SQL SELECT from multiple tables

问题:

How can I get all products from customers1 and customers2 include their customer names?

customer1 table
cid name1
1   john
2   joe

customer2 table
cid name2
p1  sandy
p2  linda

product table
pid cid pname
1   1   phone
2   2   pencil
3   p1  pen
4   p2  paper

Result should be like this

pid  cid  pname  name1 name2
1    1    phone  john  NULL
2    2    pencil joe   NULL
3    p1   pen    NULL  sandy
4    p2   paper  NULL  linda

回答1:

SELECT p.pid, p.cid, p.pname, c1.name1, c2.name2
FROM product p
LEFT JOIN customer1 c1 ON p.cid = c1.cid
LEFT JOIN customer2 c2 ON p.cid = c2.cid

回答2:

SELECT pid, cid, pname, name1, name2 
FROM customer1 c1, product p 
WHERE p.cid=c1.cid 
UNION SELECT pid, cid, pname, name1, name2 
FROM customer2 c2, product p 
WHERE p.cid=c2.cid;

回答3:

SELECT `product`.*, `customer1`.`name1`, `customer2`.`name2`
FROM `product`
LEFT JOIN `customer1` ON `product`.`cid` = `customer1`.`cid`
LEFT JOIN `customer2` ON `product`.`cid` = `customer2`.`cid`

回答4:

select p.pid, p.cid, c1.name,c2.name
from product p
left outer join customer1 c1 on c1.cid=p.cid
left outer join customer2 c2 on c2.cid=p.cid

回答5:

SELECT 
  pid, 
  cid, 
  pname, 
  name1, 
  null 
FROM 
  product p
INNER JOIN 
  customer1 c ON p.cid = c.cid
UNION
SELECT 
  pid, 
  cid, 
  pname, 
  null, 
  name2
FROM 
  product p
INNER JOIN 
  customer2 c ON p.cid = c.cid

回答6:

SELECT p.pid, p.cid, p.pname, c1.name1, c2.name2
FROM product AS p
    LEFT JOIN customer1 AS c1
        ON p.cid = c1.cid
    LEFT JOIN customer2 AS c2
        ON p.cid = c2.cid