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How to sovle Query Issue throwing MySql.Data.MySqlClient.MySqlException

I am doing a project with C# and I have this error:
MySql.Data.MySqlClient.MySqlException (0x80004005): You have an error
in your SQL syntax; check the manual that corresponds to your MariaDB
server version for the right syntax to use near ' last_name= , email=
, phone= , address= WHERE id= 6' at line 1
I know this is a query error, but I tried many things and I don't see the issue.
My query is this:
cm = new MySqlCommand("UPDATE customers SET first_name= " + txtNombre.Text + "," + " last_name= " + txtApellidos.Text + "," + " email= " + txtEmail.Text + "," + " phone= " + txtTelefono.Text + "," + " address= " + txtDireccion.Text + " WHERE id= " + dgvClient.SelectedRows[0].Cells[0].Value.ToString() , con);

There should be single quotes around the text that you want to inject into the query, so it will look like this:
var query = "UPDATE customers SET first_name= '" + txtNombre.Text + "'";
This is the easiest solution but is advised against, mostly because of a possiblity for 'sql injection'. The easiest way to show this is by using the name O'Brian, because of the quote the database will think that the name is only O and then see it followed by Brian that it doesn't know what to do with and gives an error. Some people can use this to add other things to your query to cause harm to your database (like dropping tables or the whole database)
It is advised to use parameters, this solves this whole sql injection issue. Your code will look as follows:
cm = new MySqlCommand("UPDATE customers SET first_name=#first_name, last_name=#apellidos WHERE id=#id", con) ;
cm.Parameters.AddWithValue("#first_name", txtNombre.Text);
cm.Parameters.AddWithValue("#apellidos", txtApellidos.Text);
cm.Parameters.AddWithValue("#id", dgvClient.SelectedRows[0].Cells[0].Value.ToString());
It is best to always use parameters for your query, you can also look into using a framework like Entity Framework that does this automatically for you.

How do I solve the SQL(query) missing database error?

Error Code : 1 (SQLITE_ERROR)
Caused By : SQL(query) error or missing database.
(near "=": syntax error (code 1): , while compiling: DELETE FROM notesWHEREid='1')
This is the error i am getting:
This is the part of my database handler that is the source of this error:
public void deleteNote(String id) {
SQLiteDatabase db = this.getWritableDatabase();
String deleteQuery="DELETE FROM " + DatabaseValues.TABLE_NOTES + "WHERE" + DatabaseValues.NOTES_ID + "= '" + id + "'";
db.execSQL(deleteQuery);
db.close();
}

If you are using C# 6.0, using interpolated strings makes life much easier for strings concatenation, avoiding such silly errors:
String deleteQuery= $"DELETE FROM {DatabaseValues.TABLE_NOTES} WHERE {DatabaseValues.NOTES_ID} = id";
Note: $ operator is available in C# 6.0. Also you should take a look into how to build parameterized queries because passing parameters like this can expose you to SQL injection.

Change your following statement:
String deleteQuery="DELETE FROM " + DatabaseValues.TABLE_NOTES + "WHERE" + DatabaseValues.NOTES_ID + "= '" + id + "'";
to
String deleteQuery= "DELETE FROM " + DatabaseValues.TABLE_NOTES + " WHERE " + DatabaseValues.NOTES_ID + " = '" + id + "'";
Actually you are combining the table name with where clause. You need to add a space before and after WHERE Clause like " Where "
Hope it helps.

Incorrect syntax in T-SQL statement in c#

I have the following code:
USE [DB] INSERT INTO Extract2_EventLog VALUES (" + li.userId + ", '" + li.startTime.ToString() + "', '" + li.endTime.ToString() + "', '" + li.elapsedTime.ToString() + (li.actionType == ActionType.REPORT ? "', 'report')" : "', 'extract')', '" + status + "'");
When I run this, I get the following error:
{"Incorrect syntax near ', '.\r\nUnclosed quotation mark after the
character string ''."}
I can't see what I'm doing wrong.. Anyone?

