On 7/21/19 8:53 AM, Karen Goh wrote:
On Sunday, July 21, 2019, 3:28:10 PM GMT+8, Holger Jakobs
<holger@xxxxxxxxxx> wrote:
Am 21. Juli 2019 02:58:05 MESZ schrieb Karen Goh <karenworld@xxxxxxxxx>:
[snip]
This looks like a 1:1 relationship which is to be avoided.
One ALTER TABLE command either adds a primary or a foreign key. There is
no FK without the keyword REFERENCES.
FK relationships help in keeping the data consistent, but they are totally
unrelated to/useless for SELECT statements and their JOIN operations. This
is a common misconception.
Hi Holger,
After reading your reply, I am confused now.
Cos without creating foreign key in my tutor_subject how am I going to
retrieve the zipcode at s_tutor table that meet the list of subject_name
and tutor_id in tutor_subject ? There should have some reference right ?
If not, how does the database tell this zipcode belong to which tutor_id ?
The reference is in the SELECT statement, not in the FK definition.
SELECT s_tutor.zip_code
from subject_tutor, s_tutor
where s_tutor.tutor_id = subject_tutor.tutor_id
and subject_tutor.subject_name = 'Angela Merkel';
--
Angular momentum makes the world go 'round.
