Optimizing SELECT Statements The syntax of  table_factor  is extended in comparison with the SQL Standard. The latter accepts only table_reference , not a list of them inside a pair of parentheses. This is a conservative extension if we consider each comma in a list of  table_reference  items as equivalent to an inner join. For example: Is equivalent to: In MySQL,  CROSS JOIN  is syntactically equivalent to  INNER JOIN ; they can replace each other. In standard SQL, they are not equivalent.  INNER JOIN  is used with an  ON  clause;  CROSS JOIN  is used otherwise. In general, parentheses can be ignored in join expressions containing only inner join operations. Consider this join expression: After removing parentheses and grouping operations to the left, that join expression transforms into this expression: Yet, the two expressions are not equivalent. To see this, suppose that the tables  t1 ,  t2 , and  t3  have the following state: In this case, the first expression returns a result set including the rows  (1,1,101,101) , (2,NULL,NULL,NULL) , whereas the second expression returns the rows  (1,1,101,101) , (2,NULL,NULL,101) : 1662 •Table  t1  contains rows  (1) ,  (2) •Table  t2  contains row  (1,101) •Table  t3  contains row  (101) SELECT * FROM t1 LEFT JOIN (t2, t3, t4) ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c) SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4) ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c) t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.b=t3.b OR t2.b IS NULL) ON t1.a=t2.a (t1 LEFT JOIN t2 ON t1.a=t2.a) LEFT JOIN t3 ON t2.b=t3.b OR t2.b IS NULL mysql>  SELECT * FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.b=t3.b OR t2.b IS NULL) ON t1.a=t2.a; mysql>  SELECT * FROM (t1 LEFT JOIN t2 ON t1.a=t2.a) LEFT JOIN t3 ON t2.b=t3.b OR t2.b IS NULL; +------+------+------+------+ | a | a | b | b | +------+------+------+------+ | 1 | 1 | 101 | 101 | | 2 | NULL | NULL | NULL | +------+------+------+------+ +------+------+------+------+ | a | a | b | b | +------+------+------+------+ | 1 | 1 | 101 | 101 | | 2 | NULL | NULL | 101 | +------+------+------+------+