SQL 从所有时间记录中返回第一个记录日期

Question

存储历史记录的钓鱼数据库。但是，当某人有联合记录时，我想恢复到第一次捕获作为真实记录。

select * 
from T 
inner join (select Type, 
                   Name, 
                   max(TotDrams) as maxdrams 
            from T 
            WHERE Type='Common Bream' 
            group by Type, Name 
           ) sq on T.Type = sq.Type 
                and T.Name = sq.Name 
                and sq.maxdrams = T.TotDrams 
ORDER BY Ranking ASC

上面返回每个名称的最佳捕获的所有时间记录，但是当记录是联合时，它自然会返回附加记录。我只想将最早的日期记录包含在所有时间记录中。

有没有办法修改上面的代码来删除额外的联合记录，只选择最早记录的？

Fishname     Rank            Weight           Angler            Date

Slimey       Rank 1        2 lb   3 oz      John Budd         30/11/2013
Fishy        Rank 2        1 lb   15 oz     Chris Clot        12/01/2009
Scales       Rank 3        1 lb   12 oz     John Budd         21/03/2014
Scales       Rank 3        1 lb   12 oz     Harry White       01/04/2002

对于上面的例子 - 这是目前正在发生的事情，我想删除 John Budd 的联合记录，因为它是联合的，而不是原始记录。

另一个注意事项：- 此 SQL 将与 php 一起使用。

Answer 1

您可以将另一个联接添加到每条鱼被捕获的最早日期。比如：

select * 
from T 
inner join (select Species, 
                   FishName, 
                   max(TotalDrams) as maxdrams 
            from T 
            WHERE Species='Common Bream' 
            AND DateCaught <> ''
            group by Species, FishName 
           ) sq on T.Species = sq.Species 
                and T.FishName = sq.FishName 
                and sq.maxdrams = T.TotalDrams 
inner join (select Species, 
                   FishName, 
                   min(DateCaught) as minDate 
            from T 
            WHERE Species='Common Bream' 
            AND DateCaught <> ''
            group by Species, FishName 
           ) sq2 on T.Species = sq2.Species 
                and T.FishName = sq2.FishName 
                and sq2.minDate = T.DateCaught 
where T.DateCaught <> ''
ORDER BY Rank ASC

然后，您可以将 2 个条件重构为单个连接：

select * 
from T 
inner join (select Species, 
                   FishName, 
                   max(TotalDrams) as maxdrams,
                   min(DateCaught) as minDate 
            from T 
            WHERE Species='Common Bream' 
            AND DateCaught <> ''
            group by Species, FishName 
           ) sq on T.Species = sq.Species 
                and T.FishName = sq.FishName 
                and sq.maxdrams = T.TotalDrams 
                and sq.minDate = T.DateCaught 
where T.DateCaught <> ''
ORDER BY Rank ASC

编辑：

对数据结构的进一步分析表明，上面的答案并不完全正确——它过滤掉了几条记录，因为日期是一个 varchar，而且对数据结构的假设也不正确。修改后的答案如下：

select distinct 
   T.species,
   t.fishname,
   t.rank,
   t.pounds,
   t.ounces,
   t.drams,
   t.totaldrams,
   t.peg,
   t.angler,
   sq.*,
   sq2.*
FROM (select Species, 
             FishName, 
              max(TotalDrams) as maxdrams
      from T 
      WHERE Species='Common Bream' 
      group by Species, FishName 
     ) sq 
inner join (select Species, 
                   FishName, 
                   TotalDrams,
                   min(if(DateCaught='',STR_TO_DATE('31/12/3099','%d/%m/%Y'),STR_TO_DATE(DateCaught,'%d/%m/%Y'))) as minDate 
            from T 
            WHERE Species='Common Bream' 
            group by Species, FishName, TotalDrams 
           ) sq2 on sq.Species = sq2.Species 
                 and sq.FishName = sq2.FishName 
                 and sq.MaxDrams = sq2.TotalDrams
inner join T on sq.species = T.species 
and sq.fishname = T.fishname
and sq.maxdrams = T.totaldrams
and sq2.mindate = if(DateCaught='',STR_TO_DATE('31/12/3099','%d/%m/%Y'),STR_TO_DATE(DateCaught,'%d/%m/%Y'))

