3
NULL列我有以下两个MySQL表,我需要加入:在tbl_RMySQL的左连接和右表
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
,我需要选择全部来自tbl_L日期和价值观,但也值与tbl_L中的条目具有相同的日期时间。一个不重要的加入,就像这样:
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
不会工作,因为它只会返回日期时间,它们都在tbl_L和tbl_R(因为右边的表,在WHERE子句中提到的)。
修改查询看起来像这样:
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;
显着减缓下来(从几毫秒到几秒钟长)。
编辑:并且也不实际工作。我会澄清我需要实现:
假设下列数据表中:
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,我需要的所有条目tbl_L与条目加入tbl_R匹配字词的日期时间,否则为NULL。
我的输出应该是这样的:
+---------------------+-------------+-------------+
| 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 |
+---------------------+-------------+-------------+
再次感谢!
它适用于第二个查询? – 2013-02-21 21:43:47
你需要'INNER JOIN'吗? – 2013-02-21 21:48:03
为了清楚起见,每个表中主键中的es_id是什么?描述中的tbl_F是什么? – Slowcoder 2013-02-21 22:04:32