在SQL Server中删除具有默认约束的列(IF EXISTS)

Question

我正在编写一个用于删除列和默认约束的sql脚本.以下脚本工作正常,但我想知道它是否是一种正确的方法.

我可以在一个语句中删除带有列的默认约束,而不是使用两个单独的约束吗？

IF  EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DF_Employees_EmpID]') AND type = 'D')

BEGIN    
   ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID]    
END    
GO    
BEGIN   
  ALTER TABLE [dbo].[Employees] DROP COLUMN [EmpID]    
END

Answer 1

在SQL Server 2005中,您可以在一个语句中删除约束和列.

语法是

ALTER TABLE [ database_name . [ schema_name ] . | schema_name . ] table_name 
DROP { [ CONSTRAINT ] constraint_name | COLUMN column } [ ,...n ]

重点是[,... n],表示多个术语.

NB!由于这些术语是按顺序处理的,如果要删除的列是要删除的约束的一部分,则约束必须是第一个术语,后跟列术语.

在你的例子中:

ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID], COLUMN [EmpID]

所以你的代码是:

IF  EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DF_Employees_EmpID]') AND type = 'D')
BEGIN
   ALTER TABLE [dbo].[Employees] DROP CONSTRAINT [DF_Employees_EmpID], COLUMN [EmpID]
END
GO

在SQL Server 2016中,他们引入了IF EXISTS子句,无需先检查是否存在约束,例如

ALTER TABLE [dbo].[Employees] DROP CONSTRAINT IF EXISTS [DF_Employees_EmpID], COLUMN IF EXISTS [EmpID]

Answer 2

下面是另一种删除列和默认约束的方法,在删除它们之前检查它们是否存在:

-------------------------------------------------------------------------
-- Drop COLUMN 
-- Name of Column: Column_EmployeeName 
-- Name of Table: table_Emplyee 
--------------------------------------------------------------------------
IF EXISTS (
            SELECT 1
            FROM INFORMATION_SCHEMA.COLUMNS
            WHERE TABLE_NAME = 'table_Emplyee'
              AND COLUMN_NAME = 'Column_EmployeeName'
           )
    BEGIN 

        IF EXISTS ( SELECT 1 
                    FROM sys.default_constraints 
                    WHERE object_id = OBJECT_ID('[dbo].[DF_table_Emplyee_Column_EmployeeName]') 
                      AND parent_object_id = OBJECT_ID('[dbo].[table_Emplyee]')
                  )
            BEGIN 
                ------  DROP Contraint

                ALTER TABLE [dbo].[table_Emplyee] DROP CONSTRAINT [DF_table_Emplyee_Column_EmployeeName]
            PRINT '[DF_table_Emplyee_Column_EmployeeName] was dropped'  
            END
     --    -----   DROP Column   -----------------------------------------------------------------    
        ALTER TABLE [dbo].table_Emplyee 
            DROP COLUMN Column_EmployeeName
        PRINT 'Column Column_EmployeeName in images table was dropped'    
    END  

--------------------------------------------------------------------------
-- ADD  COLUMN Column_EmployeeName IN table_Emplyee table
--------------------------------------------------------------------------
IF NOT EXISTS (
                SELECT 1
                FROM INFORMATION_SCHEMA.COLUMNS
                WHERE TABLE_NAME = 'table_Emplyee'
                  AND COLUMN_NAME = 'Column_EmployeeName'
               )
    BEGIN  
    ----- ADD Column & Contraint               
        ALTER TABLE dbo.table_Emplyee 
            ADD Column_EmployeeName BIT   NOT NULL 
            CONSTRAINT [DF_table_Emplyee_Column_EmployeeName]  DEFAULT (0) 
        PRINT 'Column [DF_table_Emplyee_Column_EmployeeName] in table_Emplyee table was Added' 
        PRINT 'Contraint [DF_table_Emplyee_Column_EmployeeName] was Added'      
     END

GO 

Answer 3

你怎么拥有它都很好。

另一种选择是

IF OBJECT_ID('DF_Employees_EmpID', 'D') IS NULL
  BEGIN
      ALTER TABLE dbo.Employees
        DROP COLUMN EmpID
  END
ELSE
  BEGIN
      ALTER TABLE dbo.Employees
        DROP CONSTRAINT DF_Employees_EmpID, 
                 COLUMN EmpID 
  END 

如果约束确实存在，则这会将两个操作合并为单个语句/事务。