Constraint symbol vs foreign key index name -> what's the difference?

user_2769590 0
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In mysql foreign keys are defined like this:

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

Why do we need CONSTRAINT and symbol? It seems like there is index_name anyway - so I totally don't get the reason for CONSTRAINT keyword. Can someone clarify what it's for?

mysql
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