我有以下一段 VB.NET 代码:
Dim conn As New MySql.Data.MySqlClient.MySqlConnection(ConnectionString)
conn.Open()
Dim sql = "SELECT * FROM users"
Dim com = New MySql.Data.MySqlClient.MySqlCommand(sql, conn)
Dim ds As New DataSet("dsUsers")
Dim da As New MySql.Data.MySqlClient.MySqlDataAdapter()
da.SelectCommand = com
da.TableMappings.Add("Table", "Users")
da.Fill(ds)
sql = "SELECT * FROM messages"
com = New MySql.Data.MySqlClient.MySqlCommand(sql, conn)
Dim da2 As New MySql.Data.MySqlClient.MySqlDataAdapter()
da2.SelectCommand = com
da2.TableMappings.Add("Table", "Messages")
da2.Fill(ds)
Dim dr As DataRelation
Dim col1 As DataColumn
Dim col2 As DataColumn
col1 = ds.Tables("Users").Columns("id")
col2 = ds.Tables("Messages").Columns("users_id")
dr = New DataRelation("UsersMessages", col1, col2)
ds.Relations.Add(dr)
基本上,我执行两个查询:第一个获取所有用户,第二个获取所有消息。我在 DataSet 中定义了两个表,并通过 DataRelation 将它们链接起来,因此 Users 表是 Messages 表的父表。
如果我想从两个表中选择所有行,这很有效,但是如果我有一个包含 4 个嵌套表和条件查询的更复杂的结构怎么办?
SELECT t1.*, t2*, t3.*, t4.*
FROM table1 t1, table2 t2, table3 t3, table4 t4
WHERE t1.id = 3
AND t2.t1_id = t1.id
AND t3.t2_id = t2.id
AND t4.t3_id = t3.id
如何通过这个单一查询创建一个包含四个表和三个 DataRelation 的 DataSet?
谢谢