MySQL左联接和右表中的空列

我需要连接以下两个MySQL表：

CREATE TABLE `tbl_L` (
  `datetime` datetime NOT NULL,
  `value` decimal(14,8) DEFAULT NULL,
  `series_id` int(11) NOT NULL,
  PRIMARY KEY (`series_id`,`datetime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `tbl_R` (
  `datetime` datetime NOT NULL,
  `value` decimal(14,8) DEFAULT NULL,
  `series_id` int(11) NOT NULL,
  PRIMARY KEY (`series_id`,`datetime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

我需要选择tbl_L中的所有日期和值，但也要选择tbl_R中与tbl_L中的条目具有相同日期时间的值。一个简单的连接，如下所示：

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
    LEFT JOIN tbl_R 
        ON tbl_L.datetime = tbl_R.datetime
WHERE 
   tbl_L.series_id = 1 AND tbl_R.series_id = 2 ORDER BY tbl_L.datetime ASC

不起作用，因为它只返回tbl_L和tbl_R中的datetime（因为WHERE子句中提到了正确的表）

将查询修改为如下所示：

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
    LEFT JOIN tbl_R 
        ON tbl_L.datetime = tbl_R.datetime 
           AND tbl_R.series_id = 2 
           AND tbl_L.series_id = 1
ORDER BY tbl_L.datetime ASC;

+---------------------+-------------+-------------+
| datetime            | val_L       | val_R       |
+---------------------+-------------+-------------+
| 2013-02-20 19:21:00 | 13.16000000 | 5.870000000 |
| 2013-02-20 19:23:00 | 13.22000000 | 5.800000000 |
| 2013-02-20 19:24:00 | 13.14000000 | NULL        |
| 2013-02-20 19:25:00 | 13.04000000 | 5.650000000 |
+---------------------+-------------+-------------+

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
LEFT JOIN tbl_R 
    ON tbl_L.datetime = tbl_R.datetime
        AND tbl_R.series_id = 2
WHERE 
tbl_L.series_id = 1 ORDER BY tbl_L.datetime ASC

显著降低速度（从几毫秒到几秒钟）

编辑：而且实际上也不起作用。我将澄清我需要实现的目标：

假设表中有以下数据：

mysql> SELECT * FROM tbl_R;
+---------------------+------------+-----------+
| datetime            | value      | series_id |
+---------------------+------------+-----------+
| 2013-02-20 19:21:00 | 5.87000000 |         2 |
| 2013-02-20 19:22:00 | 5.90000000 |         2 |
| 2013-02-20 19:23:00 | 5.80000000 |         2 |
| 2013-02-20 19:25:00 | 5.65000000 |         2 |
+---------------------+------------+-----------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM tbl_L;
+---------------------+-------------+-----------+
| datetime            | value       | series_id |
+---------------------+-------------+-----------+
| 2013-02-20 19:21:00 | 13.16000000 |         1 |
| 2013-02-20 19:23:00 | 13.22000000 |         1 |
| 2013-02-20 19:24:00 | 13.14000000 |         1 |
| 2013-02-20 19:25:00 | 13.04000000 |         1 |
+---------------------+-------------+-----------+
4 rows in set (0.00 sec)

同样，我需要将tbl\u L中的所有条目与tbl\u R中的条目连接起来，这些条目在日期时间方面匹配，否则为空

我的输出应该如下所示：

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
    LEFT JOIN tbl_R 
        ON tbl_L.datetime = tbl_R.datetime 
           AND tbl_R.series_id = 2 
           AND tbl_L.series_id = 1
ORDER BY tbl_L.datetime ASC;

+---------------------+-------------+-------------+
| datetime            | val_L       | val_R       |
+---------------------+-------------+-------------+
| 2013-02-20 19:21:00 | 13.16000000 | 5.870000000 |
| 2013-02-20 19:23:00 | 13.22000000 | 5.800000000 |
| 2013-02-20 19:24:00 | 13.14000000 | NULL        |
| 2013-02-20 19:25:00 | 13.04000000 | 5.650000000 |
+---------------------+-------------+-------------+

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
LEFT JOIN tbl_R 
    ON tbl_L.datetime = tbl_R.datetime
        AND tbl_R.series_id = 2
WHERE 
tbl_L.series_id = 1 ORDER BY tbl_L.datetime ASC

---

再次感谢

只需将

tbl\u R

的条件移动到join的

ON

子句中，即可获得所需的数据，如下所示：

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
    LEFT JOIN tbl_R 
        ON tbl_L.datetime = tbl_R.datetime 
           AND tbl_R.series_id = 2 
           AND tbl_L.series_id = 1
ORDER BY tbl_L.datetime ASC;

+---------------------+-------------+-------------+
| datetime            | val_L       | val_R       |
+---------------------+-------------+-------------+
| 2013-02-20 19:21:00 | 13.16000000 | 5.870000000 |
| 2013-02-20 19:23:00 | 13.22000000 | 5.800000000 |
| 2013-02-20 19:24:00 | 13.14000000 | NULL        |
| 2013-02-20 19:25:00 | 13.04000000 | 5.650000000 |
+---------------------+-------------+-------------+

SELECT tbl_L.datetime AS datetime, tbl_L.value AS val_L, tbl_R.value AS val_R 
FROM tbl_L 
LEFT JOIN tbl_R 
    ON tbl_L.datetime = tbl_R.datetime
        AND tbl_R.series_id = 2
WHERE 
tbl_L.series_id = 1 ORDER BY tbl_L.datetime ASC

---

此外，没有可用于

tbl\u L

的查询索引。在

tbl\u L.series\u id

上添加索引将有助于查询的性能。

第二个查询也可以使用它吗？是否需要

内部联接

？为了清楚起见，每个表的主键中的

es\u id

是什么？描述中的tbl_F是什么？请看我的编辑-谢谢！