我有一个这样的表：
// requests
+----+----------+-------------+
| id | id_user | unix_time |
+----+----------+-------------+
| 1 | 2353 | 1339412843 |
| 2 | 2353 | 1339412864 |
| 3 | 5462 | 1339412894 |
| 4 | 3422 | 1339412899 |
| 5 | 3422 | 1339412906 |
| 6 | 2353 | 1339412906 |
| 7 | 7785 | 1339412951 |
| 8 | 2353 | 1339413640 |
| 9 | 5462 | 1339413621 |
| 10 | 5462 | 1339414490 |
| 11 | 2353 | 1339414923 |
| 12 | 2353 | 1339419901 |
| 13 | 8007 | 1339424860 |
| 14 | 7785 | 1339424822 |
| 15 | 2353 | 1339424902 |
| 16 | 2353 | 1466272801 |
| 17 | 2353 | 1466272805 |
+----+----------+-------------+
我试图获取与特定用户相关的最后一行。例如对于该用户id_user = 7785我想选择这一行：
| 14 | 7785 | 1339424822 |
这是我的查询：
SELECT unix_time AS last_seen
FROM requests WHERE id = '7785'
ORDER BY unix_time DESC
LIMIT 1
但我的查询没有选择任何行。怎么了？
SQL小提琴 http://sqlfiddle.com/#!9/ad035/17
另请注意，您能否告诉我是否应该创建单个索引id_user and uinx_time或者我应该在这两列上创建多列索引(id_user, unix_time)?
您使用了错误的列（id代替id_user- 或相反亦然）：
SELECT unix_time AS last_seen
FROM requests WHERE id_user = '7785'
ORDER BY unix_time DESC
LIMIT 1
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