Answers:
据我了解，默认值约束不是ISO标准的一部分，因此它们不会出现在INFORMATION_SCHEMA中。INFORMATION_SCHEMA似乎是此类任务的最佳选择，因为它是跨平台的，但是如果信息不可用，则应使用对象目录视图（sys。*）而不是系统表视图（在SQL Server中已弃用） 2005年及以后。
以下与@ user186476的答案几乎相同。它返回给定列的默认值约束的名称。（对于非SQL Server用户，您需要使用默认名称才能删除它，如果您自己没有命名默认约束，SQL Server会创建一些疯狂的名称，例如“ DF_TableN_Colum_95AFE4B5”。为了使更改更容易您的架构，请务必明确命名约束！）
-- returns name of a column's default value constraint
SELECT
default_constraints.name
FROM
sys.all_columns
INNER JOIN
sys.tables
ON all_columns.object_id = tables.object_id
INNER JOIN
sys.schemas
ON tables.schema_id = schemas.schema_id
INNER JOIN
sys.default_constraints
ON all_columns.default_object_id = default_constraints.object_id
WHERE
schemas.name = 'dbo'
AND tables.name = 'tablename'
AND all_columns.name = 'columnname'您可以通过指定默认约束与之关联的表名和列名，使用以下内容进一步缩小结果范围：
select * from sysobjects o
inner join syscolumns c
on o.id = c.cdefault
inner join sysobjects t
on c.id = t.id
where o.xtype = 'D'
and c.name = 'Column_Name'
and t.name = 'Table_Name'下面的脚本列出了运行它的数据库中所有默认约束和用户表的默认值：
SELECT
b.name AS TABLE_NAME,
d.name AS COLUMN_NAME,
a.name AS CONSTRAINT_NAME,
c.text AS DEFAULT_VALUE
FROM sys.sysobjects a INNER JOIN
(SELECT name, id
FROM sys.sysobjects
WHERE xtype = 'U') b on (a.parent_obj = b.id)
INNER JOIN sys.syscomments c ON (a.id = c.id)
INNER JOIN sys.syscolumns d ON (d.cdefault = a.id)
WHERE a.xtype = 'D'
ORDER BY b.name, a.nameselect c.name, col.name from sys.default_constraints c
inner join sys.columns col on col.default_object_id = c.object_id
inner join sys.objects o on o.object_id = c.parent_object_id
inner join sys.schemas s on s.schema_id = o.schema_id
where s.name = @SchemaName and o.name = @TableName and col.name = @ColumnName您正在寻找INFORMATION_SCHEMA.COLUMNS的COLUMN_DEFAULT列吗？
WHILE EXISTS(
SELECT * FROM sys.all_columns
INNER JOIN sys.tables ST ON all_columns.object_id = ST.object_id
INNER JOIN sys.schemas ON ST.schema_id = schemas.schema_id
INNER JOIN sys.default_constraints ON all_columns.default_object_id = default_constraints.object_id
WHERE
schemas.name = 'dbo'
AND ST.name = 'MyTable'
)
BEGIN
DECLARE @SQL NVARCHAR(MAX) = N'';
SET @SQL = ( SELECT TOP 1
'ALTER TABLE ['+ schemas.name + '].[' + ST.name + '] DROP CONSTRAINT ' + default_constraints.name + ';'
FROM
sys.all_columns
INNER JOIN
sys.tables ST
ON all_columns.object_id = ST.object_id
INNER JOIN
sys.schemas
ON ST.schema_id = schemas.schema_id
INNER JOIN
sys.default_constraints
ON all_columns.default_object_id = default_constraints.object_id
WHERE
schemas.name = 'dbo'
AND ST.name = 'MyTable'
)
PRINT @SQL
EXECUTE sp_executesql @SQL
--End if Error
IF @@ERROR <> 0
BREAK
END 死灵法师。
如果只需要检查是否存在
默认约束（在管理不善的数据库中默认约束的名称可能不同），请
使用INFORMATION_SCHEMA.COLUMNS（column_default）：
IF NOT EXISTS(
SELECT * FROM INFORMATION_SCHEMA.COLUMNS
WHERE (1=1)
AND TABLE_SCHEMA = 'dbo'
AND TABLE_NAME = 'T_VWS_PdfBibliothek'
AND COLUMN_NAME = 'PB_Text'
AND COLUMN_DEFAULT IS NOT NULL
)
BEGIN
EXECUTE('ALTER TABLE dbo.T_VWS_PdfBibliothek
ADD CONSTRAINT DF_T_VWS_PdfBibliothek_PB_Text DEFAULT (N''image'') FOR PB_Text;
');
END 如果只想按约束名称检查：
-- Alternative way:
IF OBJECT_ID('DF_CONSTRAINT_NAME', 'D') IS NOT NULL
BEGIN
-- constraint exists, deal with it.
