If a row from the updated table is joined with a row from another table in the FROM clause, and the specified WHERE condition for the request evaluates to TRUE for that row, then the row in the updated table is updated with columns referenced in the SET clause from the joined row.
When an UPDATE request specifies a join operation, the join is more efficient if the WHERE condition uses values for indexed columns in the joined tables.
The order in which join updates run can affect the result of a transaction. See Duplicate Row Checks.