IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.TableName'))
 BEGIN 
ALTER TABLE TableName DROP CONSTRAINT CONSTRAINTNAME 
END 

IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.TableName'))
 BEGIN 
ALTER TABLE TableName DROP CONSTRAINT CONSTRAINTNAME 
END 

IF (OBJECT_ID('FK_ChannelPlayerSkins_Channels') IS NOT NULL)

如果你正在寻找其他类型的约束，例如默认值，你应该使用不同的查询 如何使用INFORMATION_SCHEMA找到默认约束?由devio回答)。使用:

SELECT * FROM sys.objects WHERE type = 'D' AND name = @name

按名称查找默认约束。

我已经把不同的“IF不存在”检查在我的帖子“DDL”IF不存在“条件使SQL脚本可重新运行”

在创建约束之前，我使用以下查询检查现有约束。

IF (NOT EXISTS(SELECT 1 FROM sysconstraints WHERE OBJECT_NAME(constid) = 'UX_CONSTRAINT_NAME' AND OBJECT_NAME(id) = 'TABLE_NAME')) BEGIN
...
END

这个函数以给定的表名为目标，通过名称查询约束。希望这能有所帮助。

SELECT tabla.name as Tabla,

        restriccion.name as Restriccion, 
        restriccion.type as Tipo, 
        restriccion.type_desc as Tipo_Desc
FROM {DATABASE_NAME}.sys.objects tabla 

INNER JOIN {DATABASE_NAME}.sys.objects restriccion

ON tabla.object_id = restriccion.parent_object_id

WHERE tabla.type = 'U' - Solo tablas creadas por el usuario.

AND restriccion.type = 'UQ' --Tipo de Restriccion UNIQUE

ORDER BY tabla.name, restriccion.type_desc