PostgreSQL的條件加入

Question

假設我有以下的表格 -

smallville=# create table contacts (name varchar(16), address_id int); 
CREATE TABLE 
smallville=# create table addresses (address_id int, address varchar(16)); 
CREATE TABLE 
smallville=# create table partners (name1 varchar(16), name2 varchar(16)); 
CREATE TABLE 

smallville=# insert into contacts values ('Clark Kent', NULL), ('Loise Lane', 1); 
INSERT 0 2 
smallville=# insert into addresses values (1, 'Manhattan'), (2, 'North Pole'); 
INSERT 0 2 
smallville=# insert into partners values ('Clark Kent', 'Loise Lane'), 
      ('Loise Lane', 'Clark Kent') ; 
INSERT 0 2 

我能得到的名稱和地址 -

smallville=# select c.name, a.address from contacts c 
      left outer join addresses a 
      on c.address_id = a.address_id ; 
    name | address 
------------+----------- 
Clark Kent | (NULL) 
Loise Lane | Manhattan 
(2 rows) 

但我怎麼得到如下，即表明他/她的夥伴的地址如果地址不見了？ -

name | address 
------------+----------- 
Clark Kent | Manhattan 
Loise Lane | Manhattan 
(2 rows) 

謝謝。

Answer 1

select c.name, coalesce(a.address, a1. address) from contacts c 
left outer join addresses a on c.address_id = a.address_id 
left outer join partners on c.name=partners.name1 
left outer join contacts c1 on c1.name=partners.name2 
left outer join addresses a1 on c1.address_id = a1.address_id;