MySQL为不存在的记录返回null答案

【问题标题】：MySQL return null for non-existent recordMySQL为不存在的记录返回null
【发布时间】：2017-12-14 12:31:59
【问题描述】：

我有 2 个 MySQL 表，我需要将它们作为派生表加入和访问。

带有一些虚拟行的基本示例：

CREATE DATABASE Test;
USE Test;

CREATE TABLE TableOne(
    Id INT auto_increment NOT NULL,
    SomeField FLOAT,
    Timestamp DATETIME,
        PRIMARY KEY(Id)
);

CREATE TABLE TableTwo(
    Id INT auto_increment NOT NULL,
    SomeField FLOAT,
    Timestamp DATETIME,
        PRIMARY KEY(Id)
);

INSERT INTO TableOne
    (Timestamp, SomeField)
VALUES
    ('2017-06-30 23:30:00', RAND()),
    ('2017-06-30 23:45:00', RAND()),
    ('2017-07-01 00:00:00', RAND()),
    ('2017-07-01 00:15:00', RAND()),
    ('2017-07-01 00:30:00', RAND()),
    ('2017-07-01 00:45:00', RAND()),
    ('2017-07-01 01:00:00', RAND()),    
    ('2017-07-01 01:15:00', RAND()),    
    ('2017-07-01 01:30:00', RAND());    


INSERT INTO TableTwo
    (Timestamp, SomeField)
VALUES
    ('2017-06-30 23:30:00', RAND()),
    ('2017-06-30 23:40:00', RAND()),
    ('2017-06-30 23:50:00', RAND()),
    ('2017-07-01 00:00:00', RAND()),
    ('2017-07-01 00:10:00', RAND()),
    ('2017-07-01 00:20:00', RAND()),
    ('2017-07-01 00:30:00', RAND()),
    ('2017-07-01 00:40:00', RAND()),
    ('2017-07-01 00:50:00', RAND()),
    ('2017-07-01 01:00:00', RAND()),
    ('2017-07-01 01:10:00', RAND()),
    ('2017-07-01 01:20:00', RAND()),
    ('2017-07-01 01:30:00', RAND());

为了阅读表格，我正在使用：

SELECT
    SomeField,
    Timestamp
FROM
    (
        SELECT
            SomeField,
            Timestamp
        FROM
            TableOne

        UNION

        SELECT
            SomeField,
            Timestamp
        FROM
            TableTwo    
    ) d1
WHERE
    d1.Timestamp BETWEEN '2017-07-01 00:00:00' AND '2017-07-01 01:00:00'
ORDER BY
    d1.Timestamp;

此查询返回下表：

+------------+---------------------+
| SomeField  | Timestamp           |
+------------+---------------------+
|   0.380433 | 2017-07-01 00:00:00 |
| 0.00938889 | 2017-07-01 00:00:00 |
|   0.963191 | 2017-07-01 00:10:00 |
|   0.290852 | 2017-07-01 00:15:00 |
|   0.674658 | 2017-07-01 00:20:00 |
|   0.483715 | 2017-07-01 00:30:00 |
|   0.426091 | 2017-07-01 00:30:00 |
|   0.394602 | 2017-07-01 00:40:00 |
|   0.257901 | 2017-07-01 00:45:00 |
|   0.521865 | 2017-07-01 00:50:00 |
|   0.425519 | 2017-07-01 01:00:00 |
|  0.0112322 | 2017-07-01 01:00:00 |
+------------+---------------------+
12 rows in set (0.00 sec)

Test.TableOne 中的时间戳（例如）每十五分钟一次。

Test.TableTwo 中的时间戳（例如）每十分钟一次。

我想要做的是返回带有 all 时间戳的派生表，其中“SomeField”列显示相应 DATETIME 的 Null。

一个示例（使用上表）如下所示：

+------------+---------------------+
| SomeField  | Timestamp           |
+------------+---------------------+
|   0.380433 | 2017-07-01 00:00:00 |
| 0.00938889 | 2017-07-01 00:00:00 |
|   0.963191 | 2017-07-01 00:10:00 |
|   NULL     | 2017-07-01 00:10:00 |
|   0.290852 | 2017-07-01 00:15:00 |
|   NULL     | 2017-07-01 00:15:00 |
|   0.674658 | 2017-07-01 00:20:00 |
|   NULL     | 2017-07-01 00:20:00 |
|   0.483715 | 2017-07-01 00:30:00 |
|   0.426091 | 2017-07-01 00:30:00 |
|   0.394602 | 2017-07-01 00:40:00 |
|   NULL     | 2017-07-01 00:40:00 |
|   0.257901 | 2017-07-01 00:45:00 |
|   NULL     | 2017-07-01 00:45:00 |
|   0.521865 | 2017-07-01 00:50:00 |
|   NULL     | 2017-07-01 00:50:00 |
|   0.425519 | 2017-07-01 01:00:00 |
|  0.0112322 | 2017-07-01 01:00:00 |
+------------+---------------------+
18 rows in set (0.00 sec)

我尝试了许多不同的 JOIN 来实现这一点，但都没有成功。

任何帮助将不胜感激。谢谢。

【问题讨论】：

如果你只有一行的时间戳和两列的值都来自 tableone 和 tabletwo 会解决你的问题吗？

标签： mysql database select join union

【解决方案1】：

如果您仅使用一行时间戳和两列包含每个表中的值，则您的查询可能会更小。

但是，如果你真的需要像你问的那样输出，你需要模拟两个完整的外连接并将它们联合起来：

select * from (
           select t1.SomeField,Timestamp from TableOne t1 left join TableTwo t2 using (Timestamp) union all 
           select t2.SomeField,Timestamp from TableOne t1 left join TableTwo t2 using (Timestamp) union all
           select t1.SomeField,Timestamp from TableOne t1 right join TableTwo t2 using (Timestamp) union all 
           select t2.SomeField,Timestamp from TableOne t1 right join TableTwo t2 using (Timestamp)

          ) tt where tt.Timestamp BETWEEN '2017-07-01 00:00:00' AND '2017-07-01 01:00:00' order by Timestamp

【讨论】：