计算 distinct 和 join over distinct 之间的差异

Question

如何计算不同行的数量减去对这些相同不同行的连接？

如果用户对提交的行没有权限，我需要在触发器之后引发错误。我可以在两个语句中做到这一点，但这似乎效率低下。

DECLARE @AccessibleCount INT =
(
    SELECT
        COUNT(DISTINCT i.[ParentId])
    FROM
        inserted i
    INNER JOIN [SuperSecret].[Parent] AS p ON
        p.[Id] = i.[ParentId] AND
        p.[LockedBy] = @UserId
);

DECLARE @ActualCount INT = (SELECT COUNT(DISTINCT [ParentId]) FROM inserted);
IF (@AccessibleCount <> @ActualCount)
BEGIN
    RAISERROR(...);
    ROLLBACK TRANSACTION;
END

出于性能考虑，我似乎应该对不同的inserted.ParentId 使用子查询来计算这两个计数。我尝试了以下方法，但结果是“无效的对象名称'i'。”

DECLARE @ActualMinusAccessible INT =
(
    SELECT
        COUNT(*)
            -
        (
            SELECT
                COUNT(*)
            FROM
                i
            INNER JOIN [SuperSecret].[Parent] AS p ON
                p.[Id] = i.[ParentId] AND
                p.[LockedBy] = @UserId
        )
    FROM
        (
            SELECT DISTINCT [ParentId] FROM inserted
        ) AS i
);

IF (@ActualMinusAccessible <> 0)
BEGIN
    RAISERROR (...);
    ROLLBACK TRANSACTION;
END

Answer 1

如果没有错，如果[ParentId] 是插入，而[SuperSecret].[Parent] 表中不存在，您想Raise Error。尝试像这样更改您的 SQL 查询。

IF EXISTS (SELECT 1
           FROM   inserted i
           WHERE  NOT EXISTS (SELECT 1
                              FROM   [SuperSecret].[Parent] a
                              WHERE  i.[ParentId] = a.[ParentId] AND a.[LockedBy] = @UserId))
  BEGIN
      RAISERROR (...);
      ROLLBACK TRANSACTION;
  END 

或

IF (SELECT Count(DISTINCT [ParentId]) - (SELECT Count(DISTINCT i.[ParentId])
                                         FROM   inserted i
                                                INNER JOIN [SuperSecret].[Parent] AS p
                                                        ON p.[Id] = i.[ParentId]
                                                           AND p.[LockedBy] = @UserId)
    FROM   inserted) <> 0
  BEGIN
      RAISERROR (...);
      ROLLBACK TRANSACTION;
  END