Answer 2

我的处理方式略有不同，而不是试图将结果限制在最大值，我会排除不是的结果。所以使用类似的东西：

SELECT  T.*
FROM    T
        LEFT JOIN T AS T2
            ON T2.Species = T.Species
            AND T2.FishName = T.FishName
            AND (T2.TotalDrams < T.TotalDrams
                OR (T.TotalDrams = T2.TotalDrams AND T2.DateCaught > T.DateCaught))
WHERE   T.Species = 'Common Bream'
AND     T2.Species IS NULL
ORDER BY T.Rank ASC;

这使用标准的LEFT JOIN/IS NULL 方法来排除相同鱼类的记录，并且：

• TotalDrams 的值较低

或

• TotalDrams 具有相同的值 AND 之后的DateCaught

Example on SQL Fiddle

由于 MySQL 实现子查询的方式，您可能还会发现这比没有给出所需结果的原始查询执行得更好！

---

编辑

好的，新方法。我认为解决此问题的最佳方法是使用变量为每条记录存储一个新的行号，然后您可以过滤前 1 个。以下将根据排序条件分配您的行号：

SELECT  @r:= CASE WHEN @f = t.FishName AND @s:= t.Species 
                    THEN @r + 1 
                ELSE 1 
            END AS RowNum,
        @f:= t.FishName AS FishName,
        @s:= t.Species AS Species,
        t.Rank,
        t.Pounds,
        t.Ounces,
        t.Drams,
        t.TotalDrams,
        t.Peg,
        t.Angler,
        STR_TO_DATE(IF(t.DateCaught = '', '31/12/2050', t.DateCaught), '%d/%m/%Y')  AS DateCaught
FROM    T
        CROSS JOIN (SELECT  @f:= '',@s:='', @r:= 0) AS v
ORDER BY t.FishName, t.Species, t.TotalDrams DESC, DateCaught ASC;

然后你可以把它放到一个子查询中，并将记录限制在前1：

SELECT  *
FROM    (   SELECT  @r:= CASE WHEN @f = t.FishName AND @s = t.Species 
                                THEN @r + 1 
                            ELSE 1 
                        END AS RowNum,
                    @f:= t.FishName AS FishName,
                    @s:= t.Species AS Species,
                    t.Rank,
                    t.Pounds,
                    t.Ounces,
                    t.Drams,
                    t.TotalDrams,
                    t.Peg,
                    t.Angler,
                    t.DateCaught
            FROM    T
                    CROSS JOIN (SELECT  @f:= '',@s:='', @r:= 0) AS v
            ORDER BY t.FishName, t.Species, t.TotalDrams DESC, STR_TO_DATE(IF(t.DateCaught = '', '31/12/2050', t.DateCaught), '%d/%m/%Y') ASC
        ) AS t
WHERE   t.RowNum = 1
ORDER BY t.Rank ASC;

Example on SQL Fiddle

这是最灵活的方法，如果您想添加更多规则，即如果您有两个相同的重量，并且在同一日期，您可以在子查询中添加进一步的排序，例如Angler。这保证了（鱼名、物种）的每个元组只有一个记录，并且在给定足够的顺序的情况下得到确定性结果。

Answer 3

排名后按日期排序，然后按鱼名分组。看起来可行？

select * from T inner join (select Species, FishName, max(TotalDrams) as maxdrams from T WHERE Species='Common Bream' group by Species, FishName ) sq on T.Species = sq.Species and T.FishName = sq.FishName and sq.maxdrams = T.TotalDrams GROUP BY T.FishName ORDER BY Rank, DateCaught ASC