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Condition to check a specific value from table asp.net c#?

string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
SqlConnection con = new SqlConnection(constr);
con.Open();
string selstatus = "select status from Status where c_email=#c_email";
SqlCommand cmd = new SqlCommand(selstatus, con);
cmd.Parameters.AddWithValue("#c_email", Session["user"].ToString());
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
DataTable dt = new DataTable();
string value = Session["user"].ToString();
DataRow[] row =dt.Select(value);
sda.Fill(ds);
sda.Fill(dt);
if(row!=null){
Label13.Text = ds.Tables[0].Rows[0]["status"].ToString();
}else{
Label13.Text = "No response from mechanic";
}
cmd.ExecuteNonQuery();
con.Close();
I have been finding way to check a specific email id exist in the table. But I can't query what is the correct format to do so. I just need that if a specific email id is available then a message should be displayed.

There is no need for the DataAdapter, DataTable etc. Simply count the number of records
select count(1) from Status where c_email=#c_email
and then on the SqlCommand just use ExecuteScalar:
string selstatus = "select count(1) from Status where c_email=#c_email";
SqlCommand cmd = new SqlCommand(selstatus, con);
cmd.Parameters.AddWithValue("#c_email", Session["user"].ToString());
var count = cmd.ExecuteScalar();
// if count=0 the email doesnt exist

how to query select data from table where first 3 character defined

i want to filter dropdownlist by first 3 charater on job code, this is my query
string result3 = Checked_By.ToString().Substring(0, 3);
SqlCommand cmd = new SqlCommand(" SELECT [Kode], [Nama]
FROM [Job] WHERE LEFT(Kode, = '" + result3 + "') ORDER BY
Nama ASC", con);
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
con.Open();
sda.Fill(dt);
con.Close();

This is straightforward fix. First, fix SQL syntax error (SQL LEFT function takes 2 parameter, a string and a number. LEFT('abcdef', 3) returns abc), next rewrite using parameters. Something like this.
string result3 = Checked_By.ToString().Substring(0, 3);
SqlCommand cmd = new SqlCommand(" SELECT [Kode], [Nama] FROM [Job] WHERE LEFT(Kode, 3) = #result3 ORDER BY Nama", con); //ASC is OK but not required as it is default option
cmd.Parameters.Add("#result3", SqlDbType.Char, 3).Value = result3;
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
//con.Open(); //not necessary. SqlDataAdapter.Fill opens connection if it needs
sda.Fill(dt);
//con.Close(); //and closed if it wasn't open before .Fill

"SELECT [Kode], [Nama]
FROM [Job] WHERE SUBSTR(Kode,1,3) = '" + result3 + "' ORDER BY
Nama ASC"

SQL Query with single quotes retrieves no record

I am currently fetching a dataset from following query select * from TableName WHERE ColumnName ='values''s' query executes without any error and return dataset was empty rows. When i execute the same in SQL Worksheets it return data
Following code for ref.
string sqlQuery = "select * from TableName WHERE Name ='McNaught''s'";
SqlConnection conn = new SqlConnection(ConnectionString);
SqlCommand cmd = conn.CreateCommand();
cmd.CommandText = sqlQuery;
SqlDataAdapter adapter = new SqlDataAdapter();
adapter.SelectCommand = cmd;
DataSet ds = new DataSet();
conn.Open();
adapter.Fill(ds);
conn.Close();

Please try this:
string sqlQuery = 'SELECT * FROM TableName WHERE Name ="McNaught\'s"'

The Problem is you need to provide Escape Sequence before the second ' so that compiler can distinguish between the previous apostrophe. Try this.
string sqlQuery = "select * from TableName WHERE Name ='McNaught'\'s'";
SqlConnection conn = new SqlConnection(ConnectionString);
SqlCommand cmd = conn.CreateCommand();
cmd.CommandText = sqlQuery;
SqlDataAdapter adapter = new SqlDataAdapter();
adapter.SelectCommand = cmd;
DataSet ds = new DataSet();
conn.Open();
adapter.Fill(ds);
conn.Close();

