Mysql如何将不同列中的值相互选择到单个列中答案

【问题标题】：Mysql how to select values from diffent columns into an single column amoung each otherMysql如何将不同列中的值相互选择到单个列中
【发布时间】：2017-02-08 21:04:18
【问题描述】：

我有一张这样的桌子：

+------+-------+
|yearIn|yearOut|
+------+-------+
|1974  |2012   |
+------+-------+
|1935  |2020   |
+------+-------+
|1980  |1999   |
+------+-------+

我需要选择所有这些年份并将它们放在这样的一列中：

+------+
|years |
+------+
|1974  |
+------+
|1935  |
+------+
|1980  |
+------+
|2012  |
+------+
|2020  |
+------+
|1999  |
+------+

在此先感谢您的帮助 :-)

【问题讨论】：

标签： mysql select multiple-columns

【解决方案1】：

你可以用这样的查询来做到这一点：

SELECT Years
FROM (
    SELECT yearIn AS Years FROM yourTable
    UNION ALL 
    SELECT yearOut FROM yourTable
) y
ORDER BY Years;

您也可以在没有子查询的情况下使用它

SELECT yearIn AS Years FROM yourTable
UNION ALL 
SELECT yearOut FROM yourTable
ORDER BY Years;

EXPLAIN 有和没有子查询

mysql> EXPLAIN SELECT Years
    -> FROM (
    ->     SELECT yearIn AS Years FROM yourTable
    ->     UNION ALL
    ->     SELECT yearOut FROM yourTable
    -> ) y
    -> ORDER BY Years;
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    6 |   100.00 | Using filesort |
|  2 | DERIVED     | yourTable  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL           |
|  3 | UNION       | yourTable  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL           |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+----------------+
3 rows in set, 1 warning (0,00 sec)

mysql>
mysql> EXPLAIN     SELECT yearIn AS Years FROM yourTable
    ->     UNION ALL
    ->     SELECT yearOut FROM yourTable
    ->     ORDER BY Years;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
|  1 | PRIMARY      | yourTable  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL                            |
|  2 | UNION        | yourTable  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL                            |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary; Using filesort |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
3 rows in set, 1 warning (0,00 sec)

mysql>

【讨论】：

为什么是子查询？另外，谁说我们想要重复？
@shmosel - 重复 ??
示例中没有，但有可能。
OP 说 我需要选择所有这些年，所以他会看到所有
我不同意这种解释，但争论它没有意义。您还没有解释对子查询的需求。