How to alter existing foreign key to add a constraint name - mysql

Question

I had previously written my create queries as follows, where the foreign keys were unnamed.
CREATE TABLE My_Table_Name (
USER_ID VARCHAR(255) NOT NULL,
CONSENT_ID VARCHAR(255) NOT NULL,
PRIMARY KEY (USER_ID, CONSENT_ID),
FOREIGN KEY (CONSENT_ID) REFERENCES ANOTHER_TABLE_NAME (CONSENT_ID) ON DELETE CASCADE
);
Now the table has data in it and I want to add the constraint name to the existing foreign keys. How can I do that?
PS: What I need is an ALTER query
Also, if the database were SQL Server instead of MySQL, would the query be different?

Answer 1

Foreign key constraints in MySQL cannot be renamed once created. You will have to drop the constraint, then recreate it with the name you want.
First you are going to have to go digging for the foreign key constraint name (there is one, you just don't know what it is):
SELECT constraint_name
FROM information_schema.REFERENTIAL_CONSTRAINTS
WHERE constraint_schema = 'your_db_name' AND table_name = 'My_Table_Name';
Now rename the constraint:
ALTER TABLE My_Table_Name
DROP FOREIGN KEY some_foreign_key -- from above query
ADD CONSTRAINT fk_my_constraint FOREIGN KEY (CONSENT_ID)
REFERENCES ANOTHER_TABLE_NAME (CONSENT_ID) ON DELETE CASCADE;

Answer 2

You use the constraint keyword:
CREATE TABLE My_Table_Name (
USER_ID VARCHAR(255) NOT NULL,
CONSENT_ID VARCHAR(255) NOT NULL,
CONSTRAINT pk_mY_table_name PRIMARY KEY (USER_ID, CONSENT_ID),
CONSTRAINT fk_my_table_name_consent_id FOREIGN KEY (CONSENT_ID) REFERENCES ANOTHER_TABLE_NAME (CONSENT_ID) ON DELETE CASCADE
);
This is standard syntax and will be the same in almost any database.
In both databases, you will need to go through the metadata tables to get the name of the existing constraint. Then add a new one:
alter table my_table_name
drop constraint x,
drop constraint y;
alter table my_table_name
CONSTRAINT pk_mY_table_name PRIMARY KEY (USER_ID, CONSENT_ID),
CONSTRAINT fk_my_table_name_consent_id FOREIGN KEY (CONSENT_ID) REFERENCES ANOTHER_TABLE_NAME (CONSENT_ID) ON DELETE CASCADE;
You can get the constraint names from INFORMATION_SCHEMA.TABLE_CONSTRAINTS (see here). This is available in both databases. I do caution that the columns are sometimes different in the INFORMATION_SCHEMA tables.

Answer 3

In MS SQL, anonymous constraints will get a system generated name.
You could use the following query to fetch the generated constraint name:
SELECT
FK.CONSTRAINT_NAME
FROM
INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS RC
INNER JOIN INFORMATION_SCHEMA.TABLE_CONSTRAINTS PK ON
PK.CONSTRAINT_CATALOG = RC.UNIQUE_CONSTRAINT_CATALOG AND
PK.CONSTRAINT_SCHEMA = RC.UNIQUE_CONSTRAINT_SCHEMA AND
PK.CONSTRAINT_NAME = RC.UNIQUE_CONSTRAINT_NAME
INNER JOIN INFORMATION_SCHEMA.TABLE_CONSTRAINTS FK ON
FK.CONSTRAINT_CATALOG = RC.CONSTRAINT_CATALOG AND
FK.CONSTRAINT_SCHEMA = RC.CONSTRAINT_SCHEMA AND
FK.CONSTRAINT_NAME = RC.CONSTRAINT_NAME
WHERE
FK.TABLE_NAME = N'My_Table_Name' AND
PK.TABLE_NAME = N'ANOTHER_TABLE_NAME'
This gives a result like 'FK__My_Table___CONSE__5535A963'.
Next, you might use the sp_rename stored procedure, like this:
EXECUTE sp_rename N'FK__My_Table___CONSE__5535A963', N'FK_MyRenamedConstraint';
Of course, you need to use the correct constraint names in your query.
(You may ignore the default warning from SQL Server about the fact that renaming might break existing scripts and stored procedures.)
When the first query is executed again, it will show you the new name.
I do not know how all this should be done in MySQL... Sorry about that...