提高大量数据时SQL语句的Count的查询效率

2024年3月17日 245次阅读 来源: 开罗尔物质

大量数据时where效率较慢，查询语句可改用case

不使用索引时：

select 
(select count(*) from testResult where Result = 'Qualified') ,
(select count(*) from testResult where Result = 'UnQualified'),
(select count(*) from testResult where Result = 'Continue'),
(select count(*) from testResult where Team = 'A'),
(select count(*) from testResult where Team = 'B'),
(select count(*) from testResult where Team = 'A' And Result = 'Qualified'),
(select count(*) from testResult where Team = 'B' And Result = 'Qualified'),
(select count(*) from testResult where Team = 'C' And Result = 'Qualified')

select
count(case when Result ='Qualified' then '1' end),
count(case when Result ='UnQualified' then '1' end),
count(case when Result ='Continue' then '1' end),
count(case when Team ='A' then '1' end),
count(case when Team ='B' then '1' end),
count(case when Team ='A' and Result ='Qualified' then '1' end),
count(case when Team ='B' and Result ='Qualified' then '1' end),
count(case when Team ='C' and Result ='Qualified' then '1' end)
from testResult

后者语句效率明显高于前者，耗时约为四分之一

    原文作者：开罗尔物质
    原文地址: https://blog.csdn.net/qq_37005350/article/details/108823048
    本文转自网络文章，转载此文章仅为分享知识，如有侵权，请联系博主进行删除。
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