I have a table containing name, surname and email. I want to retrieve them from the table and so i write:
if (LoginAs.SelectedValue == "Administrator")
{
string result;
string query = "Select * from AdminTable where ID='"+ idBox.Text +"'";
cmd1 = new SqlCommand(query, con);
result = Convert.ToString(cmd1.ExecuteScalar());
Response.Redirect("Admin.aspx");
//Admin user = new Admin(idBox.Text, "Active", mail, firstName, LastName, passwordBox.Text);
}
The problem is, it only returns the name field of the specified row even though i wrote "Select *". What is wrong here?
ExecuteScalar returns just the first column of the first row, and ignores the rest.
So you should use ExecuteReader method. An example from MSDN:
using (SqlConnection connection = new SqlConnection(
connectionString))
{
connection.Open();
SqlCommand command = new SqlCommand(queryString, connection);
SqlDataReader reader = command.ExecuteReader();
while (reader.Read())
{
Console.WriteLine(String.Format("{0}", reader[0]));
}
}
Note that the while (reader.Read()) checks whether your query returned (more) results and positions the cursor on the next record, that you can then read. This example prints the first column's value.
The using statement makes sure the connection is closed after use, whatever happens.
Also, don't build your query directly with input from the user (such as the value of a TextBox), use parameters instead to prevent SQL injection attacks.
You must try ExecuteReader() instead of using ExecuteScalar()
ExecuteScaler is used in situation where we have to read a single value.eg:
select count(*) from tablename.
while
ExecuteReader is used for any result set with multiple rows/columns
(e.g., SELECT * from TableName)
Sample code:
string myQuery="Select * from AdminTable where ID=#myid";
SqlCommand cmd=new SqlCommand(myQuery,conn);
cmd.Parameters.AddWithValue("#myid", value);
conn.Open();
SqlDataReader dreader;
dreader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
while (dreader.Read())
{
string Value1= dreader["COl1"].ToString();
string Value2= dreader["COl2"].ToString();
}
dreader.Close();
Always use parameterized Query
You may try cmd1.ExecuteReader() instead.
Related
I've created a query to select the body of a message from the message database. I'm not sure how to execute it to get the body string back and store it. I've tried using ExecuteReader, ExecuteScalar, and ExecuteNonQuery. None of them work. ExecuteScalar was the closest to working but it only returned the body of the message of the first row no matter which row you were trying to view. Anyone know how to do this? It's gotta be a easy fix.
SqlCommand com = new SqlCommand("SELECT Body FROM Messages WHERE MessageID= MessageID", conn);
com.Connection = conn;
com.Connection.Open();
String body;
body = com.ExecuteScalar.ToString;
That's what I have now. Thanks in advance!
What is messageId in your query? You should be doing something like this
SqlCommand com = new SqlCommand("SELECT Body FROM Messages WHERE MessageID = #MessageId");
com.Parameters.AddWithValue("#MessageId", 1); //Replace 1 with messageid you want to get
string s = com.ExecuteScalar().ToString()
You can use SQLDataAdapter and Datatable for this :
SqlCommand com = new SqlCommand("SELECT MessageID,Body FROM Messages WHERE MessageID= MessageID", conn);
SqlDataAdapter dadapter=new SqlDataAdapter();
DataTable dt = new DataTable();
com.Connection = conn;
com.Connection.Open();
String body;
dadapter.SelectCommand=com;
dadapter.Fill(dt);
body = dr.Rows["Body"].toString();
you should try something like this.
SqlCommand com = new SqlCommand("SELECT Body FROM Messages WHERE MessageID= MessageID", conn);
com.Connection = conn;
com.Connection.Open();
String body;
SqlDataReader dr = com.ExecuteReader();
if(dr.HasRows){
while(dr.Read()){
body+=dr["Body"].ToString();
}
}
I hope this works for you.
Based on your reply to Nudier below, you're trying to pass in the messageID of the selected message by using WHERE MessageID = MessageID
The reason this won't work, and the reason you're always getting the first row returned is that SQL doesn't know that MessageID is a variable you're trying to pass in. As far as SQL knows, MessageID is a column name, so all you're asking SQL to do is select the column "Body" of the row where the column MessageID = the column MessageID, so where MessageID equals itself, which always equates to true. And since ExecuteScalar always returns the first cell of the first row, your query will always return all rows from the Messages table, and the executeScalar will grab the first cell.
Hopefully that made sense, if not, just copy your query and run it against your SQL database, you should see what I mean about it returning all rows as the where clause always equals true.
To fix it, you need to take into account what Anuraj said about adding a parameter.
To pass in a variable to a SQL string in code, you need to parameterise it, then add the relevant parameter, so your SQL should become:
SELECT Body FROM Messages WHERE MessageID=#MessageID
(Notice the addition of the # symbol before the parameter name?)