Try this it will work with \ escape sequence.
string sqlQuery = "select * from TableName WHERE Name ='McNaught\"s'";

inserting table values from one table to another

I have two tables in SQL. I need to insert the data from one table to another.I have done it successfully, but the problem is I need to extra add one column value from text box. I will show my code
protected void Button2_Click1(object sender, EventArgs e)
{
String str1 = "insert into table2(Code,Qty,UPrice,Total) select Code,Qty,UPrice,Total from table1 ;";
SqlCommand cmd = new SqlCommand(str1, con);
con.Open();
SqlDataAdapter da1 = new SqlDataAdapter();
cmd.Parameters.AddWithValue("#Num", TextBox1.Text);
da1.SelectCommand = cmd;
DataSet ds1 = new DataSet();
da1.Fill(ds1, "Code");
GridView1.DataSource = ds1;
con.Close();
}
Here I need to add Num to my table2 that is not in the table1. So I need to add it from textbox1. How can I add that Num value to the all data from textbox.

protected void Button2_Click1(object sender, EventArgs e)
{
String str1 = "insert into table2(Code,Qty,UPrice,Total,Num) select Code,Qty,UPrice,Total,'"+TextBox1.Text+"' from table1 ;";
SqlCommand cmd = new SqlCommand(str1, con);
con.Open();
SqlDataAdapter da1 = new SqlDataAdapter();
da1.SelectCommand = cmd;
DataSet ds1 = new DataSet();
da1.Fill(ds1);
GridView1.DataSource = ds1;
con.Close();
}

Just set your dynamic value in a temporary variable.
String temp = TextBox1.Text;
Now use this variable with your insert query
String str1 = "insert into table2(Code,Qty,UPrice,Total,Num)
select Code,Qty,UPrice,Total,'"+temp+"' from table1 ;";

I guess Num is your column name, you can modify your query as
insert into table2(Code,Qty,UPrice,Total, Num)
select Code,Qty,UPrice,Total, textbox1.Text from table1 ;

OleDb Exception in c# “No value given for one or more required parameters.” while trying to Select Data from Access database

I have the following code:
public DataTable opencon(PAL.property objpal)
{
string query = "Select UserId,Firstname,UserType from TBL_USER_LOGIN where Username=#username and Password=#password and Status=1";
OleDbCommand objcmd = new OleDbCommand();
objcmd.CommandText = query;
objcmd.Connection = oldbcon;
oldbcon.Open();
objcmd.Parameters.Add("#username", OleDbType.VarChar).Value = objpal.username;
objcmd.Parameters.Add("#password", OleDbType.VarChar).Value = objpal.Password;
DataTable dt = new DataTable();
OleDbDataAdapter adp = new OleDbDataAdapter(objcmd);
adp.Fill(dt);
return dt;
}
Here I want to fetch some values from the table according to a condition,
but when I run this code it shows the following error:
Even though I passed the correct parameters value in #username and #password. How can I resolve this error? Please help.

Try This:
public DataTable opencon(PAL.property objpal)
{
string query = "Select UserId,Firstname,UserType from TBL_USER_LOGIN where Username=? and Password=? and Status=1";
OleDbCommand objcmd = new OleDbCommand();
objcmd.CommandText = query;
objcmd.Connection = oldbcon;
oldbcon.Open();
objcmd.Parameters.Add("#username", OleDbType.VarChar).Value = objpal.username;
objcmd.Parameters.Add("#password", OleDbType.VarChar).Value = objpal.Password;
DataTable dt = new DataTable();
OleDbDataAdapter adp = new OleDbDataAdapter(objcmd);
adp.Fill(dt);
return dt;
}

Try This
string query = "Select [UserId],[Firstname],[UserType] from [TBL_USER_LOGIN] where [Username]=? and [Password]=? and [Status]=1";