Add or Add Constraint for Foreign Key Constraints - mysql

Question

The term 'Foreign Key Constraint' is often tossed around. And I just want to clarify it's exact meaning. We have an employees table and a branches table. The employees table was created first but it should have a foreign key constraint on branch_id, which references the primary (surrogate) key of id on branches table:
CREATE TABLE employees (
id INT AUTO_INCREMENT,
first_name VARCHAR(40),
last_name VARCHAR(40),
birth_day DATE,
sex BOOLEAN,
salary INT,
supervisor_id INT,
branch_id INT,
PRIMARY KEY(id)
)
CREATE TABLE branches (
id INT AUTO_INCREMENT,
branch_name VARCHAR(40),
manager_id INT,
manager_start_date DATE,
PRIMARY KEY(id),
FOREIGN KEY(manager_id) REFERENCES employees(id) ON DELETE SET NULL
)
And now we add the Foreign Key Constraints:
ALTER TABLE employees
ADD FOREIGN KEY(branch_id)
REFERENCES branches(id)
ON DELETE SET NULL;
ALTER TABLE employees
ADD FOREIGN KEY(supervisor_id)
REFERENCES employees(id)
ON DELETE SET NULL;
Notice here I use Add Foreign Key and not Add Constraint constraint_name. Here would be an example of using ADD CONSTRAINT:
ALTER TABLE users
ADD CONSTRAINT check_users_age
CHECK (age>=18 AND city='Philadelphia');
Is ADD FOREIGN KEY and ADD CONSTRAINT constraint_name synonymous? Does ADD FOREIGN KEY, in effect, add a constraint without a name? And if ADD FOREIGN KEY does add a name, how can I find it in mysql?

Answer 1

Explicitly naming the constraint, i.e. use CONSTRAINT <name> FOREIGN KEY ... is optional. If you don't do it, i.e. just go with FOREIGN KEY ... the system generates a name.
The effects a constraint have are independent of its name. So both, explicitly naming the constraint and not doings so are synonym-ish -- that is except regarding the name.
You can query the constraints from the catalog, e.g. from information_schema.key_column_usage.
Consider the following example:
CREATE TABLE a
(id integer,
PRIMARY KEY (id));
CREATE TABLE b
(id integer,
PRIMARY KEY (id));
CREATE TABLE x
(a integer,
b integer,
FOREIGN KEY (a)
REFERENCES a
(id),
CONSTRAINT fancy_name
FOREIGN KEY (b)
REFERENCES b
(id));
SELECT table_name,
column_name,
constraint_name,
referenced_table_name,
referenced_column_name
FROM information_schema.key_column_usage
WHERE table_schema = database()
AND table_name = 'x';
This will result in something like:
| table_name | column_name | constraint_name | referenced_table_name | referenced_column_name |
| ---------- | ----------- | --------------- | --------------------- | ---------------------- |
| x | b | fancy_name | b | id |
| x | a | x_ibfk_1 | a | id |
DB Fiddle
You can see, the system picked a name for the constraint.