互斥查询答案

【问题标题】：Mutually Exclusive Queries互斥查询
【发布时间】：2017-09-15 22:27:58
【问题描述】：

问题

您如何将产生互斥结果的 IF...ELSE 分配到产生相同结果的查询中？

用例

作为负责人（当前登录的用户），我可以获得组织中的用户列表。如果我被分配到任何部门，则用户列表是分配给相同部门的较小集合。

示例

DECLARE @organizationId int = 421;
DECLARE @userId int = 1138;

IF EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)
  -- Get user IDs in the same department(s) as the principal.
  SELECT du.userId
  FROM dbo.DepartmentsUsers AS du
  JOIN (
        -- Get department IDs assigned to the principal.
        SELECT departmentId
        FROM dbo.DepartmentsUsers
        WHERE organizationId = @organizationId
          AND userId = @userId) AS j
    ON du.departmentId = j.departmentId
  WHERE du.organizationId = @organizationId
ELSE
  -- Get all user IDs in the organization.
  SELECT userId
  FROM dbo.OrganizationsUsers
  WHERE organizationId = @organizationId

【问题讨论】：

似乎解决方案是某种 LEFT JOIN，如果校长被分配到任何部门，则有条件地强制为 INNER JOIN。

标签： sql-server tsql

【解决方案1】：

使用在每个 where 子句中添加“if”条件的联合：

SELECT du.userId
FROM dbo.DepartmentsUsers AS du
JOIN (  SELECT departmentId
        FROM dbo.DepartmentsUsers
        WHERE organizationId = @organizationId
          AND userId = @userId) AS j
    ON du.departmentId = j.departmentId
WHERE du.organizationId = @organizationId
AND EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)
UNION
SELECT userId
FROM dbo.OrganizationsUsers
WHERE organizationId = @organizationId
AND NOT EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)

【讨论】：

【解决方案2】：

 SELECT du.userId as UserId,
            'Department' as UserType // remove this line unnecessary
      FROM dbo.DepartmentsUsers AS du
      JOIN (
            -- Get department IDs assigned to the principal.
            SELECT departmentId
            FROM dbo.DepartmentsUsers
            WHERE organizationId = @organizationId
              AND userId = @userId) AS j
        ON du.departmentId = j.departmentId
      WHERE du.organizationId = @organizationId

      UNION ALL

      -- Get all user IDs in the organization.
      SELECT userId as UserId,
             'Organization' as UserType // remove this line unnecessary
      FROM dbo.OrganizationsUsers
      WHERE organizationId = @organizationId

【讨论】：

只需要其中一个结果集。不是两者结合。商业案例是相互排斥的。