create table java (name varchar(255));
insert into java values ('java1'),('java2'),('blue'); 
create table mysql (name varchar(255));
insert into mysql values ('mysql1'),('mysql2'),('blue');

select * from java,mysql where java.name=mysql.name;
SELECT * FROM java JOIN mysql ON java.name=mysql.name;
SELECT * FROM java INNER JOIN mysql ON java.name=mysql.name;
SELECT * FROM java CROSS JOIN mysql ON java.name=mysql.name;
SELECT * FROM java STRAIGHT_JOIN mysql ON java.name=mysql.name;

+------+------+
| name | name |
+------+------+
| blue | blue |
+------+------+

join_table:
 table_reference [INNER | CROSS] JOIN table_factor [join_condition]
 | table_reference STRAIGHT_JOIN table_factor
 | table_reference STRAIGHT_JOIN table_factor ON condition

SELECT * FROM java LEFT JOIN mysql ON java.name=mysql.name;

+-------+------+
| name | name |
+-------+------+
| java1 | NULL |
| java2 | NULL |
| blue | blue |
+-------+------+

SELECT * FROM java LEFT JOIN mysql ON java.name=mysql.name WHERE mysql.name IS NULL;

SELECT * FROM java RIGHT JOIN mysql ON java.name=mysql.name;

+------+--------+
| name | name |
+------+--------+
| NULL | mysql1 |
| NULL | mysql2 |
| blue | blue |
+------+--------+

join_table:| table_reference LEFT [OUTER] JOIN table_reference join_condition
 | table_reference NATURAL [LEFT [OUTER]] JOIN table_factor
 | table_reference RIGHT [OUTER] JOIN table_reference join_condition
 | table_reference NATURAL [RIGHT [OUTER]] JOIN table_factor

SELECT java.*,mysql.* FROM java LEFT JOIN mysql USING (name);

+-------+------+
| name | name |
+-------+------+
| java1 | NULL |
| java2 | NULL |
| blue | blue |
+-------+------+

SELECT * FROM t1 LEFT JOIN (t2, t3, t4) ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c);
--相当于
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)

SELECT t1.id,t2.id,t3.id FROM t1,t2 LEFT JOIN t3 ON (t3.id=t1.id) WHERE t1.id=t2.id;
--实际上这么执行
SELECT t1.id,t2.id,t3.id FROM t1,( t2 LEFT JOIN t3 ON (t3.id=t1.id) ) WHERE t1.id=t2.id;
--应该这么写
SELECT t1.id,t2.id,t3.id FROM (t1,t2) LEFT JOIN t3 ON (t3.id=t1.id) WHERE t1.id=t2.id;