Question

SELECT * FROM Table JOIN Other ON Table.id = Other.tableId

I’m structuring a MySQL wrapper, and wondering if the “ON” keyword is used only in conjunction with joins (like above), or somewhere else as well?

Answer 1

The ON keyword is also found and can be in several other places, like CREATE TABLE and ALTER TABLE table statements (in the FOREIGN KEY definitions: ON DELETE CASCADE), in the CREATE TRIGGER syntax (ON tbl_name FOR EACH ROW), in the CREATE INDEX statements, etc.

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For joins, the JOIN syntax in the MySQL documentation has it all:

MySQL supports the following JOIN syntaxes for the table_references part of SELECT statements and multiple-table DELETE and UPDATE statements:

table_references:
    table_reference [, table_reference] ...

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )
  | { OJ table_reference LEFT OUTER JOIN table_reference
        ON conditional_expr }

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

So, ON can be used in SELECT, UPDATE and DELETE statements. It can be used with JOIN, LEFT JOIN, RIGHT JOIN (and the equivalent INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN).

Either ON or USING is to be used, not both.

It cannot be used with NATURAL JOIN (of course.)

And some differences from the ANSI/ISO SQL standard:

• you can use ON with CROSS JOIN.

• also (optionally) with STRAIGHT_JOIN (that’s a MySQL addition)

• you can use JOIN and INNER JOIN without ON.

• UPDATE and DELETE syntax has some differences with the standard, too.