如何查询sql以获取每个用户的最新记录日期

Question

我有一个表,它是用户登录时的集合条目.

username, date,      value
--------------------------
brad,     1/2/2010,  1.1
fred,     1/3/2010,  1.0
bob,      8/4/2009,  1.5
brad,     2/2/2010,  1.2
fred,     12/2/2009, 1.3

etc..

如何创建一个可以为每个用户提供最新日期的查询？

更新:我忘了我需要一个与最新日期一致的值.

Answer 1

select t.username, t.date, t.value
from MyTable t
inner join (
    select username, max(date) as MaxDate
    from MyTable
    group by username
) tm on t.username = tm.username and t.date = tm.MaxDate

Answer 2

使用窗口函数(适用于Oracle,Postgres 8.4,SQL Server 2005,DB2,Sybase,Firebird 3.0,MariaDB 10.3)

select * from (
    select
        username,
        date,
        value,
        row_number() over(partition by username order by date desc) as rn
    from
        yourtable
) t
where t.rn = 1

Answer 3

我看到大多数开发人员使用内联查询而不考虑它对大数据的影响.

简单地说,您可以通过以下方式实现

SELECT a.username, a.date, a.value
FROM myTable a
LEFT OUTER JOIN myTable b
ON a.username = b.username 
AND a.date < b.date
WHERE b.username IS NULL
ORDER BY a.date desc;

Answer 4

根据我的经验，最快的方法是获取表中没有更新行的每一行。

另一个优点是使用的语法非常简单，并且查询的含义很容易掌握（获取所有行，以便所考虑的用户名不存在较新的行）。

不存在

SELECT username, value
FROM t
WHERE NOT EXISTS (
  SELECT *
  FROM t AS witness
  WHERE witness.username = t.username AND witness.date > t.date
);

ROW_NUMBER

SELECT username, value
FROM (
  SELECT username, value, row_number() OVER (PARTITION BY username ORDER BY date DESC) AS rn
  FROM t
) t2
WHERE rn = 1

内部联接

SELECT t.username, t.value
FROM t
INNER JOIN (
  SELECT username, MAX(date) AS date
  FROM t
  GROUP BY username
) tm ON t.username = tm.username AND t.date = tm.date;

左外连接

SELECT username, value
FROM t
LEFT OUTER JOIN t AS w ON t.username = w.username AND t.date < w.date
WHERE w.username IS NULL

Answer 5

要获取包含用户最大日期的整行:

select username, date, value
from tablename where (username, date) in (
    select username, max(date) as date
    from tablename
    group by username
)

Answer 6

SELECT *     
FROM MyTable T1    
WHERE date = (
   SELECT max(date)
   FROM MyTable T2
   WHERE T1.username=T2.username
)