如何在MySQL的同一張表中加入兩個選擇

Question

Ï需要對同一張表的兩個選擇（頂行和底行）進行聯接，但是內部聯接返回一個空集。 我不知道這怎么可能，因為我正在對兩個表都相同的新列進行聯接。

這是底行表和頂行表的選擇子查詢：

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;

結果如下：

+------------+----------+------------+-------+---------+-----------+
| 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

頂行：

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;

結果如下：

+------------+----------+------------+-------+---------+-----------+
| 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 |
+------------+----------+------------+-------+---------+-----------+

我的問題是兩個表的內部聯接結果都為空集，而左聯接或右聯接都在相對側給出了空列。 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;

返回：

+------------+------------+-------------+------------+-----------------+----------------+
| 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 |
+------------+------------+-------------+------------+-----------------+----------------+

什么時候可以得到：

+------------+------------+-------------+------------+-----------------+----------------+
| 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 |
+------------+------------+-------------+------------+-----------------+----------------+

誰能告訴我我做錯了/如何實現我想要的？

提前非常感謝您:)

Answer 1

首先，這沒有任何關系，您不是在進行內聯接，而是在進行外聯接。

我希望對為什么在執行外部聯接時，計算出的行號與實際數據庫列的行為方式不同有一個明確的解釋，但事實確實如此。 因此，您應該創建臨時表，其中這些計算出的行號將變為實際的列：

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;

現在行號在實際列中，因此無需進行外部聯接； 內部聯接就足夠了。 如果您使用不帶臨時表的內部聯接，則不會返回任何行，這可能就是您訴諸外部聯接的原因。

順便說一句，如果您需要在程序中處理這些值，那么使用唯一列名的建議就變得至關重要。

如果要聯接許多行（此處每個表中只有10行），則可以考慮在臨時表的row_number列上創建索引，可能只是將它們定義為主鍵。