Question

我写了这个查询，它返回用户朋友和朋友们。由于这里有很多子查询，我认为有更多有效的编写方式，但它有点超出我的范围。

用户表

++++++++++++++++++
+ user_id + name +
++++++++++++++++++
+ 1       + bill +
+ 2       + bob  +
+ 3       + sam  +
+ 4       + ben  +
++++++++++++++++++

user_friendships表

+++++++++++++++++++++++++++++++++++++
+ sender_user_id + receiver_user_id +
+++++++++++++++++++++++++++++++++++++
+ 1              + 2                +
+ 2              + 3                +
+ 4              + 2                +
+++++++++++++++++++++++++++++++++++++

该表是双向的，因此用户1是用户2的朋友，而用户2是用户1的朋友。

用户1只有1个朋友用户2.用户2有2个朋友，用户3和用户4。

当为用户1运行以下查询时，将返回用户2,3和4。

查询

SELECT * FROM users
WHERE ( user_id IN 

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')

UNION

SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')

)

OR user_id IN

(SELECT receiver_user_id as user_id 
FROM user_friendships 
WHERE sender_user_id = '1' 
UNION 
SELECT sender_user_id as user_id 
FROM user_friendships 
WHERE receiver_user_id = '1')
)
AND user_id != '1'

为了澄清朋友和朋友的最终结果，查询应该是user_ids的单个列表，以便它可以与用户的表连接以检索名称等

Answer 1

这可能会有所帮助：它尝试尽可能少地遍历表格。

-- First generation: Friends
SELECT
  IF(firstgen.sender_user_id=<your-user-id>,firstgen.receiver_user_id,firstgen.sender_user_id) AS friend
FROM
  user_friendships AS firstgen
WHERE
  firstgen.receiver_user_id=<your-user-id>
  OR firstgen.sender_user_id=<your-user-id>

UNION

-- Second generation: Friends of friends
SELECT
  IF(secondgen.sender_user_id in(firstgen.sender_user_id,firstgen.receiver_user_id),secondgen.receiver_user_id,secondgen.sender_user_id) AS friend
FROM
  user_friendships AS firstgen
  INNER JOIN user_friendships AS secondgen ON
    (firstgen.sender_user_id=<your-user-id> AND (secondgen.sender_user_id=firstgen.receiver_user_id OR secondgen.receiver_user_id=firstgen.receiver_user_id))
  OR
    (firstgen.receiver_user_id=<your-user-id> AND (secondgen.sender_user_id=firstgen.sender_user_id OR secondgen.receiver_user_id=firstgen.sender_user_id))
WHERE
  firstgen.receiver_user_id=<your-user-id>
  OR firstgen.sender_user_id=<your-user-id>

Answer 2

这是带来所有内容的查询

SELECT * FROM (SELECT * FROM
(SELECT sender_user_id,receiver_user_id FROM user_friendships) A
UNION
(SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
UNION
SELECT * FROM
(SELECT uf1.sender_user_id,uf2.receiver_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.receiver_user_id = uf2.sender_user_id) C
UNION
SELECT * FROM
(SELECT uf1.receiver_user_id,uf2.sender_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.sender_user_id = uf2.receiver_user_id) D;

请注意

子查询A和B是第1代双向
子查询C和D是第二代双向

以下是您的示例数据：

DROP DATABASE IF EXISTS friends;
CREATE DATABASE friends;
USE friends
CREATE TABLE users
(id int not null auto_increment,
name varchar(10),primary key (id));
insert into users (name) values
('bill'),('bob'),('sam'),('ben');
CREATE TABLE user_friendships
(sender_user_id int not null,
receiver_user_id int not null,
primary key (sender_user_id,receiver_user_id),
unique key (receiver_user_id,sender_user_id));
insert into user_friendships values
(1,2),(2,3),(2,4);

