mysql innerjoin 和 leftjoin

select t1.*,t2.* from table1 t1,table2 t2;

select t1.*,t2.* from table1 t1 cross join table2 t2;

我们把上述"没有任何限制条件的连接方式"称之为"交叉连接"，"交叉连接"后得到的结果跟线性代数中的"笛卡尔乘积"一样。

select t1.*,t2.* from table1 t1,table2 t2 where t1.id = t2.id;
select t1.*,t2.* from table1 t1 inner join table2 t2 on t1.id = t2.id;

"有条件的" 交叉链接

select t1.*,t2.* from table1 t1 left join table2 t2 on t1.id = t2.id;

如果连接字段在右表中不唯一,左外链接会出现重复列情况, 
所以一般用主键进行关联,注意主表的选择;

mysql> select * from stu;
+----+-------+
| id | name  |
+----+-------+
|  1 | Kobe  |
|  2 | James |
|  3 | Bosh  |
+----+-------+
3 rows in set (0.03 sec)

mysql> select * from class;
+----+------+------+
| id | s_id | c_id |
+----+------+------+
|  1 |    1 |    1 |
|  2 |    1 |    2 |
|  3 |    2 |    5 |
+----+------+------+
3 rows in set (0.01 sec)

mysql> select * from stu left join class on stu.id= class.s_id;
+----+-------+------+------+------+
| id | name  | id   | s_id | c_id |
+----+-------+------+------+------+
|  1 | Kobe  |    1 |    1 |    1 |
|  1 | Kobe  |    2 |    1 |    2 |
|  2 | James |    3 |    2 |    5 |
|  3 | Bosh  | NULL | NULL | NULL |
+----+-------+------+------+------+
4 rows in set (0.03 sec)

1,GROUP BY 语句根据一个或多个列对结果集进行分组
2,最终查询出的结果只会显示组中一条记录
3,在mysql中没有强调select指定的字段必须属于group by后的条件(和 oracle 中不同)

mysql> select class.*,count(*) from class where id >0 group by s_id having count(*) >1;
+----+------+------+----------+
| id | s_id | c_id | count(*) |
+----+------+------+----------+
| 1 | 1 | 1 | 2 |
| 4 | 4 | 4 | 2 |
+----+------+------+----------+
2 rows in set (0.02 sec)

mysql> select class.* from class ;
+----+------+------+
| id | s_id | c_id |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 5 |
| 4 | 4 | 4 |
| 5 | 4 | 7 |
+----+------+------+
5 rows in set (0.02 sec)

1,已经筛选出的字段，having和where的效果是等效的
2,未筛出的字段 过滤只能用where
3,group by 的组内过滤, 只能用having [having 的字段必须是聚合函数或者出现在select检索中出现]

mysql> select id,s_id from class having s_id >2; 
+----+------+
| id | s_id |
+----+------+
|  4 |    4 |
|  5 |    4 |
+----+------+
2 rows in set (0.02 sec)
 
mysql> select id from class having s_id >2; 
1054 - Unknown column ‘s_id‘ in ‘having clause‘