问题
I am using these lines of code to check if the record exists or not.
SqlCommand check_User_Name = new SqlCommand(\"SELECT * FROM Table WHERE ([user] = \'\" + txtBox_UserName.Text + \"\') \", conn);
int UserExist = (int)check_User_Name.ExecuteScalar();
But I am getting an error:
Object reference not set to an instance of an object.
I want to do:
if (UserExist > 0)
// Update record
else
// Insert record
回答1:
ExecuteScalar returns the first column of the first row. Other columns or rows are ignored. It looks like your first column of the first row is null, and that's why you get NullReferenceException when you try to use the ExecuteScalar method.
From MSDN;
Return Value
The first column of the first row in the result set, or a null reference if the result set is empty.
You might need to use COUNT in your statement instead which returns the number of rows affected...
Using parameterized queries is always a good practise. It prevents SQL Injection attacks.
And Table is a reserved keyword in T-SQL. You should use it with square brackets, like [Table] also.
As a final suggestion, use the using statement for dispose your SqlConnection and SqlCommand:
SqlCommand check_User_Name = new SqlCommand("SELECT COUNT(*) FROM [Table] WHERE ([user] = @user)" , conn);
check_User_Name.Parameters.AddWithValue("@user", txtBox_UserName.Text);
int UserExist = (int)check_User_Name.ExecuteScalar();
if(UserExist > 0)
{
//Username exist
}
else
{
//Username doesn't exist.
}
回答2:
The ExecuteScalar method should be used when you are really sure your query returns only one value like below:
SELECT ID FROM USERS WHERE USERNAME = 'SOMENAME'
If you want the whole row then the below code should more appropriate.
SqlCommand check_User_Name = new SqlCommand("SELECT * FROM Table WHERE ([user] = @user)" , conn);
check_User_Name.Parameters.AddWithValue("@user", txtBox_UserName.Text);
SqlDataReader reader = check_User_Name.ExecuteReader();
if(reader.HasRows)
{
//User Exists
}
else
{
//User NOT Exists
}
回答3:
sqlConnection.Open();
using (var sqlCommand = new SqlCommand("SELECT COUNT(*) FROM Table WHERE ([user] = '" + txtBox_UserName.Text + "'", sqlConnection))
{
SqlDataReader reader = sqlCommand.ExecuteReader();
if (reader.HasRows)
{
lblMessage.Text ="Record Already Exists.";
}
else
{
lblMessage.Text ="Record Not Exists.";
}
reader.Close();
reader.Dispose();
}
sqlConnection.Close();
回答4:
MySqlCommand cmd = new MySqlCommand("select * from table where user = '" + user.Text + "'", con);
MySqlDataAdapter da = new MySqlDataAdapter(cmd);
DataSet ds1 = new DataSet();
da.Fill(ds1);
int i = ds1.Tables[0].Rows.Count;
if (i > 0) {
// Exist
}
else {
// Add
}
回答5:
I would use the "count" for having always an integer as a result
SqlCommand check_User_Name = new SqlCommand("SELECT count([user]) FROM Table WHERE ([user] = '" + txtBox_UserName.Text + "') " , conn);
int UserExist = (int)check_User_Name.ExecuteScalar();
if (UserExist == 1) //anything different from 1 should be wrong
{
//Username Exist
}
回答6:
Use try catch:
try
{
SqlCommand check_User_Name = new SqlCommand("SELECT * FROM Table WHERE ([user] = '" + txtBox_UserName.Text + "') ", conn);
int UserExist = (int)check_User_Name.ExecuteScalar();
// Update query
}
catch
{
// Insert query
}
回答7:
You can write as follows:
SqlCommand check_User_Name = new SqlCommand("SELECT * FROM Table WHERE ([user] = '" + txtBox_UserName.Text + "') ", conn);
if (check_User_Name.ExecuteScalar()!=null)
{
int UserExist = (int)check_User_Name.ExecuteScalar();
if (UserExist > 0)
{
//Username Exist
}
}
回答8:
I was asking myself the same question, and I found no clear answers, so I created a simple test.
I tried to add 100 rows with duplicate primary keys and measured the time needed to process it. I am using SQL Server 2014 Developer and Entity Framework 6.1.3 with a custom repository.
