[英]MySQL Left Join and NULL columns from right table
我有以下兩個需要加入的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
我需要從tbl_L中選擇所有日期和值,還要選擇tbl_R中與tbl_L中的條目具有相同日期時間的值。 一個簡單的聯接,像這樣:
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
將無法工作,因為它只返回tbl_L和tbl_R中的日期時間(因為在WHERE子句中提到了右表)。
將查詢修改為如下所示:
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;
顯着減慢速度(從幾毫秒到幾秒)。
編輯 :並且實際上也不起作用。 我將澄清我需要實現的目標:
假設表中的以下數據:
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)
同樣,我需要tbl_L中的所有條目與tbl_R中與datetime匹配的條目一起加入,否則為NULL。
我的輸出應該如下所示:
+---------------------+-------------+-------------+
| 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 |
+---------------------+-------------+-------------+
再次感謝!
您可以通過將tbl_R
的條件僅移動到join的ON
子句中來獲取所需的數據,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
WHERE
tbl_L.series_id = 1 ORDER BY tbl_L.datetime ASC
此外,在tbl_L
上沒有用於查詢的索引。 在tbl_L.series_id
上添加索引將有助於查詢的性能。
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