如何加入很多桌子？

Question

I am very confused on how JOIN tables work. I tried to read every possible documentation without getting anything good.

I am trying to do a big SELECT on various tables to get all the data of a specific user. The below query seems to loop forever without any good result.

Can anyone explain in the easiest way to do JOIN in a correct way? Thanks

SELECT user.id_user, name, surname, email, password, status, phone, address, city, zip_code, company_name, cf, piva, public, icon, description, note, web, facebook_id, facebook_token, twitter_id, twitter_token, linkedin_id, linkedin_token, ip, date, link, confirmed, newsletter, legal_validation FROM user,user_addfields_anagraphic,user_addfields_legal,user_addfields_public,user_addfields_social,user_registration WHERE user.id_user = '43534534534'

Answer 1

You aren't joining the tables together in any meaningful way, so you end up with a Cartesian Product, which is where every row is joined with every other row.

you have to find the columns in each table that allow then to relate to each other ( it's a relational database we're dealing with ).

So if you want to know which employees are in a department, you would do something like

select name, department 
from employee emp join Department d 
on emp.dept_no = d.dept_no
where dept.description = "SALES"

the key thing to recognise is the join condition - it specifies how you relate the contents of one table to the other.

Your statement is quite complex - I'd recommend breaking it down int much smaller chunks. Get a two table join to work and then gradually add the other tables in.

Answer 2

Try like

   SELECT table1.* , 
   table2.* , 
   table3.* 
   FROM table1 
   JOIN table2 ON .... 
   JOIN table3 ON....

Answer 3

You need to join your other tables. I can´t find this in your where clause

You could do something like this:

SELECT user.id_user, name, surname, email, password, status, phone, address, city, zip_code, company_name, cf, piva, public, icon, description, note, web, facebook_id, facebook_token, twitter_id, twitter_token, linkedin_id, linkedin_token, ip, date, link, confirmed, newsletter, legal_validation
FROM user
INNER JOIN user_addfields_anagraphic on user.user_id = user_addfields_anagraphic.user_id
...
WHERE user.id_user = '43534534534'

Or like this (which is the same)

SELECT user.id_user, name, surname, email, password, status, phone, address, city, zip_code, company_name, cf, piva, public, icon, description, note, web, facebook_id, facebook_token, twitter_id, twitter_token, linkedin_id, linkedin_token, ip, date, link, confirmed, newsletter, legal_validation
FROM user, user_addfields_anagraphic, ...
WHERE user.id_user = '43534534534' and
user.user_id = user_addfields_anagraphic.user_id and
...

In this example above I guess, that user_addfields_anagraphic.user_id is your foreign key to your primary key in table user called "user_id".

You need also to join the other tables with this syntax as shown above. But trying to select from multiple tables wihtout a where clause where to join the other tables is impossible.