联合查询

SQL联合查询中的关键语法

关于联合查询４种联合方式如果可以灵活使用，基本上复杂的语句结构也会简单起来．这４种方式是：

Inner join          left outer join           right outer join        full outer join

联合查询效率较高.以下例子来说明联合查询的好处

t1表结构(用户名,密码)              userid int                    username   varchar(20)                     password   varchar(20)

                                                  1                                       jack                                           jackpwd

                                                  2                                       owen                                         owenpwd

t2 表结构(用户积分,等级)           userid int                                  jf   int                                            dj    int

                                                 1                                         20                                                  3

                                                 3                                         50                                                  6

第一:内联(inner join)

如果想把用户信息,积分,等级都列出来.那么一般会这样写

                                          select  *   from t1  ,t2  where t1.userid = t2.userid  

其实这样的结果等同于             select  *   from t1 inner join t2  on  t1.userid=t2.userid

就是把两个表中都存在userid的行拼成一行.这是内联.但后者的效率会比前者高很多.建议用后者的写法.

运行结果:                                 userid                username            password      userid          jf                dj

                                                   1                      jack                   jacjpwd        1             20                3

第二:左联（left outer join）显示左表中的所有行

                                          select  *   from  t1   left outer join  t2 on t1.userid=t2.userid

运行结果：                                 userid               username             password     userid              jf           dj

　　　　　　                                 1                     jack                      jackpwd       1                 20           3

                                                  2                    owen                     owenpwd     NULL             NULL   NULL

第三:右联（right outer join）显示右表中的所有行

                                             select * from t1 right outer join t2 on t1.userid=t2.userid

运行结果：                                 userid                     username           password      userid          jf           dj

　　　　　                                  　1                             jack               jackpwd        1                20          3

                                                Null                              Null                   Null          3               50          6

第四:全联（full outer join）显示两边表中所有行

                                                  select * from t1 full outer join t3 on t1.userid=t3.userid

运行结果：                                   userid            username             password     userid          jf           dj

　　　　　                                      　1                jack                     jackpwd        1              20         3

                                                       2                owen                   owenpwd       NULL      NULL      NULL

                                                     Null               Null                       Null              3            50          6

 

linq 联合查询

 //查询UserId<276234的有部门Id的用户信息
                 var table = from c in ctx.Users
                            join p in ctx.Structure
                             on c.StructureID equals p.StructureID.ToString()
                             where c.UserID < 276234
                             select c;