alter table t_task alter COLUMN material_type type varchar(32) ;
-- 删除或新增非空约束
alter table table_name alter column_name drop not null;
alter table table_name alter column_name set not null;
-- 删除或设置默认值
alter table table_name alter COLUMN column_name DROP DEFAULT;
alter table table_name alter COLUMN column_name SET DEFAULT default_value;
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