以SQL Server为例，查询默认值是否存在，如果不存在，则设置默认值

IF NOT EXISTS (SELECT object_definition(default_object_id) AS definition
FROM   sys.columns
WHERE  name      ='colname'
AND    object_id = object_id('dbo.tablename'))
ALTER TABLE dbo.tablename ADD  DEFAULT your_value FOR columnname
GO

参考资料

​​SQL Server: Find out default value of a column with a query​​​](​​https://stackoverflow.com/questions/3817885/sql-server-find-out-default-value-of-a-column-with-a-query​​)