mySQL - KEY and FOREIGN KEY keywords - mysql

Question

I created my tables in workbench and forward engineered to create my database. When I backed up my database by export/dump, I came across some lines of code for CREATE TABLE that I'm not sure why they look the way they do:
I have CONSTRAINT-FOREIGN KEY()REFERENCES-() for all my foreign key constraints which makes sense. But I also have KEY which seems to be creating an index for some of my foreign keys but not for all of them in many of my tables. I don't know what this KEY line does and why it's this way? In the example below, I have KEY for 2 of my 3 FKs (one of the FKs being a composite FK made up of 3).
After the KEY and CONSTRAINT, sometimes the code just mentions the foreign key name, and some other times it has the format fk_referencingTable_referencedTable1. Accidentally in this example the composite foreign key has this second format, but really it looks like any foreign key (either single or composite) can randomly have either one of these 2 formats? Why is that and should I be concerned about it?
DROP TABLE IF EXISTS `Recommendations`;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `Recommendations` (
`client_SIN` char(9) NOT NULL
`advisor_SIN` char(9) NOT NULL,
`exchange_name` varchar(45) NOT NULL,
`security_tradeName` varchar(45) NOT NULL,
`FSI_name` varchar(45) NOT NULL,
`Date` date DEFAULT NULL,
`Quantity` int(11) DEFAULT NULL,
PRIMARY KEY (`client_SIN`,`advisor_SIN`,`exchange_name`,`security_tradeName`,
`FSI_name`),
KEY `advisor_SIN_idx` (`advisor_SIN`),
KEY `fk_Recommendations_Exchanges_has_Securities1_idx`
(`exchange_name`,`security_tradeName`,`FSI_name`),
CONSTRAINT `advisor_SIN` FOREIGN KEY (`advisor_SIN`)
REFERENCES `Advisors` (`advisor_SIN`)
ON DELETE CASCADE ON UPDATE NO ACTION,
CONSTRAINT `client_SIN` FOREIGN KEY (`client_SIN`)
REFERENCES `Clients` (`client_SIN`)
ON DELETE CASCADE ON UPDATE NO ACTION,
CONSTRAINT `fk_Recommendations_Exchanges_has_Securities1`
FOREIGN KEY (`exchange_name`, `security_tradeName`, `FSI_name`)
REFERENCES `Exchanges_has_Securities` (`Exchanges_exchange_name`,
`Securities_security_tradeName`, `Securities_issuer_name`)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

Creating a foreign key automatically adds a key for the column(s) in the FK if it's not already indexed. There's no additional KEY for client_SIN because it's the first column in the primary key, and any prefix of a composite index is an index of its own.
I believe the index names you got were assigned by MySQL Workbench. The default in MySQL is to use the first column name as the index name when you don't name an index explicitly. But it looks like MySQL Workbench appends _idx when it creates the index. And if it's creating the index for a foreign key it uses the prefix fk_ as well.

Answer 2

KEY is keyword-statement for index creation (i.e. it is sub-statement). It creates an index, changing the data storage.
FOREIGN KEY is a keyword-rule (yes, this is one keyword despite it consist from two separate words) for consistency checking subsystem, and does not cause any change in data storage itself. But it contains hidden statements - first of them checks does the index where the reference expression is a prefix exists, second creates an index by this expression if the checking fails (i.e. second hidden statement is KEY statement). CONSTRAINT is naming part of FOREIGN KEY rule (if it is absent then the name will be generated automatically).