And directly below that line, you need to add the parameter in code using:
com.AddParameterWithValue("#MessageId", MessageId);
(I think that's right, I copied it from Anuraj, I normally do it slightly differently)
Again, to see this working, you can run it directly against the database with a parameter by using
DECLARE #messsageID AS INTEGER
SET #messageID = 1
SELECT Body FROM Messages WHERE MessageID=#messageID
Have a read of this for more details (or if I haven't managed to be entirely clear) http://www.csharp-station.com/Tutorial/AdoDotNet/lesson06
I have to compare a value in a string array to that of a particular column in a database. How do i do this ?
public void setvisibility(string user_ID)
{
SqlDataReader reader = null;
SqlConnection connection = new SqlConnection(WebConfigurationManager.ConnectionStrings["ctd_prrity_dbConnectionSting"].ConnectionString);
connection.Open();\
SqlCommand cmd = new SqlCommand("Select * from Admins );
I need to compare the value of user_ID to the only column in the Admins table !
Here's how you can do it:
using( SqlConnection connection = new SqlConnection(WebConfigurationManager.ConnectionStrings["ctd_prrity_dbConnectionSting"].ConnectionString))
{
connection.Open();
SqlCommand cmd = new SqlCommand("Select 1 from Admins where User_ID=#userid",connection );
cmd.Parameters.AddWithValue("#userid",user_ID);
SqlDataReader reader= cmd.ExecuteReader();
if(reader.HasRows)
{
//user id found
}
}
This method uses a parametrized queries, which are safer than the option given in the answer by Tony since his is prone to SQL Injection attacks.
BTW: you mention "string array" in your question, yet your code only shows a single string as parameter. What did you mean by that?
sqlcommand("SELECT * FROM Admins WHERE column = " & user_ID );
That should work. What that does is it returns the value in the column if it is the same as the user_ID. Basically, if your query returns a value you have a match, if not then you don't.
I have a function which would populate a datatable with the contents of a table. But its showing an annoying invalid column name error with the value I give in the WHERE clause.
public static DataTable GetRequests(string empid)
{
DataTable dt = new DataTable();
string strConnection = ConfigurationManager.AppSettings["connStr"];
using (SqlConnection connection = new SqlConnection(strConnection))
{
connection.Open();
SqlCommand sqlcmd = new SqlCommand();
SqlDataAdapter sAdap = new SqlDataAdapter();
sqlcmd.Connection = connection;
sqlcmd.CommandType = System.Data.CommandType.Text;
sqlcmd.CommandText = "Select * from requests Where emp_id=P001";
sAdap.SelectCommand = sqlcmd;
sAdap.Fill(dt);
}
return dt;
}
Now with this i am getting the error at
sAdap.fill
and the error is
invalid column name P001
I'm stumped at this. Any ideas why I'm facing this issue?
If it's a string constant, surround it with single quotes. 'P001' or better still, paramaratise it.
You need string delimiters around the value. Change the line to:
sqlcmd.CommandText = "Select * from requests Where emp_id='P001'";
with the single quotes around P001 and you should be fine.
You need to use single quote.
where emp_id='P001'
It looks like your P001 data is a string type. Try quoting it with single quotes before feeding it in as an sql string.
sqlcmd.CommandText = "Select * from requests Where emp_id='P001'";
If that doesn't work (though it should) you can try the like statement as in:
sqlcmd.CommandText = "Select * from requests Where emp_id like 'P001'";
You need to surround your (apparrently) text criteria with single quotes:
sqlcmd.CommandText = "Select * from requests Where emp_id = 'P001'";
Trying to do a recordset, I just want one column of data, but this code is giving me an error.. I'm an ASP.NET newb, can anyone help?:
System.Data.SqlClient.SqlException: Invalid column name
'CustomerName'.
using (SqlConnection con = new SqlConnection(DB.GetDBConn()))
{
con.Open();
using (IDataReader dr = DB.GetRS("select CustomerName from Customer where CustomerID=" + Customer.CustomerID, con))
{
string CustomerName = "CustomerName";
}
}
String EncCustomerName = Encrypt(CustomerName.Replace(".", "").Replace("-", ""),"1");
Question #2: How do I bind the database content to the CustomerName string? It seems like its only returning "CustomerName" as the value for CustomerName string.. I would like it to return the database data for CustomerName string.. Help?
Suggested to use a ExecuteScalar, so i modified the request to this
using (var con = new SqlConnection(DB.GetDBConn()))
using (var cmdContrib = new SqlCommand("SELECT CustomerName FROM Customer WHERE CustomerID=" + ThisCustomer.CustomerID, con))
{
con.Open();
string CustomerName = cmdContrib.ExecuteScalar();
}
And i Get this error:
"string CustomerName = cmdCust.ExecuteScalar();"
CS0266: Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a cast?)