以下是您加载的示例数据

mysql> DROP DATABASE IF EXISTS friends;
(id int not null auto_increment,
name varchar(10),primary key (id));
insert into users (name) values
('bill'),('bob'),('sam'),('ben');
CREATE TABLE user_friendships
(sender_user_id int not null,
receiver_user_id int not null,
primary key (sender_user_id,receiver_user_id),
unique key (receiver_user_id,sender_user_id));
insert into user_friendships values
(1,2),(2,3),(2,4);
Query OK, 2 rows affected (0.08 sec)

mysql> CREATE DATABASE friends;
Query OK, 1 row affected (0.00 sec)

mysql> USE friends
Database changed
mysql> CREATE TABLE users
    -> (id int not null auto_increment,
    -> name varchar(10),primary key (id));
Query OK, 0 rows affected (0.08 sec)

mysql> insert into users (name) values
    -> ('bill'),('bob'),('sam'),('ben');
Query OK, 4 rows affected (0.07 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> CREATE TABLE user_friendships
    -> (sender_user_id int not null,
    -> receiver_user_id int not null,
    -> primary key (sender_user_id,receiver_user_id),
    -> unique key (receiver_user_id,sender_user_id));
Query OK, 0 rows affected (0.06 sec)

mysql> insert into user_friendships values
    -> (1,2),(2,3),(2,4);
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from users;
+----+------+
| id | name |
+----+------+
|  1 | bill |
|  2 | bob  |
|  3 | sam  |
|  4 | ben  |
+----+------+
4 rows in set (0.00 sec)

mysql> select * from user_friendships;
+----------------+------------------+
| sender_user_id | receiver_user_id |
+----------------+------------------+
|              1 |                2 |
|              2 |                3 |
|              2 |                4 |
+----------------+------------------+
3 rows in set (0.00 sec)

mysql>

这是“一切查询”的运行

mysql> SELECT * FROM (SELECT * FROM
    -> (SELECT sender_user_id,receiver_user_id FROM user_friendships) A
    -> UNION
    -> (SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.sender_user_id,uf2.receiver_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.receiver_user_id = uf2.sender_user_id) C
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.receiver_user_id,uf2.sender_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.sender_user_id = uf2.receiver_user_id) D;
+----------------+------------------+
| sender_user_id | receiver_user_id |
+----------------+------------------+
|              1 |                2 |
|              2 |                3 |
|              2 |                4 |
|              2 |                1 |
|              3 |                2 |
|              4 |                2 |
|              1 |                3 |
|              1 |                4 |
|              3 |                1 |
|              4 |                1 |
+----------------+------------------+
10 rows in set (0.00 sec)

mysql>

现在只查看用户1的关系，只需使用此查询

SELECT * FROM (
SELECT * FROM (SELECT * FROM
(SELECT sender_user_id,receiver_user_id FROM user_friendships) A
UNION
(SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
UNION
SELECT * FROM
(SELECT uf1.sender_user_id,uf2.receiver_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.receiver_user_id = uf2.sender_user_id) C
UNION
SELECT * FROM
(SELECT uf1.receiver_user_id,uf2.sender_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
WHERE (sender_user_id=1 and receiver_user_id<>1)
or (sender_user_id<>1 and receiver_user_id=1);

这是输出

mysql> SELECT * FROM (
    -> SELECT * FROM (SELECT * FROM
    -> (SELECT sender_user_id,receiver_user_id FROM user_friendships) A
    -> UNION
    -> (SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.sender_user_id,uf2.receiver_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.receiver_user_id = uf2.sender_user_id) C
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.receiver_user_id,uf2.sender_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
    -> WHERE (sender_user_id=1 and receiver_user_id<>1)
    -> or (sender_user_id<>1 and receiver_user_id=1);
+----------------+------------------+
| sender_user_id | receiver_user_id |
+----------------+------------------+
|              1 |                2 |
|              2 |                1 |
|              1 |                3 |
|              1 |                4 |
|              3 |                1 |
|              4 |                1 |
+----------------+------------------+
6 rows in set (0.00 sec)

mysql>

现在将用户表格中的名称连接起来，如下所示：

SELECT u1.name,u2.name FROM (
SELECT * FROM (SELECT * FROM
(SELECT sender_user_id,receiver_user_id FROM user_friendships) A
UNION
(SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
UNION
SELECT * FROM
(SELECT uf1.sender_user_id,uf2.receiver_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.receiver_user_id = uf2.sender_user_id) C
UNION
SELECT * FROM
(SELECT uf1.receiver_user_id,uf2.sender_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
INNER JOIN users u1 ON everything.sender_user_id = u1.id
INNER JOIN users u2 ON everything.receiver_user_id = u2.id
WHERE (sender_user_id=1 and receiver_user_id<>1)
or (sender_user_id<>1 and receiver_user_id=1);

