T-SQL - 循环遍历表直到满足条件的最有效方法是什么
在T-SQL.
任务：
- 人们想进入电梯，每个人都有一定的体重。
- 排队等候的人的顺序由列轮次决定。
- 电梯的最大容量 <= 1000 磅。
- 在电梯变得太重之前，返回最后一个能够进入电梯的人的名字！
- 返回类型应该是表格
问题： 解决这个问题最有效的方法是什么？如果循环是正确的，还有改进的余地吗？
我使用了一个循环和 # 临时表，这里是我的解决方案：
set rowcount 0
-- THE SOURCE TABLE "LINE" HAS THE SAME SCHEMA AS #RESULT AND #TEMP
use Northwind
go
declare @sum int
declare @curr int
set @sum = 0
declare @id int
IF OBJECT_ID('tempdb..#temp','u') IS NOT NULL
DROP TABLE #temp
IF OBJECT_ID('tempdb..#result','u') IS NOT NULL
DROP TABLE #result
create table #result(
id int not null,
[name] varchar(255) not null,
weight int not null,
turn int not null
)
create table #temp(
id int not null,
[name] varchar(255) not null,
weight int not null,
turn int not null
)
INSERT into #temp SELECT * FROM line order by turn
WHILE EXISTS (SELECT 1 FROM #temp)
BEGIN
-- Get the top record
SELECT TOP 1 @curr = r.weight FROM #temp r order by turn
SELECT TOP 1 @id = r.id FROM #temp r order by turn
--print @curr
print @sum
IF(@sum + @curr <= 1000)
BEGIN
print 'entering........ again'
--print @curr
set @sum = @sum + @curr
--print @sum
INSERT INTO #result SELECT * FROM #temp where [id] = @id --id, [name], turn
DELETE FROM #temp WHERE id = @id
END
ELSE
BEGIN
print 'breaaaking.-----'
BREAK
END
END
SELECT TOP 1 [name] FROM #result r order by r.turn desc
这是我使用 Northwind 进行测试的表的创建脚本：
USE [Northwind]
GO
/****** Object: Table [dbo].[line] Script Date: 28.05.2018 21:56:18 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[line](
[id] [int] NOT NULL,
[name] [varchar](255) NOT NULL,
[weight] [int] NOT NULL,
[turn] [int] NOT NULL,
PRIMARY KEY CLUSTERED
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
UNIQUE NONCLUSTERED
(
[turn] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[line] WITH CHECK ADD CHECK (([weight]>(0)))
GO
INSERT INTO [dbo].[line]
([id], [name], [weight], [turn])
VALUES
(5, 'gary', 800, 1),
(3, 'jo', 350, 2),
(6, 'thomas', 400, 3),
(2, 'will', 200, 4),
(4, 'mark', 175, 5),
(1, 'james', 100, 6)
;
Mar*_*ith 16
你应该尽量避免循环。它们通常比基于集合的解决方案效率低，可读性也差。
下面应该是相当有效的。
如果名称和权重列可以INCLUDE-在索引中使用d 以避免键查找，则更是如此。
它可以按顺序扫描唯一索引turn并计算Weight列的运行总计- 然后使用LEAD相同的排序标准来查看下一行中的运行总计。
一旦它找到超过 1000 或超过 1000 的第一行NULL（表示没有下一行），它就可以停止扫描。
WITH T1
AS (SELECT *,
SUM(Weight) OVER (ORDER BY turn ROWS UNBOUNDED PRECEDING) AS cume_weight
FROM [dbo].[line]),
T2
AS (SELECT LEAD(cume_weight) OVER (ORDER BY turn) AS next_cume_weight,
*
FROM T1)
SELECT TOP 1 name
FROM T2
WHERE next_cume_weight > 1000
OR next_cume_weight IS NULL
ORDER BY turn
执行计划
实际上，它似乎在绝对必要的地方之前读取了几行 - 看起来每个窗口假脱机/流聚合对都会导致读取另外两行。
对于问题中的示例数据，理想情况下，它只需要从索引扫描中读取两行，但实际上它读取 6 行，但这不是一个显着的效率问题，并且不会随着更多行添加到表中而降级（如这个演示）
对于那些有兴趣在这个问题上与由每个操作者（如由输出的行的图像query_trace_column_values扩展事件）低于，行是在输出row_id顺序（开始于47第一行读出由索引扫描，并在精加工113为TOP）
单击下面的图像将其放大，或者查看动画版本以使流程更易于理解。
在右手流聚合发出其第一行的点暂停动画（对于 gary - turn = 1）。很明显，它正在等待接收具有不同 WindowCount 的第一行（对于 Jo - turn = 2）。并且窗口线轴不会释放第一个“Jo”行，直到它以不同的方式读取下一行turn（对于 thomas - turn = 3）
因此，窗口假脱机和流聚合都会导致读取一个额外的行，并且计划中有四个这样的行 - 因此有 4 个额外的行。
上面显示的列的解释如下（基于此处的信息）
- NodeName：索引扫描，NodeId：15，ColumnName：索引覆盖的id基表列
- NodeName：索引扫描，NodeId：15，ColumnName：转索引覆盖的基表列
