SQL查询的几种方式

1  /*  左连接 left join 或者 left outer join   */
 2  /* 左连接 Table_A表数据全部显示，Table_B根据条件匹配Table_A 匹配上显示，否则显示NULL */
 3  SELECT * from Table_A
 4  SELECT * from Table_B
 5  SELECT * from Table_A a  left outer join Table_B b on a.Code = b.Code
 6  
 7  
 8  
 9  /*  右连接 right join 或者 right outer join   */
10  /* 右连接 Table_B表数据全部显示，Table_A根据条件匹配Table_B 匹配上显示，否则显示NULL */
11  SELECT * from Table_A
12  SELECT * from Table_B
13  SELECT * from Table_A a right join Table_B b on a.Code = b.Code
14  
15  
16  /*完整外部联接:full join或full outer join*/
17  /*完整外部联接返回Table_A和Table_b中的所有行。当Table_A在Table_B表中没有匹配行时，
18  则Table_B的选择列表列包含空值。如果表之间有匹配行，则整个结果集行包含基表的数据值。*/
19  SELECT * from Table_A
20  SELECT * from Table_B
21  SELECT * from Table_A a full join Table_B b on a.Code = b.Code
22  
23 /*内连接:join或 inner join*/
24 /*返回Table_A和Table_B中匹配的列*/
25  SELECT * from Table_A
26  SELECT * from Table_B
27  SELECT * from Table_A a  join Table_B b on a.Code = b.Code
28  
29  /* 交差连接 cross join */
30  /*交差连接返回的结果是 Table_A 和Table_B表的笛卡尔积(Table_A的行数乘以Table_B的行数) */
31  SELECT * from Table_A
32  SELECT * from Table_B
33  SELECT Count(*) from Table_A a cross join Table_B 
34  SELECT * from Table_A a cross join Table_B