Question

Does the following in MySQL:

alter table TABLE_A 
ADD CONSTRAINT fk_id (id) REFERENCES TABLE_B(id) 
ON DELETE CASCADE ON UPDATE CASCADE;

do the same as

alter table TABLE_A 
ADD FOREIGN KEY (id) REFERENCES TABLE_B(id);

except that it also adds a BTREE index on the relationship? Or do they complement each other?

Answer 1

Both queries are more or less the same, though according to the manual the FOREIGN KEY should be present in both queries.

[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]

The benefit of CONSTRAINT [symbol] is that you can name the constraint rather than MySQL generating the name for you; this symbol must be unique within the database and can be used to later remove the constraint.

Generated symbol names can be seen by checking the table structure after creation:

SHOW CREATE TABLE TABLE_A;