如何从 sql 查询中获取第一条和最后一条记录？

Question

我在PostgreSQL 中有一个表，我对它运行了一个查询，其中包含几个返回多行的条件，按其中一列排序。一般是：

SELECT <some columns> 
FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date DESC

现在我只对从此查询中获取第一行和最后一行感兴趣。我可以将它们放在数据库之外，在我的应用程序中（这就是我实际所做的），但我想知道是否为了获得更好的性能，我不应该只从数据库中获取我真正感兴趣的那两条记录。

如果是这样，我该如何修改我的查询？

Answer 1

[警告：可能不是最有效的方法]：

(SELECT <some columns>
FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date DESC
LIMIT 1)

UNION ALL

(SELECT <some columns>
FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date ASC    
LIMIT 1)

Answer 2

您可能想试试这个，可能比执行两个查询更快：

select <some columns>
from (
    SELECT <some columns>,
           row_number() over (order by date desc) as rn,
           count(*) over () as total_count
    FROM mytable
    <maybe some joins here>
    WHERE <various conditions>
) t
where rn = 1
   or rn = total_count
ORDER BY date DESC

Answer 3

第一条记录：

SELECT <some columns> FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date ASC
LIMIT 1

最后一条记录：

SELECT <some columns> FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date DESC
LIMIT 1

Answer 4

最后一条记录：

SELECT * FROM `aboutus` order by id desc limit 1

第一条记录：

SELECT * FROM `aboutus` order by id asc limit 1

Answer 5

到目前为止所有暴露的do方式，都必须扫描两次，第一行一次，最后一行一次。

使用窗口函数“ROW_NUMBER() OVER (...)”加上“WITH Queries”，您可以只扫描一次并获得两项。

窗口功能： https://www.postgresql.org/docs/9.6/static/functions-window.html

有查询： https://www.postgresql.org/docs/9.6/static/queries-with.html

例子：

WITH scan_plan AS (
SELECT
    <some columns>,
    ROW_NUMBER() OVER (ORDER BY date DESC) AS first_row, /*It's logical required to be the same as major query*/
    ROW_NUMBER() OVER (ORDER BY date ASC) AS last_row /*It's rigth, needs to be the inverse*/
FROM mytable
<maybe some joins here>
WHERE <various conditions>
ORDER BY date DESC)

SELECT
    <some columns>
FROM scan_plan
WHERE scan_plan.first_row = 1 OR scan_plan.last_row = 1;

这样，您将只进行一次关系、过滤和数据操作。

在这两种方式上尝试一些 EXPLAIN ANALYZE。

Answer 6

SELECT <rows> FROM TABLE_NAME WHERE ROWID=(SELECT MIN(ROWID) FROM TABLE_NAME) 
UNION
SELECT <rows> FROM TABLE_NAME WHERE ROWID=(SELECT MAX(ROWID) FROM TABLE_NAME)

或

SELECT * FROM TABLE_NAME WHERE ROWID=(SELECT MIN(ROWID) FROM TABLE_NAME) 
                            OR ROWID=(SELECT MAX(ROWID) FROM TABLE_NAME)

Answer 7

我知道这是一个有 7 年历史的线程，但问题几乎相同，并且接受的答案是我开始使用并最终优化为以下内容，在我的情况下，它始终返回 85ms +-5ms 是一个索引 int 字段。

note1：已接受答案中的 UNION ALL 示例也有效，但在我的情况下性能较差，时间为 300 毫秒 +-20 毫秒。

note2：下一个最受好评的答案（行计数器示例）也有效，但在我的情况下性能最低，时间为 800 毫秒 +-70 毫秒。

select
  (select <some_column> from <some_table>
    order by <some_field> limit 1)        as oldest,
  (select <some_column> from <some_table> 
    order by <some_field> desc limit 1)   as newest
;

