需求：
MSSQL，列出服務實例中全部數據庫的例如以下信息： 數據庫ID、數據庫名、創建日期、數據文件類型、數據文件大小、數據庫總大小、文件所在路徑。
寫法(後面的百分比為所花時間占比)：
-- 连接子查询 (47%)WITH cte_TotalSize AS( SELECT database_id ,CAST(SUM(size) AS FLOAT)/128 AS [TotalSize(MB)] FROM sys.master_files GROUP BY database_id)SELECT a.database_id AS [DB_ID] ,b.name AS [DB_Name] ,b.create_date ,CASE a.type WHEN 1 THEN 'Log' ELSE 'Data' END AS File_Type ,CAST(a.size AS FLOAT)/128 AS [Size(MB)] -- Size以页（8 KB）为单位 ,c.[TotalSize(MB)] ,a.physical_Name AS File_PathFROM sys.master_files aINNER JOIN sys.databases b ON a.database_id = b.database_idINNER JOIN cte_TotalSize c ON a.database_id = c.database_id-- 标量子查询 (34%)SELECT a.database_id AS [DB_ID] ,b.name AS [DB_Name] ,b.create_date ,CASE a.type WHEN 1 THEN 'Log' ELSE 'Data' END AS File_Type ,CAST(a.size AS FLOAT)/128 AS [Size(MB)] -- Size以页（8 KB）为单位 ,(SELECT CAST(SUM(size) AS FLOAT)/128 FROM sys.master_files WHERE database_id = a.database_id) AS [TotalSize(MB)] ,a.physical_Name AS File_Path FROM sys.master_files aINNER JOIN sys.databases b ON a.database_id = b.database_id-- 窗体函数 (19%)SELECT a.database_id AS [DB_ID] ,b.name AS [DB_Name] ,b.create_date ,CASE a.type WHEN 1 THEN 'Log' ELSE 'Data' END AS File_Type ,CAST(a.size AS FLOAT)/128 AS [Size(MB)] -- Size以页（8 KB）为单位 ,CAST(SUM(size) OVER(PARTITION BY a.database_id) AS FLOAT)/128 AS [TotalSize(MB)] ,a.physical_Name AS File_Path FROM sys.master_files aINNER JOIN sys.databases b ON a.database_id = b.database_id
結果：
当中第三種是使用了窗体函數，相關文檔：