同事说永远不要在 SQL 中使用 OR 语句，这是真的吗？

Question

我现在负责 SQL 开发的同事说我永远不应该使用OR语句，因为它会弄乱查询优化器并忽略产生慢查询的表索引。我在谷歌搜索时找不到任何这样的例子。以下查询的替代方案变得非常难看，其中有十几个代码块看起来几乎相同（对于示例）使用 if else 语句为每个变量状态。请注意检查为短路的变量，如果值为 2，则返回所有结果，否则按字段过滤。

我询问了一些包含这些关于为什么不使用OR语句的声明的资源，并收到了以下链接（我们使用的是 MS SQL Server）。

• /sf/ask/394779731/
• https://bertwagner.com/2018/02/20/or-vs-union-all-is-one-better-for-performance/
• http://sqlserverplanet.com/optimization/using-union-instead-of-or

这些示例似乎都不像当前的实现，如下所示。我发现很难相信这段代码有问题，但如果有问题，请告诉我。我还想了解更多信息，其中关于不使用的评论OR实际上可能适用以及为什么如此，以便更好地理解该问题。

SELECT
    e.EmployeeName,
    e.DepartmentName,
    crs.Title,
    c.Name as CompanyName
FROM Employee E
Left Outer Join Company c ON c.Id = @companyId
    INNER JOIN Department d on e.DepartmentId = d.Id 
WHERE   
    c.Id = @companyId           
    AND (@Active = 2 OR  crs.IsActive = @Active)
    AND (@Dot = 2 OR IsDot = @Dot)
    AND crs.CompanyId = @companyId
    AND d.CompanyId = @companyId
ORDER BY EmployeeName, Title, PassedDate

我相信复制代码总是不好的，除非有很好的理由。测试查询后，我确认使用了正确的索引。提到这一点后，我被告知他会使用最佳实践。我还没有看到任何告诉我不要使用OR. 任何人都可以引导我到这些吗？

---

这是我很久以前更新之前的可憎之处。如果您发现@Department并对此感到疑惑，这不是错误。Telerik 报告组件正在对这段代码做一些事情，并在它到达服务器之前在后台扩展一个数组。

IF @Active = 2
BEGIN
--ACTIVE AND INACTIVE
    IF 0 IN (@Department)
    BEGIN
        IF @DOT = 1 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 1 

            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 0 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,           
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    
            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 2 
            BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,           
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D       ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 0 
                    
            ORDER BY EmployeeName, Title, PassedDate
        END 
    END
    ELSE
    BEGIN
        IF @DOT = 1 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 1
                    AND A.DepartmentId IN (@Department)

            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 0 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND A.DepartmentId IN (@Department)
                    
            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 2 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE   A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 0 
                    AND A.DepartmentId IN (@Department)
                    
            ORDER BY EmployeeName, Title, PassedDate
        END 
    END
END
ELSE
BEGIN
--ACTIVE OR INACTIVE
    IF 0 IN (@Department)
    BEGIN
        IF @DOT = 1 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE  A.IsActive = @Active 
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 1 

            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 0 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE  A.IsActive = @Active 
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    
            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 2 
            BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D       ON @companyId = D.Id

            WHERE  A.IsActive = @Active 
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 0 
                    
            ORDER BY EmployeeName, Title, PassedDate
        END 
    END
    ELSE
    BEGIN
        IF @DOT = 1 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE  A.IsActive = @Active
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 1
                    AND A.DepartmentId IN (@Department)

            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 0 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE  A.IsActive = @Active 
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND A.DepartmentId IN (@Department)
                    
            ORDER BY EmployeeName, Title, PassedDate
        END

        IF @DOT = 2 
        BEGIN
            Select 
            A.LastName + ', ' + A.FirstName as EmployeeName,
            A.DepartmentName,
            C.Title,
            ISNULL(B.Comments, ' ') as Remarks,
            CONVERT(varchar, B.PassedDate, 101) as DateOut,
            D.Name as CompanyName

            FROM Employee A
            Left Outer Join EmployeeCourse B    ON A.Id = B.EmployeeId
            Left Outer Join CompanyCourse C     ON B.CompanyCourseId = C.Id
            Left Outer Join Company D           ON @companyId = D.Id

            WHERE  A.IsActive = @Active 
                    AND A.CompanyId = @companyId 
                    AND B.PassedDate IS Not NULL 
                    AND C.DotCourse = 0 
                    AND A.DepartmentId IN (@Department)
                    
            ORDER BY EmployeeName, Title, PassedDate
        END 
    END
END

注意：为了简化，我在初始代码示例中删除了一些。

根据他的解释、链接和我的搜索，我发现很难相信我当前的解决方案在性能和可读性方面不是最好的。我承认在某些情况下可能存在性能下降的风险，但我非常怀疑我的实施情况是否如此。OR当我没有看到任何说明这一点的文档时，我不喜欢永远不应该使用的想法。

当我显示查询时，我被告知永远不要使用OR. 我现在正在做功课，看看这是否是有效信息。我真的很讨厌使用以前的代码，因为它在很多方面都是错误的。

Answer 1

永远不要听任何人说你永远不应该做 X。

一般来说，如果没有一些很好的理由，你不应该试图超越查询优化器。

确实，在某些情况下，太多的ORs 会导致不理想的¹计划，但是您应该单独考虑每个这样的情况，并且只有在原始查询性能不可接受的情况下才寻找解决方法。

如果您确实需要解决您发布的查询的性能问题，请考虑提出不同的问题。

1 - 从人的角度来看。该计划实际上将是该特定查询变体的最佳选择（在优化器功能的范围内）；我的意思是重写查询可能会产生一个执行速度更快或消耗更少资源的不同计划。

Answer 2

并不是你永远不应该在 where 子句中使用 OR，而是你在这种情况下遵循的特定模式是一个非常糟糕的模式。我已经写并记录了这个主题：

• 可选参数和缺失索引请求
• SQL Server 性能品尝菜单：可选参数

此外，您选择替换它的模式不会按照您的想法工作。看这里：

• SQL Server--如果存储过程和计划缓存中的逻辑
• 不是很强大的 IF 分支

用 or 子句连接也很困难：

• 与 OR 子句连接 =

在不同的情况下，例如不使用可选参数，只要您有索引来支持谓词，就可以使用 OR 来损害很小。我并不是说它总是最好的，但它是可行的。

很多时候，通过使用 UNION ALL 替换 OR，您会发现自己处于更好的位置，但在特定情况下，您最好使用动态 SQL 来构建适当的查询并执行它。我链接到的视频中介绍了该技术。