Dim newE As New Employee With {.Name = "e"}
For index = 1 To 100
Dim e = employees.Select(Function(item) item.Name = "e").FirstOrDefault()
If e Is Nothing Then
employees.Insert(newE)
End If
Next
2.1 seconds
Dim newE As New Employee With {.Name = "e"}
For index = 1 To 100
Try
employees.Insert(newE)
Catch ex As Exception
End Try
Next
3.1 seconds
回答9:
sda = new SqlCeDataAdapter("SELECT COUNT(regNumber) AS i FROM tblAttendance",con);
sda.Fill(dt);
string i = dt.Rows[0]["i"].ToString();
int bar = Convert.ToInt32(i);
if (bar >= 1){
dt.Clear();
MetroFramework.MetroMessageBox.Show(this, "something");
}
else if(bar <= 0) {
dt.Clear();
MetroFramework.MetroMessageBox.Show(this, "empty");
}
回答10:
try this
public static bool CheckUserData(string phone, string config)
{
string sql = @"SELECT * FROM AspNetUsers WHERE PhoneNumber = @PhoneNumber";
using (SqlConnection conn = new SqlConnection(config)
)
{
conn.Open();
using (SqlCommand cmd = new SqlCommand(sql, conn))
{
cmd.Parameters.AddWithValue("@PhoneNumber", phone);
SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
if (reader.HasRows)
{
return true; // data exist
}
else
{
return false; //data not exist
}
}
}
}
回答11:
Use the method Int.Parse() instead. It will work.
回答12:
protected void btnsubmit_Click(object sender, EventArgs e)
{
string s = @"SELECT * FROM tbl1 WHERE CodNo = @CodNo";
SqlCommand cmd1 = new SqlCommand(s, con);
cmd1.Parameters.AddWithValue("@CodNo", txtid.Text);
con.Open();
int records = (int)cmd1.ExecuteScalar();
if (records > 0)
{
Response.Write("<script>alert('Record not Exist')</script>");
}
else
{
Response.Write("<script>alert('Record Exist')</script>");
}
}
private void insert_data()
{
SqlCommand comm = new SqlCommand("Insert into tbl1(CodNo,name,lname,fname,gname,EmailID,PhonNo,gender,image,province,district,village,address,phonNo2,DateOfBirth,school,YearOfGraduation,exlanguage,province2,district2,village2,PlaceOfBirth,NIDnumber,IDchapter,IDpage,IDRecordNumber,NIDCard,Kankur1Year,Kankur1ID,Kankur1Mark,Kankur2Year,Kankur2ID,Kankur2Mark,Kankur3Year,Kankur3ID,Kankur3Mark) values(@CodNo,N'" + txtname.Text.ToString() + "',N'" + txtlname.Text.ToString() + "',N'" + txtfname.Text.ToString() + "',N'" + txtgname.Text.ToString() + "',N'" + txtemail.Text.ToString() + "','" + txtphonnumber.Text.ToString() + "',N'" + ddlgender.Text.ToString() + "',@image,N'" + txtprovince.Text.ToString() + "',N'" + txtdistrict.Text.ToString() + "',N'" + txtvillage.Text.ToString() + "',N'" + txtaddress.Value.ToString() + "','" + txtphonNo2.Text.ToString() + "',N'" + txtdbo.Text.ToString() + "',N'" + txtschool.Text.ToString() + "','" + txtgraduate.Text.ToString() + "',N'" + txtexlanguage.Text.ToString() + "',N'" + txtprovince1.Text.ToString() + "',N'" + txtdistrict1.Text.ToString() + "',N'" + txtvillage1.Text.ToString() + "',N'" + txtpbirth.Text.ToString() + "','" + txtNIDnumber.Text.ToString() + "','" + txtidchapter.Text.ToString() + "', '" + txtidpage.Text.ToString() + "','" + txtrecordNo.Text.ToString() + "',@NIDCard,'" + txtkankuryear1.Text.ToString() + "','" + txtkankurid1.Text.ToString() + "','" + txtkankurscore1.Text.ToString() + "','" + txtkankuryear2.Text.ToString() + "','" + txtkankurid2.Text.ToString() + "','" + txtkankurscore2.Text.ToString() + "','" + txtkankuryear3.Text.ToString() + "','" + txtkankurid3.Text.ToString() + "','" + txtkankurscore3.Text.ToString() + "')", con);
flpimage.SaveAs(Server.MapPath("~/File/") + flpimage.FileName);
string img = @"~/File/" + flpimage.FileName;
flpnidcard.SaveAs(Server.MapPath("~/Tazkiera/") + flpnidcard.FileName);
string img1 = @"~/Tazkiera/" + flpnidcard.FileName;
comm.Parameters.AddWithValue("CodNo", Convert.ToInt32(txtid.Text));
comm.Parameters.AddWithValue("image", flpimage.FileName);
comm.Parameters.AddWithValue("NIDCard", flpnidcard.FileName);
comm.ExecuteNonQuery();
con.Close();
Response.Redirect("~/SecondPage.aspx");
//Response.Write("<script>alert('Record Inserted')</script>");
}
}
来源：https://stackoverflow.com/questions/21302244/check-if-a-record-exists-in-the-database