SQL constraints

Put into production today , Because the constraints are stuck for a long time
cause :
Change the table structure , Change the fields that cannot be empty into fields that can be empty .
sql：
alter table tablename modify Field name null;
In this way, the corresponding non empty constraints should be deleted together , I don't know why not , You need to delete one by one manually .

external conditions ：
1.plsql Do not show constraints
2. Production environment sql The table results are different from the table structure of the test environment

The root cause ：
Understanding of constraints
The operation of querying and creating constraints is unclear

mysql Check constraint _MySQL Database access sql Statement to add constraints and view constraints

oracle Query constraints

oracle constraint_type problem

The leader said not to change the database structure casually
If it is not empty, it can be written to death , Then the code determines that the value is not displayed