Question

我在数据库中有以下表格。

<plugin>
    <artifactId>maven-resources-plugin</artifactId>
    <version>2.6</version>
    <executions>
        <execution>
            <id>copy-mstrweb-jars</id>
            <phase>validate</phase>
            <goals>
                <goal>copy-resources</goal>
            </goals>
            <configuration>
                <outputDirectory>${project.basedir}/lib</outputDirectory>
                <resources>
                    <resource>
                        <directory>${build.location}/${version}/java/jar</directory>
                        <filtering>false</filtering>
                        <includes>
                            <include>some.jar</include>
                        </includes>
                    </resource>
                    <resource>
                        <directory>${build.location}/${version}/jars</directory>
                        <filtering>false</filtering>
                        <includes>
                            <include>somejar.jar</include>
                        </includes>
                    </resource>
                </resources>
            </configuration>
        </execution>
    </executions>
</plugin>

如何为每个user_id仅提取按+----+---------+---------------------+-----------+ | id | user_id | timeid | course_id | +----+---------+---------------------+-----------+ | 02 | 2 | 2016-01-27 19:24:17 | 14 | | 03 | 2 | 2016-02-27 19:24:27 | 15 | | 04 | 2 | 2016-03-27 19:24:37 | 16 | | 05 | 3 | 2016-01-27 19:24:17 | 19 | | 06 | 3 | 2016-02-27 19:24:27 | 18 | | 07 | 3 | 2016-03-27 19:24:37 | 17 | +----+---------+---------------------+-----------+排序的最新行？

换句话说：

timeid

Answer 1

一种简单的方法是使用=和相关的子查询：

select t.*
from t
where t.timeid = (select max(t2.timeid) from t t2 where t2.user_id = t.user_id);

Answer 2

您可以使用以下查询：

SELECT  t1.id, t1.user_id, t1.timeid, t1.course_id
FROM mytable as t1
JOIN (
   SELECT user_id, MAX(timeid) AS timeid
   FROM mytable
   GROUP BY user_id
) AS t2 ON t1.user_id = t2.user_id AND t1.timeid = t2.timeid

t2是一个派生表，其中包含每timeid的最新user_id值。如果您将原始表格加入此表格，则可以取回具有最新每组值的记录的所有字段。

Answer 3

您可以使用像

这样的mysql查询

SELECT id, user_id, timeid, course_id FROM tablename ORDER BY timeid DESC GROUP BY user_id;

如何从数据库中提取特定列的最新行？

3 个答案: