如何加入 mysql 中相同 table 的两个选择
How to join two selects from the same table in mysql
Ï 需要连接相同 table 的两个 select（顶部和底部行），但内部连接 returns 是一个空集。我不明白这是怎么可能的，因为我正在对一个新列进行连接，这两个 tables.
是相同的
下面是底行和顶行 table 的 select 子查询：
底行：
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10;
结果为：
+------------+----------+------------+-------+---------+-----------+
| row_number | username | timestamp | game | viewers | followers |
+------------+----------+------------+-------+---------+-----------+
| 1 | user1 | 1517443140 | game1 | 3 | 44669 |
| 2 | user2 | 1517443081 | game2 | 5 | 44668 |
| 3 | user1 | 1517443080 | game1 | 3 | 44668 |
| 4 | user2 | 1517443021 | game2 | 5 | 44667 |
| 5 | user1 | 1517443020 | game1 | 3 | 44667 |
| 6 | user2 | 1517442961 | game2 | 5 | 44666 |
| 7 | user1 | 1517442960 | game1 | 3 | 44666 |
| 8 | user2 | 1517442901 | game2 | 5 | 44665 |
| 9 | user1 | 1517442900 | game1 | 3 | 44665 |
| 10 | user2 | 1517442841 | game2 | 5 | 44664 |
+------------+----------+------------+-------+---------+-----------+
和
前几行：
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10;
结果为：
+------------+----------+------------+-------+---------+-----------+
| row_number | username | timestamp | game | viewers | followers |
+------------+----------+------------+-------+---------+-----------+
| 1 | user1 | 1514764860 | game1 | 3 | 31 |
| 2 | user2 | 1514764861 | game2 | 5 | 31 |
| 3 | user1 | 1514764920 | game1 | 3 | 32 |
| 4 | user2 | 1514764921 | game2 | 5 | 32 |
| 5 | user1 | 1514764980 | game1 | 3 | 33 |
| 6 | user2 | 1514764981 | game2 | 5 | 33 |
| 7 | user1 | 1514765040 | game1 | 3 | 34 |
| 8 | user2 | 1514765041 | game2 | 5 | 34 |
| 9 | user1 | 1514765100 | game1 | 3 | 35 |
| 10 | user2 | 1514765101 | game2 | 5 | 35 |
+------------+----------+------------+-------+---------+-----------+
我的问题是，两个 table 的内部联接结果给出了一个空集，并且左联接或右联接在另一侧给出了空列。例如：
select late.row_number, early.row_number, early.username as early_users, late.username as late_users, early.followers as early_followers, late.followers as late_followers from
(select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10) late
left join
(select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10) early
on late.row_number = early.row_number;
returns:
+------------+------------+-------------+------------+-----------------+----------------+
| row_number | row_number | early_users | late_users | early_followers | late_followers |
+------------+------------+-------------+------------+-----------------+----------------+
| 1 | NULL | NULL | user1 | NULL | 44669 |
| 2 | NULL | NULL | user2 | NULL | 44668 |
| 3 | NULL | NULL | user1 | NULL | 44668 |
| 4 | NULL | NULL | user2 | NULL | 44667 |
| 5 | NULL | NULL | user1 | NULL | 44667 |
| 6 | NULL | NULL | user2 | NULL | 44666 |
| 7 | NULL | NULL | user1 | NULL | 44666 |
| 8 | NULL | NULL | user2 | NULL | 44665 |
| 9 | NULL | NULL | user1 | NULL | 44665 |
| 10 | NULL | NULL | user2 | NULL | 44664 |
+------------+------------+-------------+------------+-----------------+----------------+
什么时候，我想得到的是：
