EXISTS/NOT EXISTS CASE WHEN等使用方法

最新推荐文章于 2021-11-27 01:11:11 发布

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最新推荐文章于 2021-11-27 01:11:11 发布

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原文链接：http://www.cnblogs.com/taiyonghai/p/5806260.html

本文通过具体实例介绍了SQL中EXISTS、NOT EXISTS及CASE WHEN等语句的使用技巧，包括如何用EXISTS替代IN操作符进行高效查询，以及CASE WHEN在不同场景下的应用。

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--简单判断用法
SELECT 1 WHERE EXISTS (SELECT * FROM cpay..System_Setting)
--可以替换count
SELECT COUNT(1) FROM cpay..System_Setting

USE YSB_PayDB
--1 练习NOT EXISTS的用法
--（Exists使用心得：1、先查目标表的全部；2、再用Exists关联目标表过滤掉不要的）
SELECT c.* FROM dbo.YSB_Distributor AS d 
INNER JOIN dbo.YSB_CustomerInfo AS c
ON d.DistributorID = c.DistributorID
WHERE d.DistributorName='tests'
AND NOT EXISTS (
SELECT 1 FROM dbo.YSB_DistributorCustomerServiceFee AS s 
WHERE c.CustomerNo = s.CustomerNo
)

--2 练习NOT EXISTS
SELECT * FROM dbo.YSB_PayProductInfo AS p
WHERE NOT EXISTS (
SELECT 1 FROM dbo.YSB_PayProductSupportBusProduct AS s
WHERE p.PayProductCode = s.PayProductCode AND s.BusProductCode = 'UATP'
)

--3 练习NOT EXISTS的用法
SELECT * FROM dbo.Sys_Company
SELECT * FROM dbo.YSB_CustomerInfo
SELECT * FROM dbo.YSB_CustomerAndFactoringCompanyRelation

SELECT * FROM dbo.YSB_CustomerInfo AS c
WHERE NOT EXISTS  (
SELECT 1 FROM dbo.YSB_CustomerAndFactoringCompanyRelation AS f
INNER JOIN dbo.Sys_Company AS s
ON f.FactoringCompanyID = s.CompanyID
WHERE s.CompanyName = '北京海淀科技企业融资担保有限公司' AND c.CustomerNo = f.CustomerNo
)

--4 练习 CASE WHEN的用法
USE MySchool
SELECT * FROM dbo.tblScore
SELECT * FROM dbo.tblCourse
SELECT * FROM dbo.tblStudent
SELECT * FROM dbo.tblTeacher

SELECT st.StuId,st.StuName,co.CourseName,sc.Score ,
CASE 
    WHEN sc.Score >= 90 THEN '优秀'
    WHEN sc.Score >= 80 THEN '良好'
    WHEN sc.Score >= 60 THEN '及格'
    WHEN sc.Score < 60 THEN '不及格'
    ELSE '其他'
END AS '成绩',
CASE st.StuSex
    WHEN '男' THEN '男生'
    WHEN '女' THEN '女生'
    ELSE '其他'
END AS 性别
FROM dbo.tblStudent AS st
INNER JOIN dbo.tblScore AS sc
ON st.StuId = sc.StuId
INNER JOIN dbo.tblCourse AS  co
ON co.CourseId = sc.CourseId

--5 练习CASW WHEN GROUP BY的用法
--（此写法类似Switch...Case...）
SELECT b.BusProductCode, b.BusProductName,
CASE b.BusProductType
    WHEN 1 THEN '机票类'
    WHEN 2 THEN '平台类'
    WHEN 3 THEN '票务类'
END
FROM YSB_PayDB.dbo.YSB_BusProductInfo AS b
--（此写法类似If...Else...）
SELECT b.BusProductCode, b.BusProductName,
CASE 
    WHEN b.BusProductType = 1 THEN '机票类'
    WHEN b.BusProductType = 2 THEN '平台类'
    WHEN b.BusProductType = 3 THEN '票务类'
END
FROM YSB_PayDB.dbo.YSB_BusProductInfo AS b

SELECT 
CASE BusProductType
    WHEN 1 THEN '机票类'
    WHEN 2 THEN '平台类'
    WHEN 3 THEN '票务类'
END AS 业务类型,
COUNT(1) AS 产品数量
FROM YSB_PayDB.dbo.YSB_BusProductInfo
GROUP BY BusProductType