更改列默认值

Question

我知道您可以更改现有列的默认值,如下所示:

ALTER TABLE Employee ADD CONSTRAINT DF_SomeName DEFAULT N'SANDNES' FOR CityBorn;

但根据这个我的查询应该工作:

ALTER TABLE MyTable ALTER COLUMN CreateDate DATETIME NOT NULL 
      CONSTRAINT DF_Constraint DEFAULT GetDate()

所以我在这里尝试使我的列不为Null并设置默认值.但在CONSTRAINT附近获取Incoorect语法错误.我错过了吗？

Answer 1

我觉得这里的问题是与之间的混乱Create Table和Alter Table命令.如果我们看一下Create table那么我们可以同时添加默认值和默认约束:

<column_definition> ::= 
column_name <data_type>
    [ FILESTREAM ]
    [ COLLATE collation_name ] 
    [ SPARSE ]
    [ NULL | NOT NULL ]
    [ 
        [ CONSTRAINT constraint_name ] DEFAULT constant_expression ] 
      | [ IDENTITY [ ( seed,increment ) ] [ NOT FOR REPLICATION ] 
    ]
    [ ROWGUIDCOL ]
    [ <column_constraint> [ ...n ] ] 
    [ <column_index> ]
 ex: 
CREATE TABLE dbo.Employee 
(
     CreateDate datetime NOT NULL 
     CONSTRAINT DF_Constraint DEFAULT (getdate())
) 
ON PRIMARY;

你可以在这里检查完整的定义:http: //msdn.microsoft.com/en-IN/library/ms174979.aspx

但是如果我们看一下Alter Table定义那么ALTER TABLE ALTER COLUMN你就无法添加 CONSTRAINT可用的选项ADD:

 | ADD 
    { 
        <column_definition>
      | <computed_column_definition>
      | <table_constraint> 
      | <column_set_definition> 
    } [ ,...n ]

请点击此处:http://msdn.microsoft.com/en-in/library/ms190273.aspx

因此,您必须为Altering列编写两个不同的语句:

ALTER TABLE MyTable ALTER COLUMN CreateDate DATETIME NOT NULL;

另一个用于更改表并添加默认约束

ALTER TABLE MyTable ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR CreateDate;

希望这可以帮助!!!

Answer 2

要更改现有列,您需要:

ALTER TABLE MyTable ALTER COLUMN CreateDate DATETIME NOT NULL;
ALTER TABLE MyTable ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR CreateDate;

Answer 3

没有直接的方法可以更改SQL Server中列的默认值，但是以下参数化脚本可以完成此工作：

DECLARE @table nvarchar(100)
DECLARE @column nvarchar(100)
DECLARE @newDefault nvarchar(100)
SET @table='TableName'
SET @column='ColumnName'
SET @newDefault='0'

IF EXISTS (select name from sys.default_constraints 
    where parent_object_id = object_id(@table) 
    and parent_column_id = columnproperty(object_id(@table), @column, 'ColumnId'))
BEGIN
    DECLARE @constraintName as nvarchar(200)
    DECLARE @constraintQuery as nvarchar(2000)

    SELECT @constraintName = name from sys.default_constraints 
        where parent_object_id = object_id(@table) and parent_column_id = columnproperty(object_id(@table),@column, 'ColumnId')

    SET @constraintQuery = 'ALTER TABLE '+@table+' DROP CONSTRAINT '+@constraintName +'; ALTER TABLE '+ @table 
        + ' ADD CONSTRAINT ' + @constraintName +' DEFAULT '+@newDefault+' FOR '+@column 

    EXECUTE sp_executesql @constraintQuery  
END 

只需填写参数并执行即可。该脚本将删除现有约束，并使用指定的默认值创建一个新约束。

Answer 4

它应该是 -

ALTER TABLE MyTable
ALTER COLUMN CreateDate DATETIME NOT NULL;

ALTER TABLE MyTable 
ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR CreateDate;