TOP 1 或 EXISTS - 哪个更快？

Posted 2023-05-08

【中文标题】TOP 1 或 EXISTS - 哪个更快？

【英文标题】：TOP 1 or EXISTS - Which is faster?

【发布时间】：2014-04-14 13:57:19

【问题描述】：

我在数据库中有一个表，其中包含近 200 万条记录。我需要确定是否有超过 1 条记录可供客户使用。我有以下两个脚本：

SELECT TOP 1 ex.exceptionId
FROM exception AS ex
WHERE ex.customerId='1BF87745-DE00-47FF-9AC0-430BAF87679A'
AND ex.loanId IS NULL
AND ex.statusType = 'required'
AND ex.exceptionState <> 'N'

SELECT (CASE WHEN EXISTS ( SELECT ex.exceptionId FROM exception AS ex
WHERE ex.customerId = '1BF87745-DE00-47FF-9AC0-430BAF87679A'
AND ex.loanId IS NULL
AND ex.statusType = 'required'
AND ex.exceptionState <> 'N') THEN 1 ELSE 0
END) AS CustomerExceptionCount

哪个会更快，为什么..？

非常感谢..！

【问题讨论】：

第一个查询不会回答您关于有多少条记录的问题吗？

这些是用作子查询吗？因为他们看起来都不会告诉你你想要什么。

您是否尝试将查询与服务器查询分析器进行比较？

您已经编写了两个查询。为什么不试试看呢？您可以查看执行计划以找出原因。

这些查询都没有回答more than 1 record 的问题。他们都只回答1 or more records。

【参考方案1】：

你不想要类似的东西，

DECLARE @MoreThanOne Bit;

SELECT @MoreThanOne = CASE WHEN count(*) > 1 THEN 1 ELSE 0 END
FROM
(
    SELECT TOP 2 1
    FROM [exception]
    WHERE [customerId] ='1BF87745-DE00-47FF-9AC0-430BAF87679A'
       AND [loanId] IS NULL
       AND [statusType] = 'required'
       AND [exceptionState] <> 'N'
) [FirstTwo]

【参考方案2】：

尝试运行这些查询，您将获得查询所花费的时间

SET STATISTICS TIME ON
SELECT TOP 1 ex.exceptionId
FROM exception AS ex
WHERE ex.customerId='1BF87745-DE00-47FF-9AC0-430BAF87679A'
AND ex.loanId IS NULL
AND ex.statusType = 'required'
AND ex.exceptionState <> 'N'
SET STATISTICS TIME OFF
GO
SET STATISTICS TIME ON
SELECT (CASE WHEN EXISTS ( SELECT ex.exceptionId FROM exception AS ex
WHERE ex.customerId = '1BF87745-DE00-47FF-9AC0-430BAF87679A'
AND ex.loanId IS NULL
AND ex.statusType = 'required'
AND ex.exceptionState <> 'N') THEN 1 ELSE 0
END) AS CustomerExceptionCount
SET STATISTICS TIME OFF