To answer your second question:
// Set it here so you can access it outside the scope of the using statement
string CustomerName = "";
using (SqlConnection con = new SqlConnection(DB.GetDBConn()))
{
con.Open();
using (IDataReader dr = DB.GetRS("select CustomerName from Customer where CustomerID=" + Customer.CustomerID, con))
{
while (dr.Read())
CustomerName = dr["CustomerName"].ToString();
}
}
}
If you're sure you'll only get one CustomerName result, using a DataReader is a bit of an overkill.
SqlCommand.ExecuteScalar Example
string CustomerName = "";
using (SqlConnection con = new SqlConnection(DB.GetDBConn()))
{
SqlCommand cmd = new SqlCommand("SELECT CustomerName FROM Customer WHERE CustomerID = " + Customer.CustomerID, con);
cmd.CommandType = CommandType.Text;
con.Open();
CustomerName = Convert.ToString(cmd.ExecuteScalar());
}
SqlCommand.ExecuteScalar Method
Additional Info
ExecuteScalar returns an object, so you'll need to convert the returned value to the proper type (in this case, string).
Also, you should declare your CustomerName value outside of the using blocks (as I did in my example) - otherwise it will be scoped to the using blocks and not available outside of them.
It means that either CustomerName or CustomerID is not a valid column within your database. Check your table again.
Make sure you are trying to connect correct database.
See CustomerName column should be in Customer table. check spelling also
First, debug and check the value of:
DB.GetDBConn()
You will verify that you are going to the same in Studio as you are in the program.
I think it is the spelling somewhere between the db and your code.
Once you get past the error, you need to fix this:
{
string CustomerName = "CustomerName";
}
You are not accessing the reader, try some kind of tutorial for that stuff.
Try doing a select * from customer where ... and put a breakpoint on your using datareader statement. Then use quick-watch on the datareader object to investigate the columns exposed in the recordset.
Or you could run the select statement on your db of choice to ensure that the column name is the same.
I agree with Madhur above, your column name is not spelled correctly. Or you are not connecting to the correct db.
Hope this helps
Suppose I am calling a query "SELECT name, city, country FROM People". Once I execute my SqlDataReader do columns come in the same order as in my sql query?
In other words can I rely that the following code will always work correctly:
SqlConnection connection = new SqlConnection(MyConnectionString);
SqlCommand command = new SqlCommand();
command.Connection = connection;
command.CommandText = "SELECT [name], [city], [country] WHERE [id] = #id";
try
{
connection.Open();
SqlDataReader reader = command.ExecuteReader(System.Data.CommandBehavior.SingleRow);
if (reader.Read())
{
// Read values.
name = reader[0].ToString();
city = reader[1].ToString();
country = reader[2].ToString();
}
}
catch (Exception)
{
throw;
}
finally
{
connection.Close();
}
Also how much performance do I lose if I use column names instead of ordinals (reader["name"])?
Are there any official microsoft documents describing the behavior of column ordering in SqlDataReader?
Yes they do but you can also use SqlDataReader.GetName(ordinal) and SqlDataReader.GetOrdinal(name).
As for performance, I think it's probably extremely insignificant compared to the overhead of say, retrieving the next row of data.
I totally agree with Josh - the positions of the fields are indeed such as you specify them in your SQL query text.
BUT: I would still prefer to use the column names, since it's more robust. E.g. what if you need to add a field to your SQL query?
command.CommandText = "SELECT [name], [jobtitle], [city], [country] WHERE [id] = #id";
Now suddenly you have to rewrite all your code to change the positions....
What I normally do outside the loop that enumerates through all the rows returned by the data reader is determine the positions of each field I'm interested in:
int namePosition = reader.GetOrdinal("name");
int cityPosition = reader.GetOrdinal("city");
and then I use these positions inside my loop handling the data to get quick access to the individual fields. That way you determine the positions only once, but you're using positions in your looping over the data - the best of both worlds! :-)
Marc
This example is the most maintainable and easiest to read:
int? quantity = reader.Get<int?>("Quantity");
Guid token = reader.Get<Guid>("Token");
It relies on the following extension method I created. It performs DB null checks, provides an informative error message when field is not found, and does not break when columns are re-aligned.
internal static T Get<T>(this SqlDataReader reader, string fieldName)
{
int ordinal;
try
{
ordinal = reader.GetOrdinal(fieldName);
}
catch (IndexOutOfRangeException)
{
throw new IndexOutOfRangeException(string.Format("Field name '{0}' not found.", fieldName));
}
return !reader.IsDBNull(ordinal) ? (T)reader.GetValue(ordinal) : default(T);
}