如果不是EXISTS似乎不起作用

I have the following script:

我有以下脚本:

BEGIN
    IF NOT EXISTS (SELECT SessionID FROM SessionData WHERE SessionID = @SessionID)
    BEGIN
    SELECT @RegionID = RegionID
    FROM Region
    WHERE Domain = @Domain
    INSERT INTO SessionData (
    SessionID,
    SystemID,
    RegionID,
    RegionDomain,
    RemoteAddr,
    CreatePage)
    VALUES (
    @SessionID,
    @SystemID,
    @RegionID,
    @RegionDomain,
    @RemoteAddr,
    @CreatePage)
END
END
BEGIN