END 最后但并非最不重要的一点是，您可以创建一个名为
INFORMATION_SCHEMA.DEFAULT_CONSTRAINTS 的视图：
CREATE VIEW INFORMATION_SCHEMA.DEFAULT_CONSTRAINTS
AS
SELECT
DB_NAME() AS CONSTRAINT_CATALOG
,csch.name AS CONSTRAINT_SCHEMA
,dc.name AS CONSTRAINT_NAME
,DB_NAME() AS TABLE_CATALOG
,sch.name AS TABLE_SCHEMA
,syst.name AS TABLE_NAME
,sysc.name AS COLUMN_NAME
,COLUMNPROPERTY(sysc.object_id, sysc.name, 'ordinal') AS ORDINAL_POSITION
,dc.type_desc AS CONSTRAINT_TYPE
,dc.definition AS COLUMN_DEFAULT
-- ,dc.create_date
-- ,dc.modify_date
FROM sys.columns AS sysc -- 46918 / 3892 with inner joins + where
-- FROM sys.all_columns AS sysc -- 55429 / 3892 with inner joins + where
INNER JOIN sys.tables AS syst
ON syst.object_id = sysc.object_id
INNER JOIN sys.schemas AS sch
ON sch.schema_id = syst.schema_id
INNER JOIN sys.default_constraints AS dc
ON sysc.default_object_id = dc.object_id
INNER JOIN sys.schemas AS csch
ON csch.schema_id = dc.schema_id
WHERE (1=1)
AND dc.is_ms_shipped = 0
/*
WHERE (1=1)
AND sch.name = 'dbo'
AND syst.name = 'tablename'
AND sysc.name = 'columnname'
*/我认为它不在INFORMATION_SCHEMA中-您可能必须使用sysobjects或相关的不推荐使用的表/视图。
您可能会在INFORMATION_SCHEMA.TABLE_CONSTRAINTS中找到一种类型，但是我没有看到。
可能是因为在其他一些SQL DBMS上，“默认约束”并不是真正的约束，因此您不会在“ INFORMATION_SCHEMA.TABLE_CONSTRAINTS”中找到其名称，因此最好的选择是“ INFORMATION_SCHEMA.COLUMNS”，正如其他人已经提到的那样。
（SQLServer-ignoramus在这里）
当您必须知道“默认约束”的名称时，我想到的唯一原因是SQLServer不支持"ALTER TABLE xxx ALTER COLUMN yyy SET DEFAULT..."命令。但是，那时您已经处在非标准区域，则必须使用特定于产品的方式来获得所需的东西。
如何结合使用CHECK_CONSTRAINTS和CONSTRAINT_COLUMN_USAGE：
select columns.table_name,columns.column_name,columns.column_default,checks.constraint_name
from information_schema.columns columns
inner join information_schema.constraint_column_usage usage on
columns.column_name = usage.column_name and columns.table_name = usage.table_name
inner join information_schema.check_constraints checks on usage.constraint_name = checks.constraint_name
where columns.column_default is not null我正在使用以下脚本检索带有以下脚本的所有默认值（sp_binddefaults）和所有默认约束：
SELECT
t.name AS TableName, c.name AS ColumnName, SC.COLUMN_DEFAULT AS DefaultValue, dc.name AS DefaultConstraintName
FROM
sys.all_columns c
JOIN sys.tables t ON c.object_id = t.object_id
JOIN sys.schemas s ON t.schema_id = s.schema_id
LEFT JOIN sys.default_constraints dc ON c.default_object_id = dc.object_id
LEFT JOIN INFORMATION_SCHEMA.COLUMNS SC ON (SC.TABLE_NAME = t.name AND SC.COLUMN_NAME = c.name)
WHERE
SC.COLUMN_DEFAULT IS NOT NULL
--WHERE t.name = '' and c.name = ''对象目录视图：sys.default_constraints
信息架构视图INFORMATION_SCHEMA符合ANSI，但是默认约束不是ISO标准的一部分。Microsoft SQL Server提供系统目录视图，以获取有关SQL Server对象元数据的信息。
sys.default_constraints 系统目录视图，用于获取有关默认约束的信息。
SELECT so.object_id TableName,
ss.name AS TableSchema,
cc.name AS Name,
cc.object_id AS ObjectID,
sc.name AS ColumnName,
cc.parent_column_id AS ColumnID,
cc.definition AS Defination,
CONVERT(BIT,
CASE cc.is_system_named
WHEN 1
THEN 1
ELSE 0
END) AS IsSystemNamed,
cc.create_date AS CreationDate,
cc.modify_date AS LastModifiednDate
FROM sys.default_constraints cc WITH (NOLOCK)
INNER JOIN sys.objects so WITH (NOLOCK) ON so.object_id = cc.parent_object_id
LEFT JOIN sys.schemas ss WITH (NOLOCK) ON ss.schema_id = so.schema_id
LEFT JOIN sys.columns sc WITH (NOLOCK) ON sc.column_id = cc.parent_column_id
AND sc.object_id = cc.parent_object_id
ORDER BY so.name,
cc.name;