Develop Reference

How to check if a table within a .sqlite file exists using C#

I am creating a WPF program where the user can create lists of products that are stored in an .sqlite database. Each product list has it's own table inside the database, who's name is chosen by the user. I need to figure out if the table already exists when I try to create them so that if they do I can ask the user to choose a different name.
I know
SELECT name FROM sqlite_master WHERE type='table' AND name='{table_name}';
is the query used to check if a table exists but how do I get this statement in boolean form using c#?
I would assume something similar to
SQLiteConnection connection = new SQLiteConnection("Data Source=filepath;Version=3");
sql = "SELECT name FROM sqlite_master WHERE type='table' AND name='{table_name}';"
SQLiteCommand command = new SQLiteCommand(sql, connection);
command.something();
But I am unsure what function of command to use to get the Boolean value of this query and how to format it.

ExecuteNonQuery() returns -1 for SELECT statements, so it would be inappropriate to use it in this case.
try using ExecuteReader(), which returns a SqliteDataReader class that contains the HasRows property, in which should be true in case of a non-empty result set.
the code:
SQLiteConnection connection = new SQLiteConnection("Data Source=filepath;Version=3");
sql = "SELECT name FROM sqlite_master WHERE type='table' AND name='{table_name}';"
SQLiteCommand command = new SQLiteCommand(sql, connection);
try
{
var result = command.ExecuteReader();
if (result.HasRows)
{
// INSERT statement here
}
}
catch (Exception e)
{
throw e;
}

How to fix a Select Statement returning -1

I'm trying to return the InvoiceID from the database but it keeps returning -1.
I'm trying to select the invoice ID From tblInvoice to display it on a form and also use it to insert it into a second table that has a one to many relationship with tblInvoice.
try
{
conn = new SqlConnection(conString);
conn.Open();
string select = "SELECT TOP 1 FId FROM tblFaktuur ORDER BY FId DESC";
SqlCommand cmd1 = new SqlCommand(select, conn);
i = (int)cmd1.ExecuteNonQuery();
}
catch (Exception e)
{
System.Windows.Forms.MessageBox.Show("ReadInvoiceNumber()" + e.Message);
}
return i;
When Running the query SELECT TOP 1 FId FROM tblFaktuur ORDER BY FId DESC in SQL Server it returns the value of 6 which is the last Invoice ID.

Think about that code. How do you expect a method called ExecuteNonQuery to handle a Query? Change that to ExecuteScalar. This work Perfectly Thanks to #Camilo Terevinto, i will be doing research to improve my skills atleast i know what is wrong.

asp.net : INSERT in oracle table only if the clause WHERE of the UPDATE find zero records

I am developping an asp.net application. I would like to know how to update a field of an oracle table record, and if the record is not present, inserting it.
I have a table with the following fields NAME and SURNAME.
I would like to change the SURNAME to "new_surname" of the record where the NAME equals="name". However, if none of the records in table contains a field NAME equals to name I would like to insert a new record (SURNAME=new_surname and NAME=name).
This is my code :
OracleConnection connection = new OracleConnection(connectionstring);
try
{
connection.Open();
OracleCommand command = connection.CreateCommand();
string sql = "UPDATE TABLE SET SURNAME=\'new_surname\' WHERE NAME=\'name\'";
command.CommandText = sql;
command.ExecuteNonQuery();
connection.Close();
}
catch (Exception exp)
{
}
Is there an optimal way to do the insert only if the update find zero records matching the "where" clause. I was thinking of first doing a select count of the record matching the "where" clause, and then if I found zero results I would do an insert, and if I found at least one result I would do an update. But I find this solution a little bit heavy.

Cannot test but you could try this
using(OracleConnection connection = new OracleConnection(connectionstring))
using(OracleCommand command = connection.CreateCommand())
{
connection.Open();
string sql = #"MERGE INTO TABLE t USING dual on(name='name')
WHEN NOT MATCHED THEN INSERT (name, surname) values ('name', 'new_surname')
WHEN MATCHED THEN UPDATE SET surname = 'new_surname'";
command.CommandText = sql;
command.ExecuteNonQuery();
}

you can modify the query as :
string sql = #"IF EXISTS (SELECT * FROM \'Given Table\' WHERE NAME=\'name\') THEN
BEGIN
--your update query
END
ELSE
BEGIN
-- your Insert Query
END
END IF";

How to display values into gridview from sql databese

In my database i have three tables. One For employs where i keep their names, ids, salary... In the second one named Project i keep id, location, name. in the third one named WorksOn i keep the id from employs and the id from project. In my Asp .Net web site in gridview i need to display the employee's name and the name of the project that he is working.
string connect =
WebConfigurationManager.ConnectionStrings["ApplicationServices"].ConnectionString;
SqlConnection con = new SqlConnection(connect);
try
{
con.Open();
}
catch (Exception err)
{
string error = err.Message;
con.Close();
}
SqlCommand command = new SqlCommand();
command.Connection = con;
SqlDataReader reader;
command.CommandText = "SELECT * FROM WorksON ";
reader= command.ExecuteReader();
In data source in gridview if i choose to display the values from WorksOn table it shows the id from employs and the id from project but what i need is to show the names on the employs and project.
I know that i need to do something with dataset but i don't know who.

Your SQL command must JOIN the related tables. Something like:
SELECT * FROM WorksOn JOIN Employee on WorksOn.EmployeeId = Employee.Id
Note that you should not SELECT "*" (all columns). You should only SELECT those columns that are necessary to your data view for performance.

On your SQL command, you don't mention anything about Employs. You need to use a JOIN SQL command in order to get both the employee name and the company name.
And instead of using "SELECT * FROM...", consider using the columns name instead, because you're not trying to get all the columns display. And it will help us understand the name of the columns to further help you.

Use a JOIN query like this:
SELECT project.projectName, employee.Name
FROM (worksOn
INNER JOIN employee
ON (worksOn.employeeId = employee.employeeId))
INNER JOIN project
ON (project.projectId = employee.employeeId)
AND (worksOn.projectId = project.projectId)

How to check if mysql table is empty?

How to check if my table is empty from C#?
I have something like:
public MySqlConnection con;
public MySqlCommand cmd;
con = new MySqlConnection(GetConnectionString());
con.Open();
cmd = new MySqlCommand("SELECT * FROM data;", con);
Or I don't need to call SELECT statement?

You can use COUNT(*) with no WHERE close and see if exactly how many rows exist with the result.
Or you can do a SELECT (id) FROM tablename with no WHERE clause and if no rows are returned then the table is empty.

I'll give you an example in C# good luck
public bool checkEmptyTable(){
try
{
MySql.Data.MySqlClient.MySqlCommand com = new MySql.Data.MySqlClient.MySqlCommand();
conn = new MySql.Data.MySqlClient.MySqlConnection("YOUR CONNECTION");
com.Connection = conn;
com.CommandText = "SELECT COUNT(*) from data";
int result = int.Parse(com.ExecuteScalar().ToString());
return result == 0; // if result equals zero, then the table is empty
}
finally
{
conn.Close();
}
}

If 'data' might be a big table you would be better with this (where pkdata is your primary key field)
SELECT COUNT(*) FROM data WHERE pkdata = (SELECT pkdata FROM data LIMIT 1);
This will run very quickly whether you have 0 rows in 'data' or millions of rows. Using SELECT with no WHERE or ORDER BY means it just pulls the first row available, LIMIT 1 stops it getting more than 1.
Maybe something to look for if you have a program that ran very quickly six months ago but now runs like a dog in treacle!

SELECT COUNT(*)
FROM table
WHERE `col_name` IS NOT NULL