Check if record in a table exist in a database through ExecuteNonQuery

问题

in my program i need to check if a record in the database already exists in the table using the if statement. using c# i am trying to do this through an sql connection. as i supposed that the ExecuteNonQuery(); command returns an integer value, if my supposing is true, i want to know what value is true to know that a certain record exists in the table or not. here's a sample of my code:

using (SqlConnection sqlConnection = dbUtil.GetSqlConnection(dbUtil.GetConnectionStringByName("NonConnectionString")))
{
    using (SqlCommand sqlCommand = new SqlCommand("SELECT * from users where user_name like 'Adam' AND password like '123456'", sqlConnection))
    {
        sqlresult = sqlCommand.ExecuteNonQuery();
    }
}

considering sqlresult has been initialized previously in the main as int sqlresult; so i would like to know, that if this user 'Adam' exists in the database or not. and if he exists, then i want to proceed with an 'if' statement saying for example:

if(sqlresult == 0)
{
   MessageBox.Show("Adam exists!");
}

so i just don't know the integer that it should return, and i am either not sure that this is the proper way to do it so.

thank you.

回答1:

If you want to check if the user exists, you have to change your sql and use COUNT or EXISTS:

So instead of

SELECT * from users where user_name like 'Adam' AND password like '123456'

this

SELECT COUNT(*) from users where user_name like 'Adam' AND password like '123456'

Now you can use ExecuteScalar to retrieve the count of users with this username and password:

int userCount = (int) sqlCommand.ExecuteScalar();
if(userCount > 0)
    // user exists ....

Note that you should use sql-parameters to prevent sql-injection:

using (SqlCommand sqlCommand = new SqlCommand("SELECT COUNT(*) from users where user_name like @username AND password like @password", sqlConnection))
{
    sqlConnection.Open();
    sqlCommand.Parameters.AddWithValue("@username", userName);
    sqlCommand.Parameters.AddWithValue("@password", passWord);
    int userCount = (int) sqlCommand.ExecuteScalar();
    ...
}

回答2:

You should be using ExecuteScalar for cheking if the record exists. ExecuteNonQuery runs a transact-SQL statement against the connection and returns the number of rows affected for an UPDATE, INSERT, or DELETE. It doesn't apply for SELECT statements

回答3:

I would use Select Top 1 Id rather than the count(*) because it can be much faster

回答4:

You should do a count(1) on the table instead of a select * and then executescalar to get that integer value.

Using your existing code I would change it to be:

using (SqlConnection sqlConnection = dbUtil.GetSqlConnection(dbUtil.GetConnectionStringByName("NonConnectionString")))
        {
            using (SqlCommand sqlCommand = new SqlCommand("SELECT count(1) from users where user_name = 'Adam' AND password = '123456'", sqlConnection))
            {
                sqlresult = sqlCommand.ExecuteNonQuery();
            }
        }

Please note that I have used equals values instead of like values.

Also if I were do to this I would change your inline sql to use a stored procedure.

回答5:

If someday you want to use EF just do:

private MyDb db = new MyDb();

public bool UserExists(string userName, string password){

   return db.Users.Any(x => x.user_name.Equals(userName, StringComparison.InvariantCultureIgnoreCase)
                         && x.password.Equals(password, StringComparison.InvariantCultureIgnoreCase));
}

Or do a generic method, so you can handle multiple entites:

public bool EntityExists<T>(Expression<Func<T, bool>> predicate) where T : class, new()
{
   return db.Set<T>().Any(predicate);
}

Usage:

EntityExists<Users>(x => x.user_name.Equals(userName, StringComparison.InvariantCultureIgnoreCase)
                      && x.password.Equals(password, StringComparison.InvariantCultureIgnoreCase));

来源：https://stackoverflow.com/questions/17903092/check-if-record-in-a-table-exist-in-a-database-through-executenonquery

标签

sqlcommand