这是输出

mysql> SELECT u1.name,u2.name FROM (
    -> SELECT * FROM (SELECT * FROM
    -> (SELECT sender_user_id,receiver_user_id FROM user_friendships) A
    -> UNION
    -> (SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.sender_user_id,uf2.receiver_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.receiver_user_id = uf2.sender_user_id) C
    -> UNION
    -> SELECT * FROM
    -> (SELECT uf1.receiver_user_id,uf2.sender_user_id
    -> FROM user_friendships uf1 INNER JOIN user_friendships uf2
    -> ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
    -> INNER JOIN users u1 ON everything.sender_user_id = u1.id
    -> INNER JOIN users u2 ON everything.receiver_user_id = u2.id
    -> WHERE (sender_user_id=1 and receiver_user_id<>1)
    -> or (sender_user_id<>1 and receiver_user_id=1);
+------+------+
| name | name |
+------+------+
| bob  | bill |
| sam  | bill |
| ben  | bill |
| bill | bob  |
| bill | sam  |
| bill | ben  |
+------+------+
6 rows in set (0.00 sec)

mysql>

试一试!!!

CAVEAT

使用LEFT JOIN连接名称而不是INNER JOIN保留返回数字的顺序

SELECT u1.name,u2.name FROM (
SELECT * FROM (SELECT * FROM
(SELECT sender_user_id,receiver_user_id FROM user_friendships) A
UNION
(SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
UNION
SELECT * FROM
(SELECT uf1.sender_user_id,uf2.receiver_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.receiver_user_id = uf2.sender_user_id) C
UNION
SELECT * FROM
(SELECT uf1.receiver_user_id,uf2.sender_user_id
FROM user_friendships uf1 INNER JOIN user_friendships uf2
ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
LEFT JOIN users u1 ON everything.sender_user_id = u1.id
LEFT JOIN users u2 ON everything.receiver_user_id = u2.id
WHERE (sender_user_id=1 and receiver_user_id<>1)
or (sender_user_id<>1 and receiver_user_id=1);

这是输出

mysql>     SELECT u1.name,u2.name FROM (
    ->     SELECT * FROM (SELECT * FROM
    ->     (SELECT sender_user_id,receiver_user_id FROM user_friendships) A
    ->     UNION
    ->     (SELECT receiver_user_id,sender_user_id FROM user_friendships)) B
    ->     UNION
    ->     SELECT * FROM
    ->     (SELECT uf1.sender_user_id,uf2.receiver_user_id
    ->     FROM user_friendships uf1 INNER JOIN user_friendships uf2
    ->     ON uf1.receiver_user_id = uf2.sender_user_id) C
    ->     UNION
    ->     SELECT * FROM
    ->     (SELECT uf1.receiver_user_id,uf2.sender_user_id
    ->     FROM user_friendships uf1 INNER JOIN user_friendships uf2
    ->     ON uf1.sender_user_id = uf2.receiver_user_id) D) everything
    ->     LEFT JOIN users u1 ON everything.sender_user_id = u1.id
    ->     LEFT JOIN users u2 ON everything.receiver_user_id = u2.id
    ->     WHERE (sender_user_id=1 and receiver_user_id<>1)
    ->     or (sender_user_id<>1 and receiver_user_id=1);
+------+------+
| name | name |
+------+------+
| bill | bob  |
| bob  | bill |
| bill | sam  |
| bill | ben  |
| sam  | bill |
| ben  | bill |
+------+------+
6 rows in set (0.00 sec)

mysql>

Answer 3

这不会得到你的名字，只有匹配的ID：

user_friendships
UNION
SELECT UF1.sender_user_id AS sender_user_id, 
      UF2.receiver_user_id AS receiver_user_id
FROM user_friendships as UF1, 
     user_friendships as UF2
WHERE UF1.receiver_user_id = UF2.sender_user_id 
   AND UF1.sender_user_id != UF2.receiver_user_id;

即：user_friendships为您提供了朋友，SELECT为您提供了朋友的朋友（确保我们不会将某人视为他们自己的朋友）。

更高效的方式写这个朋友的sql查询朋友

3 个答案: