SQL——快速计算所有表的方法答案

【问题标题】：TSQL -- quick way to get a count across all tablesSQL——快速计算所有表的方法
【发布时间】：2019-06-25 00:17:12
【问题描述】：

如果数据库d1 的表T1,T2,T3,T4 都带有"Date1" 字段。

获取所有表中日期早于 3 天的所有记录的最佳方法是什么？

我知道可以做联合，我认为没有漂亮的语法可以省略所有表 [就像 C++ 中的“父”对象]。

这里最好可能意味着更高效，或者只是 T-SQL 中令人愉悦的语法。

这适用于 SSMS 17.7。微软 SQL Server 2014 (SP2)

【问题讨论】：

使用适当的软件（MySQL、Oracle、DB2...）和版本标记数据库问题很有帮助，例如sql-server-2014。语法和功能的差异通常会影响答案。请注意，tsql 缩小了选择范围，但没有指定数据库。
根据请求，更新为 17.7 -- 这是我产品的“帮助-->关于”。多年来，微软在其所有产品上都使用了该功能，但现在某些 Microsoft 产品已缺少此功能！
SQL Server Management Studio (SSMS) 不是数据库，但它可以连接到一个（或多个）。如果您使用的是 SQL Server，您可以使用 select @@Version; 获取数据库软件的版本。

标签： tsql count sql-server-2014 ssms-17

【解决方案1】：

如果你事先知道表名，一个简单的 union all 查询可能是最简单的方法：

SELECT COUNT(*)
FROM
(
     SELECT Date1
     FROM T1
     UNION ALL
     SELECT Date1
     FROM T2
     SELECT Date1
     FROM T3
     SELECT Date1
     FROM T4
) As t
WHERE Date1 <= DATEADD(DAY, -3, GETDATE())

如果事先不知道表名，可以使用information_schema.columns动态构建联合查询。

【讨论】：

【解决方案2】：

没有联合？

既然没有GROUP BY 的COUNT 返回1 个值，为什么不使用一次CROSS JOIN？

SELECT 
 t1.Cnt AS [T1], 
 t2.Cnt AS [T2], 
 t3.Cnt AS [T3],
 t4.Cnt AS [T4],
 (t1.Cnt + t2.Cnt + t3.Cnt + t4.Cnt) AS [T1234]
FROM 
(SELECT COUNT(*) AS Cnt FROM T1 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t1
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM T2 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t2
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM T3 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t3
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM T4 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t4

或CROSS APPLY

SELECT 
 t1.Cnt AS [T1], 
 t2.Cnt AS [T2], 
 t3.Cnt AS [T3],
 t4.Cnt AS [T4],
 (t1.Cnt + t2.Cnt + t3.Cnt + t4.Cnt) AS [T1234]
FROM (SELECT CAST(GetDate()-3 AS DATE) as Dt) d
CROSS APPLY (SELECT COUNT(*) AS Cnt FROM T1 WHERE [Date1] < d.Dt) AS t1
CROSS APPLY (SELECT COUNT(*) AS Cnt FROM T2 WHERE [Date1] < d.Dt) AS t2
CROSS APPLY (SELECT COUNT(*) AS Cnt FROM T3 WHERE [Date1] < d.Dt) AS t3
CROSS APPLY (SELECT COUNT(*) AS Cnt FROM T4 WHERE [Date1] < d.Dt) AS t4

Sql Server 的示例 sn-p：

declare @T1 table (id int primary key identity(1,1), [Date1] date);
declare @T2 table (id int primary key identity(1,1), [Date1] date);
declare @T3 table (id int primary key identity(1,1), [Date1] date);
declare @T4 table (id int primary key identity(1,1), [Date1] date);

insert into @T1 ([Date1]) values (getdate()-6),(getdate()-5),(getdate()-4),(getdate()-3),(getdate()-2),(getdate()-1),(getdate()-0);
insert into @T2 ([Date1]) select top 6 [Date1] from @T1 order by [Date1] desc;
insert into @T3 ([Date1]) select top 5 [Date1] from @T1 order by [Date1] desc;
insert into @T4 ([Date1]) select top 4 [Date1] from @T1 order by [Date1] desc;

SELECT 
 t1.Cnt AS [T1], 
 t2.Cnt AS [T2], 
 t3.Cnt AS [T3],
 t4.Cnt AS [T4],
 (t1.Cnt + t2.Cnt + t3.Cnt + t4.Cnt) AS [T1234]
FROM 
(SELECT COUNT(*) AS Cnt FROM @T1 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t1
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM @T2 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t2
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM @T3 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t3
CROSS JOIN
(SELECT COUNT(*) AS Cnt FROM @T4 WHERE [Date1] < CAST(GetDate()-3 AS DATE)) AS t4

返回：

T1  T2  T3  T4  T1234
3   2   1   0   6

【讨论】：

【解决方案3】：

嗯，你对一个父对象感兴趣，那就是一个视图。您可以将它重用于各种查询。或者，如果需要，添加更多列：

CREATE VIEW parent AS
SELECT Date1 FROM t1 UNION ALL
SELECT Date1 FROM t2 UNION ALL
SELECT Date1 FROM t3 UNION ALL
SELECT Date1 FROM t4;

现在，可以按照你想要的方式查询了

SELECT COUNT(*) FROM parent WHERE Date1 <= DATEADD(DAY, -3, GETDATE())

【讨论】：

【解决方案4】：

您可以使用 CTE（通用表表达式）来代替创建视图。它像视图一样工作，但不会保留在数据库上。请试一试：

WITH CteDate( Date1 )
AS ( SELECT Date1 FROM t1 UNION ALL
     SELECT Date1 FROM t2 UNION ALL
     SELECT Date1 FROM t3 UNION ALL
     SELECT Date1 FROM t4
    )
SELECT COUNT(*) FROM CteDate WHERE Date1 <= DATEADD(DAY, -3, GETDATE())

它适用于所有大于或等于 2005 的 SQL Server。

【讨论】：