How to select rows from a table while ignoring the duplicate field values?
如何从表中选择行而忽略重复的字段值？
Here is an example:
这是一个例子：
id user_id message
1 Adam "Adam is here."
2 Peter "Hi there this is Peter."
3 Peter "I am getting sick."
4 Josh "Oh, snap. I'm on a boat!"
5 Tom "This show is great."
6 Laura "Textmate rocks."
What i want to achive is to select the recently active users from my db. Let's say i want to select the 5 recently active users. The problem is, that the following script selects Peter twice.
我想要实现的是从我的数据库中选择最近活跃的用户。假设我想选择5个最近活跃的用户。问题是，以下脚本选择了Peter两次。
mysql_query("SELECT * FROM messages ORDER BY id DESC LIMIT 5 ");
What i want is to skip the row when it gets again to Peter, and select the next result, in our case Adam. So i don't want to show my visitors that the recently active users were Laura, Tom, Josh, Peter, and Peter again. That does not make any sense, instead i want to show them this way: Laura, Tom, Josh, Peter, (skipping Peter) and Adam.
我想要的是当它再次到达Peter时跳过该行，并选择下一个结果，在我们的例子中是Adam。因此，我不想向访问者展示最近活跃的用户是Laura，Tom，Josh，Peter和Peter。这没有任何意义，相反，我想以这种方式向他们展示：劳拉，汤姆，乔希，彼得，（跳过彼得）和亚当。
Is there an SQL command i can use for this problem?
是否有可以用于此问题的SQL命令？
2 个解决方案
#1
26
Yes. "DISTINCT".
是。 “不同”。
SELECT DISTINCT(user_id) FROM messages ORDER BY id DESC LIMIT 5
#2
2
Maybe you could exclude duplicate user using GROUP BY.
也许你可以使用GROUP BY排除重复的用户。
SELECT * FROM messages GROUP BY user_id ORDER BY id DESC LIMIT 5;
#1
26
Yes. "DISTINCT".
是。 “不同”。
SELECT DISTINCT(user_id) FROM messages ORDER BY id DESC LIMIT 5
#2
2
Maybe you could exclude duplicate user using GROUP BY.
也许你可以使用GROUP BY排除重复的用户。
SELECT * FROM messages GROUP BY user_id ORDER BY id DESC LIMIT 5;