如何从sql查询中获取第一条记录和最后一条记录？

我在PostgreSQL中有一个表，我用几种条件对它运行查询，该条件返回按行之一排序的多行。通常是：

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

现在，我只想从此查询中获取第一行和最后一行。我可以将它们从数据库之外移到我的应用程序中（这是我实际上所做的），但想知道是否为了获得更好的性能，我不应该仅从我真正感兴趣的那两条记录中获取数据。

如果是这样，如何修改查询？

[注意：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)

您可能想尝试一下，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

第一记录：

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

上一条记录：

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

上一条记录：

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

第一条记录：

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

到目前为止，在所有公开的方式中，都必须进行两次扫描，一次扫描到第一行，最后一次扫描。

使用窗口功能“ ROW_NUMBER（）OVER（...）”加上“ WITH Queries”，您只能扫描一次并获得两个项目。

视窗功能：https : //www.postgresql.org/docs/9.6/static/functions-window.html

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

例：

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;

这样一来，您将只进行一次关系，筛选和数据处理。

尝试尝试两种方式的EXPLAIN ANALYZE。

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)

要么

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

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}));

-- 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）

在某些情况下（当列数不多时），WINDOW函数FIRST_VALUE（）和LAST_VALUE（）很有用。

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

该查询仅对数据排序一次。

它可以用来通过一些ID获取第一个和最后一行

SELECT DISTINCT
    order_id,
    FIRST_VALUE(timestamp) over (PARTITION BY order_id ORDER BY timestamp ASC) as created_dt,
    LAST_VALUE(timestamp) over (PARTITION BY order_id ORDER BY timestamp ASC) as last_update_dt,
    LAST_VALUE(action) over (PARTITION BY order_id ORDER BY timestamp ASC) as last_action

FROM events as x

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

要么

SELECT        
    MAX(Column) 
FROM            
    TableName
WHERE        
    (Filter)

UNION ALL

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

如何在c＃中获取数据库的第一条记录和最后一条记录。

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; 

我认为这段代码变得相同并且更易于阅读。

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

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