redundant constraint_schema

Hi,

I have a patching script that is supposed to add column if not existing :

ALTER TABLE myschem.table1

ADD COLUMN IF NOT EXISTS col1 VARCHAR(254) REFERENCES
myschem.table2(col2)

When col1 exists, I expected that nothing would happen. But, when applying
this query

select constraint_name from information_schema.key_column_usage where
constraint_schema='myschem'

I notice that a new constraint "table1_col2_fkeyxxx" is created each time
the previous ALTER TABLE is called (with xxx being a new number each time)

Can someone explain why, when a column is not created (if not exists), a
redundant constraint is still created from the REFERENCES part ?

Thanks,

Olivier