MySql JOIN syntax with multiple tables-CodePudding

Can someone explain why in the first solution for this SO answer the LEFT JOIN can be nested between the INNER JOIN table name and ON clause?

SELECT Persons.Name, Persons.SS, Fears.Fear FROM Persons
LEFT JOIN Person_Fear
    INNER JOIN Fears
    ON Person_Fear.FearID = Fears.FearID
ON Person_Fear.PersonID = Persons.PersonID

I can't find anything describing this syntax and I'm not sure what the point is. The answer has 2 solutions, and I would've naturally done the second solution having never seen the first solution's syntax before.

Is there performance benefits for either?

CodePudding user response：

The parameters to the FROM keyword are what the MySQL documentation calls table_references, and its syntax is described here. There's lots of recursive references in the syntax, and I think this is what allows that syntax. I've copied what I think are relevant excerpts from the BNF.

table_references:
    escaped_table_reference [, escaped_table_reference] ...

escaped_table_reference: {
    table_reference
  | { OJ table_reference }
}

table_reference: {
    table_factor
  | joined_table
}

joined_table: {
    table_reference {[INNER | CROSS] JOIN | STRAIGHT_JOIN} table_factor [join_specification]
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_specification
  | table_reference NATURAL [INNER | {LEFT|RIGHT} [OUTER]] JOIN table_factor
}

The nested

    Person_Fear
    INNER JOIN Fears
    ON Person_Fear.FearID = Fears.FearID

is a joined_table, which can be used as the table_reference in the first LEFT JOIN.