我有一个问题,我有超过100名学生的名单,但读者只检索表中的最后一名学生。
string selectQuery = "SELECT * FROM Students WHERE firstName = @first AND lastName = @last";
using (SqlCommand sqlCommand = new SqlCommand(selectQuery, sqlConnection))
{
sqlCommand.Parameters.Add("@first", SqlDbType.VarChar);
sqlCommand.Parameters.Add("@last", SqlDbType.VarChar);
foreach (Student student in studentList)
{
sqlCommand.Parameters["@first"].Value = student.first;
sqlCommand.Parameters["@last"].Value = student.last;
}
using (SqlDataReader sqlReader = sqlCommand.ExecuteReader())
{
while (sqlReader.Read())
{
Console.WriteLine(sqlReader["firstName"].ToString());
Console.WriteLine(sqlReader["lastName"].ToString());
}
}
}
如果要检索所有学生的值,则需要在foreach Student
循环中执行SQL:
string selectQuery = "SELECT * FROM Students WHERE firstName = @first AND lastName = @last";
using (SqlCommand sqlCommand = new SqlCommand(selectQuery, sqlConnection))
{
sqlCommand.Parameters.Add("@first", SqlDbType.VarChar);
sqlCommand.Parameters.Add("@last", SqlDbType.VarChar);
foreach (Student student in studentList)
{
sqlCommand.Parameters["@first"].Value = student.first;
sqlCommand.Parameters["@last"].Value = student.last;
using (SqlDataReader sqlReader = sqlCommand.ExecuteReader())
{
while (sqlReader.Read())
{
Console.WriteLine(sqlReader["firstName"].ToString());
Console.WriteLine(sqlReader["lastName"].ToString());
}
}
}
}
但这真的没有多大意义......你真正想要实现的是什么?您按名字和姓氏搜索 - 您从studentList
提供 - 但是您也只输出您检索到的名字和姓氏 - 这与您传入的名称相同 - 那么为什么要进入数据库呢?
更新:正如@PatrickArtner正确评论 - 创建和处理100个SqlDataReader
实例并不理想。将这个选择过程粘贴到一个存储过程中会更好,这个过程包含一个SELECT
和一个table-valued parameter,它将保存100(或更多)学生ID或信息,根据您的学生ID或信息选择您的学生,然后迭代使用单个SqlDataReader
对象从C#中的存储过程得到的结果集。
问题的原因是您反复覆盖单个2个参数。查看代码中的注释:
string selectQuery = "SELECT * FROM Students WHERE firstName=@first and lastName=@last";
using (SqlCommand sqlCommand = new SqlCommand(selectQuery, sqlConnection))
{
sqlCommand.Parameters.Add("@first", SqlDbType.VarChar);
sqlCommand.Parameters.Add("@last", SqlDbType.VarChar);
foreach (Student student in studentList) // addds 100 students
{
// overwrites the same parameter again and again.... so its only the last one
sqlCommand.Parameters["@first"].Value = student.first;
sqlCommand.Parameters["@last"].Value = student.last;
}
using (SqlDataReader sqlReader = sqlCommand.ExecuteReader())
{// the query uses only 2 params so you need to change the query and the adding
while (sqlReader.Read())
{
Console.WriteLine(sqlReader["firstName"].ToString());
Console.WriteLine(sqlReader["lastName"].ToString());
}
}
}
解决方案使用一个查询搜索所有这些查询,使用来自您的数据的更复杂的WHERE
条件构建以及您的数据所需的更多SqlParameters:
// create all the variable names
var firstVarName = Enumerable.Range(1, studentList.Count)
.Select(i => $"@first_{i}")
.ToList();
var lastVarName = Enumerable.Range(1, studentList.Count)
.Select(i => $"@last_{i}")
.ToList();
var cond = "firstName = {0} and lastName = {1}";
var whereOred = new StringBuilder("WHERE 1 = 0 -- false, just ease of formatting\n");
for (int i = 0; i < firstVarName.Count; i++)
{
whereOred.AppendLine(" OR " + string.Format(cond, firstVarName[i], lastVarName[i]));
}
// adapt query to search all variable names
string selectQuery = $@"
SELECT *
FROM Students
{whereOred.ToString()}";
Console.WriteLine(selectQuery);
using (SqlCommand sqlCommand = new SqlCommand(selectQuery, sqlConnection))
{
// add all the variable names with values from studentList
for(int i=0;i<studentList.Count;i++)
{
sqlCommand.Parameters.Add(firstVarName[i], SqlDbType.VarChar);
sqlCommand.Parameters.Add(lastVarName[i], SqlDbType.VarChar);
sqlCommand.Parameters[firstVarName[i]].Value = studentList[i].first;
sqlCommand.Parameters[lastVarName[i]].Value = studentList[i].last;
}
using (SqlDataReader sqlReader = sqlCommand.ExecuteReader())
{
if (sqlReader.HasRows)
while (sqlReader.Read())
{
Console.WriteLine(sqlReader["firstName"].ToString());
Console.WriteLine(sqlReader["lastName"].ToString());
}
else Console.WriteLine("No match.");
}
}
为我的demodata创建的SqlStatement:
SELECT *
FROM Students
WHERE 1 = 0 -- false, just ease of formatting
OR firstName = @first_1 and lastName = @last_1
OR firstName = @first_2 and lastName = @last_2
OR firstName = @first_3 and lastName = @last_3
OR firstName = @first_4 and lastName = @last_4
OR firstName = @first_5 and lastName = @last_5
OR firstName = @first_6 and lastName = @last_6
OR firstName = @first_7 and lastName = @last_7
OR firstName = @first_8 and lastName = @last_8
OR firstName = @first_9 and lastName = @last_9
OR firstName = @first_10 and lastName = @last_10
OR firstName = @first_11 and lastName = @last_11
OR firstName = @first_12 and lastName = @last_12
OR firstName = @first_13 and lastName = @last_13
OR firstName = @first_14 and lastName = @last_14
OR firstName = @first_15 and lastName = @last_15
OR firstName = @first_16 and lastName = @last_16
OR firstName = @first_17 and lastName = @last_17
OR firstName = @first_18 and lastName = @last_18
OR firstName = @first_19 and lastName = @last_19
OR firstName = @first_20 and lastName = @last_20
添加了SqlParameters以执行我的demodata:
我用了
public class Student { public string first; public string last; }
和
var studentList = Enumerable.Range(1, 20)
.Select(i => new Student { first = $"firstname {i}", last = $"lastname {i}" })
.ToList();
创建一些演示学生列表。