我确实注意到 op 引用了可能的连接。我不需要为了我自己的目的而包含连接（只是在相当动态的视图中获取当前的低 ID 和高 ID），但是使用这个模型，最旧和最新的子查询应该能够成为完整的查询。尚未测试，因此不确定它是否有效或最佳。

我确实测试过这个模型（上面可能也已经建议过），它可能更容易加入，但性能只是比上面示例的一半少一点，始终返回 220 毫秒 +在我的情况下为 -10 毫秒。

select oldest.<some_field> as old, 
       newest.<some_field> as new  
from
  (select <some_column> from <some_table>
    order by <some_field> limit 1)        as oldest,
  (select <some_column> from <some_table> 
    order by <some_field> desc limit 1)   as newest
;

Answer 8

在某些情况下，WINDOW 函数 FIRST_VALUE() 和 LAST_VALUE() 很有用。主要优势 - 此查询是可读的，仅对数据进行一次排序，并且它只有一个查询可用于多列。

 SELECT
    FIRST_VALUE(timestamp) over w as created_dt,
    LAST_VALUE(timestamp) over w as last_update_dt,
    LAST_VALUE(action) over w as last_action
FROM events
WINDOW w as (ORDER BY timestamp ASC)

它可以用于通过某个ID获取第一行和最后一行

SELECT DISTINCT
    order_id,
    FIRST_VALUE(timestamp) over w as created_dt,
    LAST_VALUE(timestamp) over w as last_update_dt,
    LAST_VALUE(action) over w as last_action
    
FROM events as x
WINDOW w as (PARTITION BY order_id ORDER BY timestamp ASC)

Answer 9

select *
from {Table_Name}
where {x_column_name}=(
    select d.{x_column_name} 
    from (
        select rownum as rno,{x_column_name}
        from {Table_Name})d
        where d.rno=(
            select count(*)
            from {Table_Name}));

Answer 10

-- Create a function that always returns the first non-NULL item
CREATE OR REPLACE FUNCTION public.first_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT AS $$
        SELECT $1;
$$;


-- And then wrap an aggregate around it
CREATE AGGREGATE public.FIRST (
        sfunc    = public.first_agg,
        basetype = anyelement,
        stype    = anyelement
);

-- Create a function that always returns the last non-NULL item
CREATE OR REPLACE FUNCTION public.last_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT AS $$
        SELECT $2;
$$;

-- And then wrap an aggregate around it
CREATE AGGREGATE public.LAST (
        sfunc    = public.last_agg,
        basetype = anyelement,
        stype    = anyelement
);

从这里得到它： https://wiki.postgresql.org/wiki/First/last_(aggregate)

Answer 11

SELECT 
    MIN(Column), MAX(Column), UserId 
FROM 
    Table_Name
WHERE 
    (Conditions)
GROUP BY 
    UserId DESC

或

SELECT        
    MAX(Column) 
FROM            
    TableName
WHERE        
    (Filter)

UNION ALL

SELECT        
    MIN(Column)
FROM            
    TableName AS Tablename1
WHERE        
    (Filter)
ORDER BY 
    Column

Answer 12

为什么不使用order by asc limit 1 和相反的order by desc limit 1？

Answer 13

如何在c#中获取DB的第一条和最后一条记录。

SELECT TOP 1 * 
  FROM ViewAttendenceReport 
 WHERE EmployeeId = 4 
   AND AttendenceDate >='1/18/2020 00:00:00' 
   AND AttendenceDate <='1/18/2020 23:59:59'
 ORDER BY Intime ASC
 UNION
SELECT TOP 1 * 
  FROM ViewAttendenceReport 
 WHERE EmployeeId = 4 
   AND AttendenceDate >='1/18/2020 00:00:00' 
   AND AttendenceDate <='1/18/2020 23:59:59' 
 ORDER BY OutTime DESC; 

Answer 14

我认为这段代码是一样的并且更容易阅读。

SELECT <some columns> 
FROM mytable
<maybe some joins here>
WHERE date >= (SELECT date from mytable)
OR date <= (SELECT date from mytable);