前言 1.左连接 table1居左，故谓之左连接。这种情况下，以table1为主，即table1中的所有记录均会被列出。有一下三种情况： 1、对于table1中的每一条记录对应的城市如果在table2中也恰好存
前言
1.左连接
table1居左，故谓之左连接。这种情况下，以table1为主，即table1中的所有记录均会被列出。有一下三种情况：
1、对于table1中的每一条记录对应的城市如果在table2中也恰好存在而且刚好只有一条，那么就会在
返回的结果中形成一条新的记录。如上面Person A和Person B对应的情况。
2、对于table1中的每一条记录对应的城市如果在table2中也恰好存在而且有N条，那么就会在返回的结果中形成N条新的记录。如上面的Person C对应的情况。
3、对于table1中的每一条记录对应的城市如果在table2中不存在，那么就会在返回的结果中形成一条
条新的记录，且该记录的右边全部NULL。如上面的Person D对应的情况。
不符合上面三条规则的记录不会被列出。
2.右连接
table2居右，故谓之右连接。这种情况下，以table2为主，即table2中的所有记录均会被列出。有一下三种情况：
1、对于table2中的每一条记录对应的城市如果在table1中也恰好存在而且刚好只有一条，那么就会在
返回的结果中形成一条新的记录。如上面Person X和Person Y对应的情况。
2、对于table2中的每一条记录对应的城市如果在table1中也恰好存在而且有N条，那么就会在返回的结果中形成N条新的记录。如上面的Person W对应的情况。
3、对于table2中的每一条记录对应的城市如果在table1中不存在，那么就会在返回的结果中形成一条
条新的记录，且该记录的左边全部NULL。如上面的Person Z对应的情况。
不符合上面三条规则的记录不会被列出。
3. 内连接
内连接的数据记录中，不会存在字段为NULL的情况。可以简单地认为，内链接的结果就是在左连接或者右连接的结果中剔除存在字段为NULL的记录后所得到的结果。甚至可以认为，如果两个表中仅分别剩下内连接运算后所得的数据记录，如table1中只有Person A、Person B和Person C，table2中只有Person W、Person X和Person Y，那么这两个表的之间的左连接和右连接的返回的结果是一样的。
注意：select * from table1 a inner join table2 b on a.city = b.city 和select * from table1 a join table2 b on a.city = b.city 的效果是一样的，即如果join的左边没有诸如left、right或者inner这样的关键字时，缺省的是内连接。另，MySQL不支持full join。
一、LINQ和lambda实现左、右、内链接
1.LINQ实现左、右、内链接
1.1 左链接
static List<Customer> GetCustomer()//客户
{
List<Customer> list = new List<Customer>();
list.Add(new Customer { id = 1, name = "刘德华", email = "ldh@net.cn" });
list.Add(new Customer { id = 2, name = "张学友", email = "zxy@net.cn" });
list.Add(new Customer { id = 3, name = "黎明", email = "lm@net.cn" });
list.Add(new Customer { id = 4, name = "郭富城", email = "gfc@net.cn" });
list.Add(new Customer { id = 4, name = "古天乐", email = "gtl@net.cn" });
return list;
}
static List<CustomerContact> GetCustomerContact()//联系人
{
List<CustomerContact> list = new List<CustomerContact>();
list.Add(new CustomerContact { ccid = 1, CustomerId = 1, Phone = "13888888888", Address = "北京" });
list.Add(new CustomerContact { ccid = 2, CustomerId = 1, Phone = "13899999999", Address = "天津" });
list.Add(new CustomerContact { ccid = 3, CustomerId = 2, Phone = "13866666666", Address = "香港" });
list.Add(new CustomerContact { ccid = 4, CustomerId = 8, Phone = "13877777777", Address = "上海" });
return list;
}
//左链接
var LeftJoin = from cusetomer in GetCustomer()
join cc in GetCustomerContact()
on cusetomer.id equals cc.CustomerId into JoinCC
from cc in JoinCC.DefaultIfEmpty()
select new
{
CustomerName = cusetomer.name,
phone = cc != null ? cc.Phone : null
};
1.2 右链接
static List<Customer> GetCustomer()//客户
{
List<Customer> list = new List<Customer>();
list.Add(new Customer { id = 1, name = "刘德华", email = "ldh@net.cn" });
list.Add(new Customer { id = 2, name = "张学友", email = "zxy@net.cn" });
list.Add(new Customer { id = 3, name = "黎明", email = "lm@net.cn" });
list.Add(new Customer { id = 4, name = "郭富城", email = "gfc@net.cn" });
list.Add(new Customer { id = 4, name = "古天乐", email = "gtl@net.cn" });
return list;
}
static List<CustomerContact> GetCustomerContact()//联系人
{
List<CustomerContact> list = new List<CustomerContact>();
list.Add(new CustomerContact { ccid = 1, CustomerId = 1, Phone = "13888888888", Address = "北京" });
list.Add(new CustomerContact { ccid = 2, CustomerId = 1, Phone = "13899999999", Address = "天津" });
list.Add(new CustomerContact { ccid = 3, CustomerId = 2, Phone = "13866666666", Address = "香港" });
list.Add(new CustomerContact { ccid = 4, CustomerId = 8, Phone = "13877777777", Address = "上海" });
return list;
}
//右链接
var LightJoin = from cc in GetCustomerContact()
join cusetomer in GetCustomer()
on cc.CustomerId equals cusetomer.id into JoinCC
from cusetomer in JoinCC.DefaultIfEmpty()
select new
