我有2个表:table_a和table_b。两者都包含一个名为“open”的列。
table_a
+-------+
| open |
+-------+
| 36.99 |
| 36.85 |
| 36.40 |
| 36.33 |
| 36.33 |
+-------+
table_b
+------+
| open |
+------+
| 4.27 |
| 4.46 |
| 4.38 |
| 4.22 |
| 4.18 |
+------+
我想编写一个返回以下内容的查询
+-------++------+
| open || open |
+-------++------+
| 36.99 || 4.27 |
| 36.85 || 4.46 |
| 36.40 || 4.38 |
| 36.33 || 4.22 |
| 36.33 || 4.18 |
+-------++------+
我尝试以下查询:
select a.open, b.open from table_a a, table_b b;
这将为table_a.open的每个值返回一个table_b.open的每个值的表
+-------++------+
| open || open |
+-------++------+
| 36.99 || 4.27 |
| 36.99 || 4.46 |
| 36.99 || 4.38 |
| 36.99 || 4.22 |
| ... || 4.18 |
+ ... ++------+
我可以看到我在这里误解了别名的正确用法。有什么想法吗?
答案 0 :(得分:6)
这不是你的别名问题。您正在表格上执行CROSS JOIN
,以创建笛卡尔结果集。
这会使您的结果集倍增,因此table_a
的每一行都会直接与table_b
中的每一行匹配。
如果你想将JOIN
表放在一起,那么你需要一些列来加入表。
如果您有JOIN
列,则您的查询将是:
select a.open as a_open,
b.open as b_open
from table_a a
inner join table_b b
on a.yourCol = b.yourCol
如果您没有可用于加入的列,则可以创建用户定义的变量来执行此操作,这将为每行创建行号。
select
a.open a_open,
b.open b_open
from
(
select open, a_row
from
(
select open,
@curRow := @curRow + 1 AS a_row
from table_a
cross join (SELECT @curRow := 0) c
) a
) a
inner join
(
select open, b_row
from
(
select open,
@curRow := @curRow + 1 AS b_row
from table_b
cross join (SELECT @curRow := 0) c
) b
) b
on a.a_row = b.b_row;
答案 1 :(得分:2)
您需要一个可用于连接这两个表的列。
您可以尝试将伪列生成为行号,但我不确定这是您要实现的目标。这看起来应该是这样(可以立即测试,但想法很明确):
SELECT
a.open, b.open
FROM
(SELECT
open, @curRow := @curRow + 1 AS row_number
FROM
table_a
JOIN
(SELECT @curRow := 0)
) a
JOIN
(SELECT
open, @curRow := @curRow + 1 AS row_number
FROM
table_b
JOIN
(SELECT @curRow := 0)
) b
ON
a.row_number = b.row_number