主键约束
删除数据表的主键
ALTER TABLE <table_name> DROP CONSTRAINT <cons_name>;
添加主键约束
ALTER TABLE <table_name> ADD PRIMARY KEY (col_name);
非空约束
删除非空约束
ALTER TABLE <table_name> ALTER COLUMN <col_name> DROP NOT NULL;
添加非空约束
ALTER TABLE <table_name> ALTER COLUMN <col_name> SET NOT NULL;