{
phone = cc.Phone,
CustomerName = cusetomer != null ? cusetomer.name : null,
};
1.3 内链接
static List<Customer> GetCustomer()//客户
{
List<Customer> list = new List<Customer>();
list.Add(new Customer { id = 1, name = "刘德华", email = "ldh@net.cn" });
list.Add(new Customer { id = 2, name = "张学友", email = "zxy@net.cn" });
list.Add(new Customer { id = 3, name = "黎明", email = "lm@net.cn" });
list.Add(new Customer { id = 4, name = "郭富城", email = "gfc@net.cn" });
list.Add(new Customer { id = 4, name = "古天乐", email = "gtl@net.cn" });
return list;
}
static List<CustomerContact> GetCustomerContact()//联系人
{
List<CustomerContact> list = new List<CustomerContact>();
list.Add(new CustomerContact { ccid = 1, CustomerId = 1, Phone = "13888888888", Address = "北京" });
list.Add(new CustomerContact { ccid = 2, CustomerId = 1, Phone = "13899999999", Address = "天津" });
list.Add(new CustomerContact { ccid = 3, CustomerId = 2, Phone = "13866666666", Address = "香港" });
list.Add(new CustomerContact { ccid = 4, CustomerId = 8, Phone = "13877777777", Address = "上海" });
return list;
}
//内链接
var InnerJoin = from cc in GetCustomerContact()
join cusetomer in GetCustomer()
on cc.CustomerId equals cusetomer.id
select new
{
phone = cc.Phone,
CustomerName = cusetomer.name
};
2.lambda实现左、右、内链接
2.1 左链接和右链接
左链接和有链接一样，只是位置不一样，这边就整合一个案例
static List<Customer> GetCustomer()//客户
{
List<Customer> list = new List<Customer>();
list.Add(new Customer { id = 1, name = "刘德华", email = "ldh@net.cn" });
list.Add(new Customer { id = 2, name = "张学友", email = "zxy@net.cn" });
list.Add(new Customer { id = 3, name = "黎明", email = "lm@net.cn" });
list.Add(new Customer { id = 4, name = "郭富城", email = "gfc@net.cn" });
list.Add(new Customer { id = 4, name = "古天乐", email = "gtl@net.cn" });
return list;
}
static List<CustomerContact> GetCustomerContact()//联系人
{
List<CustomerContact> list = new List<CustomerContact>();
list.Add(new CustomerContact { ccid = 1, CustomerId = 1, Phone = "13888888888", Address = "北京" });
list.Add(new CustomerContact { ccid = 2, CustomerId = 1, Phone = "13899999999", Address = "天津" });
list.Add(new CustomerContact { ccid = 3, CustomerId = 2, Phone = "13866666666", Address = "香港" });
list.Add(new CustomerContact { ccid = 4, CustomerId = 8, Phone = "13877777777", Address = "上海" });
return list;
}
//左链接和右链接
var LMLeftJoin = GetCustomer().GroupJoin(GetCustomerContact(),
a => a.id, b => b.CustomerId, (a, b) => new { a, b })
.SelectMany(c => c.b.DefaultIfEmpty(), (c, b)
=> new { CustomerName = c.a.name, Phone = b != null ? b.Phone : null });
2.2 内链接
static List<Customer> GetCustomer()//客户
{
List<Customer> list = new List<Customer>();
list.Add(new Customer { id = 1, name = "刘德华", email = "ldh@net.cn" });
list.Add(new Customer { id = 2, name = "张学友", email = "zxy@net.cn" });
list.Add(new Customer { id = 3, name = "黎明", email = "lm@net.cn" });
list.Add(new Customer { id = 4, name = "郭富城", email = "gfc@net.cn" });
list.Add(new Customer { id = 4, name = "古天乐", email = "gtl@net.cn" });
return list;
}
static List<CustomerContact> GetCustomerContact()//联系人
{
List<CustomerContact> list = new List<CustomerContact>();
list.Add(new CustomerContact { ccid = 1, CustomerId = 1, Phone = "13888888888", Address = "北京" });
list.Add(new CustomerContact { ccid = 2, CustomerId = 1, Phone = "13899999999", Address = "天津" });
list.Add(new CustomerContact { ccid = 3, CustomerId = 2, Phone = "13866666666", Address = "香港" });
list.Add(new CustomerContact { ccid = 4, CustomerId = 8, Phone = "13877777777", Address = "上海" });
return list;
}
//内链接
var LMInnerJoin = GetCustomer().Join(GetCustomerContact(), a => a.id, b => b.CustomerId
, (a, b) => new { CustomerName = a.name, Phone = b.Phone });
