if exists(SELECT 1
FROM sys.tables t
INNER JOIN sys.columns c ON t.object_id = c.object_id
INNER JOIN sys.default_constraints dc ON c.default_object_id = dc.object_id
WHERE t.name = '你的表名称'
AND c.name = '你的列名称')
存在默认值约束的处理
else 不存在默认值约束的处理