+------------+------------+-------------+------------+-----------------+----------------+
| row_number | row_number | early_users | late_users | early_followers | late_followers |
+------------+------------+-------------+------------+-----------------+----------------+
| 1 | 1 | user1 | user1 | 31 | 44669 |
| 2 | 2 | user2 | user2 | 31 | 44668 |
| 3 | 3 | user1 | user1 | 32 | 44668 |
| 4 | 4 | user2 | user2 | 32 | 44667 |
| 5 | 5 | user1 | user1 | 33 | 44667 |
| 6 | 6 | user2 | user2 | 33 | 44666 |
| 7 | 7 | user1 | user1 | 34 | 44666 |
| 8 | 8 | user2 | user2 | 34 | 44665 |
| 9 | 9 | user1 | user1 | 35 | 44665 |
| 10 | 10 | user2 | user2 | 35 | 44664 |
+------------+------------+-------------+------------+-----------------+----------------+
任何人都可以告诉我我正在做什么 wrong/how 来实现我想要的吗？
非常感谢您:)
首先，这与任何事情都没有关系，您不是在进行 INNER JOIN，而是在进行 OUTER JOIN。
我希望我对为什么计算的行号在执行外部连接时的行为方式与实际数据库列不同的原因有明确的解释，但事实似乎确实如此。因此，您应该创建临时 tables，其中这些计算的行号将成为实际列：
CREATE TEMPORARY TABLE late as
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10;
CREATE TEMPORARY TABLE early as
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10;
select late.row_number, early.row_number, early.username as early_users, late.username as late_users, early.followers as early_followers, late.followers as late_followers from
late l JOIN early on late.row_number = early.row_number;
既然行号在实际的列中，就没有必要进行外连接了；内部连接就足够了。如果您在没有临时 table 的情况下使用内部联接，您将不会返回任何行，这可能就是您求助于外部联接的原因。
顺便说一句，如果您需要在程序中处理这些值，使用唯一列名的建议就变得至关重要。
如果您要连接很多行（这里每个 table 只有 10 个行），您会考虑在临时 table 的 row_number 列上创建索引s，可能只是将它们定义为主键。
Ï 需要连接相同 table 的两个 select（顶部和底部行），但内部连接 returns 是一个空集。我不明白这是怎么可能的，因为我正在对一个新列进行连接，这两个 tables.
是相同的下面是底行和顶行 table 的 select 子查询：
底行：
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10;
结果为：
+------------+----------+------------+-------+---------+-----------+
| row_number | username | timestamp | game | viewers | followers |
+------------+----------+------------+-------+---------+-----------+
| 1 | user1 | 1517443140 | game1 | 3 | 44669 |
| 2 | user2 | 1517443081 | game2 | 5 | 44668 |
| 3 | user1 | 1517443080 | game1 | 3 | 44668 |
| 4 | user2 | 1517443021 | game2 | 5 | 44667 |
| 5 | user1 | 1517443020 | game1 | 3 | 44667 |
| 6 | user2 | 1517442961 | game2 | 5 | 44666 |
| 7 | user1 | 1517442960 | game1 | 3 | 44666 |
| 8 | user2 | 1517442901 | game2 | 5 | 44665 |
| 9 | user1 | 1517442900 | game1 | 3 | 44665 |
| 10 | user2 | 1517442841 | game2 | 5 | 44664 |
+------------+----------+------------+-------+---------+-----------+
和
前几行：
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10;
结果为：
+------------+----------+------------+-------+---------+-----------+
| row_number | username | timestamp | game | viewers | followers |
+------------+----------+------------+-------+---------+-----------+
| 1 | user1 | 1514764860 | game1 | 3 | 31 |
| 2 | user2 | 1514764861 | game2 | 5 | 31 |
| 3 | user1 | 1514764920 | game1 | 3 | 32 |
| 4 | user2 | 1514764921 | game2 | 5 | 32 |
| 5 | user1 | 1514764980 | game1 | 3 | 33 |
| 6 | user2 | 1514764981 | game2 | 5 | 33 |
| 7 | user1 | 1514765040 | game1 | 3 | 34 |
| 8 | user2 | 1514765041 | game2 | 5 | 34 |