- NodeName：聚集索引查找，NodeId：17，ColumnName：从查找中检索的权重基表列
- NodeName：聚集索引查找，NodeId：17，ColumnName：从查找中检索的名称基表列
- NodeName：Segment，NodeId：13，ColumnName：Segment1010在新组开始时返回 1，否则返回 null。由于没有
Partition By中SUM只有第一排得1 - NodeName：序列项目，NodeId：12，ColumnName：RowNumber1009
row_number()在 Segment1010 标志指示的组内。由于所有行都在同一组中，因此这是从 1 到 6 的升序整数。将用于在rows between 5 preceding and 2 following. （或LEAD以后） - NodeName：Segment，NodeId：11，ColumnName：Segment1011在新组开始时返回 1，否则返回 null。由于没有
Partition By中SUM只有第一排得1（同Segment1010） - NodeName：Window Spool，NodeId：10，ColumnName：WindowCount1012将属于一个窗口框架的行组合在一起的属性。此窗口线轴正在使用“快速跟踪”案例
UNBOUNDED PRECEDING。每个源行发出两行。一种是累积值，一种是详细值。尽管暴露的行没有明显差异，但query_trace_column_values我假设累积列实际上存在。 - NodeName: Stream Aggregate, NodeId: 9, ColumnName: Expr1004
Count(*)按 WindowCount1012 按计划分组但实际上是一个运行计数 - NodeName: Stream Aggregate, NodeId: 9, ColumnName: Expr1005
SUM(weight)按 WindowCount1012 按计划分组，但实际上是权重的运行总和（即cume_weight） - NodeName: Segment, NodeId: 7, ColumnName: Expr1002
CASE WHEN [Expr1004]=(0) THEN NULL ELSE [Expr1005] END- 不知道怎么COUNT(*)会是 0 所以总是运行 sum (cume_weight) - NodeName: Segment, NodeId: 7, ColumnName: Segment1013 No
partition by在LEAD第一行得到 1. 剩下的所有得到 null - NodeName：序列项目，NodeId：6，ColumnName：RowNumber1006
row_number()在 Segment1013 标志指示的组内。由于所有行都在同一组中，这是从 1 到 4 的升序整数 - NodeName: Segment, NodeId: 4, ColumnName: BottomRowNumber1008 RowNumber1006 + 1 因为
LEAD需要下一行 - NodeName: Segment, NodeId: 4, ColumnName: TopRowNumber1007 RowNumber1006 + 1 因为
LEAD需要下一行 - NodeName: Segment, NodeId: 4, ColumnName: Segment1014 No
partition by在LEAD第一行得到 1. 剩下的所有得到 null - NodeName: Window Spool, NodeId: 3, ColumnName: WindowCount1015使用前面的行号将属于一个窗口框架的行组合在一起的属性。窗口框架
LEAD最多有 2 行（当前行和下一行） - NodeName: Stream Aggregate, NodeId: 2, ColumnName: Expr1003
LAST_VALUE([Expr1002])for theLEAD(cume_weight)
出于好奇（因为问题说明了 T-SQL），也可以使用 SQLCLR 有效地解决此问题。
这个想法是一次读取一行，turn直到weight超过 1000（或者我们用完行），然后返回最后一次name读取。
源代码是：
using Microsoft.SqlServer.Server;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
public partial class UserDefinedFunctions
{
[SqlFunction(DataAccess = DataAccessKind.Read,
SystemDataAccess = SystemDataAccessKind.None,
IsDeterministic = true, IsPrecise = true)]
[return: SqlFacet(IsFixedLength = false, IsNullable = true, MaxSize = 255)]
public static SqlString Elevator()
{
const string query =
@"SELECT L.[name], L.[weight]
FROM dbo.line AS L
ORDER BY L.turn;";
using (var con = new SqlConnection("context connection = true"))
{
con.Open();
using (var cmd = new SqlCommand(query, con))
{
var rdr = cmd.ExecuteReader(CommandBehavior.SingleResult);
var name = SqlString.Null;
var total = 0;
while (rdr.Read() && (total += rdr.GetInt32(1)) <= 1000)
{
name = rdr.GetSqlString(0);
}
return name;
}
}
}
}
编译后的程序集和 T-SQL 函数：
CREATE ASSEMBLY Elevator AUTHORIZATION [dbo]
FROM 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WITH PERMISSION_SET = SAFE;
GO
CREATE FUNCTION dbo.Elevator ()
RETURNS nvarchar(255)
AS EXTERNAL NAME Elevator.UserDefinedFunctions.Elevator;
得到结果：
SELECT dbo.Elevator();
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