Can't run NOCHECK CONSTRAINT on mysql - SQL syntax error - mysql

Question

I've create a table called Students.
It has 4 fields: Name, Age, Class, Years.
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| Name | varchar(50) | NO | PRI | NULL | |
| Age | smallint | YES | | 18 | |
| Class | varchar(10) | YES | | NULL | |
| Years | smallint | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
I have added a CHECK CONSTRAINT on Years field that means that the value in Years column can't be lower than 3.
I did it by
mysql> alter table Students
-> add constraint CK_Years check (Years >= 3);
My goal here is to insert a query with a record which has a Years value of 2, which means it won't pass the CK_Years check constraint.
I want to do it without changing the constraint or the fields in the insert statement.
Is it possible?
mysql> insert into Students
-> (Name, Age, Class, Years) values ("Eric","25","110","2");
I have tried:
mysql> alter table Students
-> NOCHECK CONSTRAINT CK_Years;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NOCHECK CONSTRAINT CK_Years' at line 2
I want to disable it just for this insert, and then bring it back (the check constraint).
As you can see, I got this error.
I tried to write it in couple ways. Wit 'CK_Years' or CK_Years or
`CK_Years`
got the same error for all of these ways.
I have verified that I have the constraint in the table:
mysql> select * from INFORMATION_SCHEMA.TABLE_CONSTRAINTS
-> where table_name='Students';
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | TABLE_SCHEMA | TABLE_NAME | CONSTRAINT_TYPE | ENFORCED |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
| def | School | PRIMARY | School | Students | PRIMARY KEY | YES |
| def | School | CK_Years | School | Students | CHECK | YES |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
So how can I do it without changing the constraint or the fields in the insert statement?
How can I fix the syntax error?
And what is the reason for it?
Thanks.

Answer 1

MySQL does support syntax to alter a check constraint so it is not enforced, but it's different from the Microsoft SQL Server syntax.
mysql> alter table students alter check CK_Years NOT ENFORCED;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> insert into students values ('Eric',25,'110',2);
Query OK, 1 row affected (0.00 sec)
It's documented here: https://dev.mysql.com/doc/refman/8.0/en/alter-table.html
If you re-enable it, the constraint is enforced, and if you had inserted any rows that violate the constraint, you get an error trying to re-enable it.
mysql> alter table students alter check CK_Years ENFORCED;
ERROR 3819 (HY000): Check constraint 'CK_Years' is violated.
Also once you make the constraint not enforced, it would take effect for all clients, not just for one INSERT.
So I don't think you can do what you intend with a CHECK constraint. You may have to write a trigger that enforces a similar condition on that column, but has logic to enforce it conditionally.
Or do what most developers did before MySQL supported CHECK constraints: enforce it in the client app, before doing the INSERT. Then you can apply any conditions you want.