查询第一个元素出现的 select 行

Question

我有以下 SQL 查询来自我正在尝试编译的一些数据：

SELECT State, CountyName, Month, SUM(Sum_Confirmed) AS Max_Sum_Confirmed 
FROM `covid_by_countynmonth` 
GROUP BY State, Month 
ORDER BY State, Max_Sum_Confirmed DESC

它给了我以下 table:

我怎样才能让它只显示每个州的第一条记录？

我想要这样的东西（我通过添加 WHERE Month="July" 得到它，但这只是为了这个例子）：

Answer 1

显然这只适用于 postgresql。

将 DISTINCT ON 用于 select 不同的值，如下所示：

SELECT DISTINCT ON (COLUMN_OF_INTEREST) COLUMN_ALIAS, ...

然后初始查询变成这样：

SELECT DISTINCT ON (SUB_.State) SUB_.State, SUB_.Month, SUB_.Max_Sum_Confirmed 
FROM (SELECT State, Month, SUM(Sum_Confirmed) AS Max_Sum_Confirmed 
FROM `covid_by_countynmonth` 
GROUP BY State, Month) AS SUB_ ORDER BY SUB_.State, SUB_.Max_Sum_Confirmed DESC

Answer 2

How can I have it so that it only shows the first record of each State?

使用window函数：

SELECT sm.*
FROM (SELECT State, Month,
             SUM(Sum_Confirmed) AS Max_Sum_Confirmed,
             ROW_NUMBER() OVER (PARTITION BY State ORDER BY SUM(SUM_Confirmed) DESC) as seqnum
      FROM `covid_by_countynmonth` 
      GROUP BY State, Month 
     ) sm
WHERE seqnum = 1;
ORDER BY State, Max_Sum_Confirmed DESC ;

CountyName 似乎是多余的所以我删除了它。

Answer 3

这是一个包含两个输出的示例：来自月的值和来自州的 SUM

SELECT
    c.State , c.CountyName
    , MONTHNAME(minMonth) as 'Month'
    , c.Sum_Confirmed -- SUM othe State der
    , f.Sum_Confirmed -- only from the first Month
      FROM (
      SELECT
          State, CountyName
        , MIN(str_to_date(`Month`,'%M')) as minMonth
        , SUM(Sum_Confirmed) as Sum_Confirmed
      FROM covid_by_countynmonth
      GROUP BY State
    ) AS c
LEFT JOIN covid_by_countynmonth AS f ON c.State = f.State AND f.`Month` = MONTHNAME(minMonth);

样本

MariaDB [Bernd]> SELECT * FROM covid_by_countynmonth;
+----+-------+------------+---------+---------------+
| id | State | CountyName | Month   | Sum_Confirmed |
+----+-------+------------+---------+---------------+
|  1 | AR    | Arkansas   | July    |         21502 |
|  2 | AR    | Arkansas   | June    |         13244 |
|  3 | LA    | Acadia     | July    |         40500 |
|  4 | LA    | Acadia     | May     |         11997 |
|  5 | LA    | Acadia     | January |           999 |
+----+-------+------------+---------+---------------+
5 rows in set (0.10 sec)

MariaDB [Bernd]> SELECT
    -> c.State , c.CountyName
    -> , MONTHNAME(minMonth) as 'Month'
    -> , c.Sum_Confirmed -- SUM othe State der
    -> , f.Sum_Confirmed -- only from the first Month
    ->   FROM (
    ->   SELECT
    ->       State, CountyName
    ->     , MIN(str_to_date(`Month`,'%M')) as minMonth
    ->     , SUM(Sum_Confirmed) as Sum_Confirmed
    ->   FROM covid_by_countynmonth
    ->   GROUP BY State
    -> ) AS c
    -> LEFT JOIN covid_by_countynmonth AS f ON c.State = f.State AND f.`Month` = MONTHNAME(minMonth);
+-------+------------+---------+---------------+---------------+
| State | CountyName | Month   | Sum_Confirmed | Sum_Confirmed |
+-------+------------+---------+---------------+---------------+
| AR    | Arkansas   | June    |         34746 |         13244 |
| LA    | Acadia     | January |         53496 |           999 |
+-------+------------+---------+---------------+---------------+
2 rows in set (0.05 sec)

MariaDB [Bernd]>

查询第一个元素出现的 select 行

Query to select row with first occurrence of an element

mysql

sql

database

greatest-n-per-group