如何限制表中的行数？如何在顶部插入行？ SQL 服务器

Question

首先 - 我想限制我在数据库中插入的行。 IE。我想在我的表中最多有 100 行（我不想有版本计数（*），如果 >100，删除...）

次要的 - 我想要将（插入时）推入表格之类的东西。即：

ID |  Name    |      City
 0 |   Mike   |    New York


ID |  Name    |      City
1  |   David  |      Pekin   -> as insert on the top!
0  |   Mike   |    New York


ID |  Name    |      City
2  |   Marcus |     Warsaw  -> as next
1  |   David  |      Pekin   
0  |   Mike   |    New York

最好的问候 大卫

Answer 1

您的两个条件都可以用于 100 行表。或者，甚至是 1,000 行的表。但是，下面的示例不是生产表的推荐操作。

RE：“最多有 100 行” 创建表，然后添加一个 INSTEAD OF INSERT 触发器，在允许插入之前检查表的大小。

RE：“类似于将（插入时）推入表中。” 使用聚集主键（推荐）创建表，但使用 DESC 键（不推荐用于除了小的、很少使用的表之外的任何东西）。 CLUSTERED 索引是物理排序的。因此，添加下一个更大的数字（如在 IDENTITY 序列中）会将新条目推到索引的顶部。 SELECT 将按磁盘顺序返回行。对于具有聚集索引的 100 行表而言，磁盘顺序和索引顺序是相同的。

代码：
注意：下面的代码将 max_size 限制为 3 以进行演示。为您的示例设置 @max_size = 100

-->-- create table with descending order for primary key.  will act like a push down stack
IF OBJECT_ID('dbo.so_limit_size') IS NOT NULL DROP TABLE dbo.so_limit_size
CREATE TABLE dbo.so_limit_size ( 
    id TINYINT IDENTITY(1,1)
    , name VARCHAR(100) 
    , city VARCHAR(100)
    , CONSTRAINT pk_so_limit_size PRIMARY KEY CLUSTERED (id DESC) -- DESC makes it work like a push down stack
    )

INSERT INTO dbo.so_limit_size (name, city)
VALUES
( 'Mike', 'New York City' )
, ( 'David', 'Pekin' )
, ( 'Marcus', 'Warsaw' )


SELECT * FROM dbo.so_limit_size  -- 3 rows

GO
CREATE TRIGGER dbo.limit_size ON dbo.so_limit_size
INSTEAD OF INSERT AS
    SET NOCOUNT ON
    -- purpose:  limit size of table to @max_size.  insert batch of 1 or more that exceeds @max_size will not be allowed

    DECLARE @max_size TINYINT = 3  -- size limit of table

    DECLARE @existing_count TINYINT = (SELECT COUNT(*) FROM dbo.so_limit_size)
        , @insert_count TINYINT = (SELECT COUNT(*) FROM Inserted )
    PRINT 'existing_count = ' + LOWER(@existing_count) + ' new insert count = ' + LOWER(@insert_count)

    IF @existing_count + @insert_count >= 3
    BEGIN
        PRINT 'insert will cause table count to exceed max_size.  insert aborted.  max_size = ' + LOWER(@max_size) 
    END     
    ELSE
    BEGIN
        PRINT 'table count less than max_size.  insert allowed.  max_size = ' + LOWER(@max_size)  --<<-- demonstration, print is not a recommended practice for a trigger
        INSERT INTO dbo.so_limit_size (name, city)
        SELECT name, city FROM inserted
    END
GO
INSERT INTO dbo.so_limit_size (name, city)
VALUES
( 'Zorba', 'Athens' ) -- will not be allowed if @max_size = 3


SELECT * FROM dbo.so_limit_size

Answer 2

@Hans Kesting 指出，他对 100 行限制的解释“应该推出最旧的”。这是一个有效的解释。所以，我创建了一个答案。以下与我之前的答案相同，只是触发器现在删除插入之前最旧的行。表最大大小保持不变。有关使用降序键而不是触发器的聚集索引的解释，请参阅我的其他答案。

代码：

-->-- create table with descending order for primary key.  will act like a push down stack
IF OBJECT_ID('dbo.so_limit_size') IS NOT NULL DROP TABLE dbo.so_limit_size
CREATE TABLE dbo.so_limit_size ( 
    id TINYINT IDENTITY(1,1)
    , name VARCHAR(100) 
    , city VARCHAR(100)
    , CONSTRAINT pk_so_limit_size PRIMARY KEY CLUSTERED (id DESC) -- DESC makes it work like a push down stack
    )

INSERT INTO dbo.so_limit_size (name, city)
VALUES
( 'Mike', 'New York City' )
, ( 'David', 'Pekin' )
, ( 'Marcus', 'Warsaw' )


SELECT * FROM dbo.so_limit_size  -- 3 rows

GO
CREATE TRIGGER dbo.limit_size ON dbo.so_limit_size
INSTEAD OF INSERT AS
    SET NOCOUNT ON
    -- purpose:  limit size of table to @max_size.  insert batch of 1 or more that exceeds @max_size will not be allowed

    DECLARE @max_size TINYINT = 3  -- size limit of table

    DECLARE @existing_count TINYINT = (SELECT COUNT(*) FROM dbo.so_limit_size)
        , @insert_count TINYINT = (SELECT COUNT(*) FROM Inserted )
    PRINT 'existing_count = ' + LOWER(@existing_count) + ' new insert count = ' + LOWER(@insert_count)

    IF @existing_count + @insert_count >= 3
    BEGIN
        -- for FIFO stack, delete oldest rows before insert if @max_size exceeded
        PRINT 'insert will cause table count to exceed max_size.  oldest row(s) deleted before insert.  max_size = ' + LOWER(@max_size) 
        DELETE FROM dbo.so_limit_size
        WHERE Id IN ( SELECT TOP (@insert_count) Id FROM dbo.so_limit_size ORDER BY Id ASC )
        
        INSERT INTO dbo.so_limit_size (name, city)
        SELECT name, city FROM inserted

    END     
    ELSE
    BEGIN
        PRINT 'table count less than max_size.  insert allowed.  max_size = ' + LOWER(@max_size)  --<<-- demonstration, print is not a recommended practice for a trigger
        INSERT INTO dbo.so_limit_size (name, city)
        SELECT name, city FROM inserted
    END
GO
INSERT INTO dbo.so_limit_size (name, city)
VALUES
( 'Zorba', 'Athens' ) -- will not be allowed if @max_size = 3
, ('Sriram', 'Chennai')


SELECT * FROM dbo.so_limit_size