MySQL如何从友谊表中选择朋友？

Question

嗨我有一个简单的数据库来存储友谊，只有两列person_a，person_b，表示person_a和person_b是朋友。从这样的大列表中，我应该运行什么查询来获得'sam'的所有朋友。谢谢。

Answer 1

使用UNION：

SELECT person_b 
FROM friendship 
WHERE person_a = 'sam' 
UNION 
SELECT person_a 
FROM friendship 
WHERE person_b = 'sam' 

Answer 2

你可能想尝试像

SELECT * 
FROM users u INNER JOIN 
     (
      SELECT CASE 
         WHEN person_a = yourid THEN 
          person_b 
         ELSE person_a 
        END person_id 
      FROM FRIENDSHIPS 
      WHERE person_a = yourid 
      OR  person_b = yourid 
     ) myFriends ON u.userID = myFriends.person_id 

Answer 3

可能尝试这样，

SELECT person_a, person_b 
FROM friendship 
WHERE person_a = 'sam' or person_b = 'sam' 

Use the query like this, after that 

if(person_a=='sam') { 
    $frnd = $rowfetch["person_b"]; 
} else if(person_b=='sam') { 
    $frnd = $rowfetch["person_a"]; 
}