| 9 | user1 | 1514765100 | game1 | 3 | 35 |
| 10 | user2 | 1514765101 | game2 | 5 | 35 |
+------------+----------+------------+-------+---------+-----------+
我的问题是，两个 table 的内部联接结果给出了一个空集，并且左联接或右联接在另一侧给出了空列。例如：
select late.row_number, early.row_number, early.username as early_users, late.username as late_users, early.followers as early_followers, late.followers as late_followers from
(select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10) late
left join
(select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10) early
on late.row_number = early.row_number;
returns:
+------------+------------+-------------+------------+-----------------+----------------+
| row_number | row_number | early_users | late_users | early_followers | late_followers |
+------------+------------+-------------+------------+-----------------+----------------+
| 1 | NULL | NULL | user1 | NULL | 44669 |
| 2 | NULL | NULL | user2 | NULL | 44668 |
| 3 | NULL | NULL | user1 | NULL | 44668 |
| 4 | NULL | NULL | user2 | NULL | 44667 |
| 5 | NULL | NULL | user1 | NULL | 44667 |
| 6 | NULL | NULL | user2 | NULL | 44666 |
| 7 | NULL | NULL | user1 | NULL | 44666 |
| 8 | NULL | NULL | user2 | NULL | 44665 |
| 9 | NULL | NULL | user1 | NULL | 44665 |
| 10 | NULL | NULL | user2 | NULL | 44664 |
+------------+------------+-------------+------------+-----------------+----------------+
什么时候，我想得到的是：
+------------+------------+-------------+------------+-----------------+----------------+
| row_number | row_number | early_users | late_users | early_followers | late_followers |
+------------+------------+-------------+------------+-----------------+----------------+
| 1 | 1 | user1 | user1 | 31 | 44669 |
| 2 | 2 | user2 | user2 | 31 | 44668 |
| 3 | 3 | user1 | user1 | 32 | 44668 |
| 4 | 4 | user2 | user2 | 32 | 44667 |
| 5 | 5 | user1 | user1 | 33 | 44667 |
| 6 | 6 | user2 | user2 | 33 | 44666 |
| 7 | 7 | user1 | user1 | 34 | 44666 |
| 8 | 8 | user2 | user2 | 34 | 44665 |
| 9 | 9 | user1 | user1 | 35 | 44665 |
| 10 | 10 | user2 | user2 | 35 | 44664 |
+------------+------------+-------------+------------+-----------------+----------------+
任何人都可以告诉我我正在做什么 wrong/how 来实现我想要的吗？
非常感谢您:)
首先，这与任何事情都没有关系，您不是在进行 INNER JOIN，而是在进行 OUTER JOIN。
我希望我对为什么计算的行号在执行外部连接时的行为方式与实际数据库列不同的原因有明确的解释，但事实似乎确实如此。因此，您应该创建临时 tables，其中这些计算的行号将成为实际列：
CREATE TEMPORARY TABLE late as
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp desc, username limit 10;
CREATE TEMPORARY TABLE early as
select @n := @n + 1 row_number, st.* from (select @n:=0) init, strmrs st where timestamp between 1514764860 and 1517443140 order by timestamp, username limit 10;
select late.row_number, early.row_number, early.username as early_users, late.username as late_users, early.followers as early_followers, late.followers as late_followers from
late l JOIN early on late.row_number = early.row_number;
既然行号在实际的列中，就没有必要进行外连接了；内部连接就足够了。如果您在没有临时 table 的情况下使用内部联接，您将不会返回任何行，这可能就是您求助于外部联接的原因。
顺便说一句，如果您需要在程序中处理这些值，使用唯一列名的建议就变得至关重要。
如果您要连接很多行（这里每个 table 只有 10 个行），您会考虑在临时 table 的 row_number 列上创建索引s，可能只是将它们定义为主键。