Inhabitants, would it then be feasible? Join moon and inhabitants and then join the result to planet?

no, "join moon and inhabitants" is the part that is going to give you the cross-join effect

Even if they were related? So that means that joining more than two tables is not ever advisable?

no, no, no

if they were related, you wouldn't have the cross-join problem

you can join as many tables together as you like, provided they are related

you really ought to try it yourself with a few test tables

you say you are using sql server 2000, so get busy and create some tables and some joins...

I'll do that. I appreciate all of your help!