Man....Where to start with this...
First off, you should be using stored procedures that accept parameters (variables from your application code). Second, you should have a dataaccess layer in your application separating database calls and your user interface. I can't possible stress enough how important this is and how bad your current approach is. You will forever be fighting problems like this until you correct it.
But to address the question as it was asked...Your error is because your query string is malformatted. Use the debugging tools to view the string before it is sent to the database and then you should be able to quickly determine what is wrong with it. To troubleshoot, you can always cut and paste that string into SSMS, refine it there, and then make the necessary changes to your c# code.

First of all look at the answer of Stan Shaw, next take a look at the comment of Jon Skeet!
The first thing to do is stop building SQL like that... right now. Use parameterized SQL and you may well find the problem just goes away... and you'll be preventing SQL Injection Attacks at the same time.
They sayed everything that's important and just for the sake of giving you a direct answer:
You have a status + "'"); at your code which needs to be changed to status + "')"; ...
...like this one:
string statement = "USE [DB] INSERT INTO Extract2_EventLog VALUES (" + li.userId + ", '" + li.startTime.ToString() + "', '" + li.endTime.ToString() + "', '" + li.elapsedTime.ToString() + (li.actionType == ActionType.REPORT ? "', 'report')" : "', 'extract')', '" + status + "')";

Instead of concatenating values into your query you should use a parameterized query or a stored procedure.
A rewrite of your code could be something like (depending on datatypes, etc):
string commandText = "INSERT INTO Extract2_EventLog (userId, startTime, endTime, elapsedTime, actionType, [status]) VALUES (#userId, #startTime, #endTime, #elapsedTime, #actionType, #status)";
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = new SqlCommand(commandText, connection);
command.Parameters.AddWithValue("#userId", li.userId);
command.Parameters.AddWithValue("#startTime", li.startTime);
command.Parameters.AddWithValue("#endTime", li.endTime);
command.Parameters.AddWithValue("#elapsedTime", li.elapsedTime);
command.Parameters.AddWithValue("#actionType", li.actionType == ActionType.REPORT ? "report" : "extract");
command.Parameters.AddWithValue("#status", status);
connection.Open();
int rowsAffected = command.ExecuteNonQuery();
Console.WriteLine("RowsAffected: {0}", rowsAffected);
}

You've forgot the " at the beginning. So your code reverts sql with non sql.
AND your example seems to be incomplete.

String field with single quotation mark is causing an error when inserting record in table

I have below code:
query = "insert into tblB2B_OrderStatusTopStillInRB (LSRNbr, ShipName, Units, DroppedInRB, EPT, Status, OnTimeStatus, ShipVia, DroppedInRB_Order, RealEPT) ";
query += "values ('"
+ ListOrdStatusTopInRB[i].LSRNbr + "','"
+ ListOrdStatusTopInRB[i].ShipName + "',"
+ ListOrdStatusTopInRB[i].Units + ",'"
+ ListOrdStatusTopInRB[i].DroppedInRB + "','"
+ ListOrdStatusTopInRB[i].EPT + "','"
+ ListOrdStatusTopInRB[i].Status + "','"
+ ListOrdStatusTopInRB[i].OnTimeStatus + "','"
+ ListOrdStatusTopInRB[i].ShipVia + "','"
+ ListOrdStatusTopInRB[i].DroppedInRB_Order + "','"
+ ListOrdStatusTopInRB[i].RealEPT + "')";
cmd.CommandText = query;
cmd.ExecuteNonQuery();
And I just realized, that when the ShipName has a value with a single quotation mark, is causing an error in the insert statement, for instance: int'l Transp.
Is there any way to fix that, without removing the single quotation mark from the string?
I was trying using the following but didn't work:
cmd.CommandText = #query
+ #ListOrdStatusTopInRB[i].ShipName + "',"
Any ideas?

Is there any way to fix that, without removing the single quotation mark from the string?
Yes - use parameterized SQL instead. You should never use variable values directly in your SQL like this. It can allow SQL injection attacks, cause conversion oddities, and generally make the SQL more confusing to read.
See the documentation for SqlCommand.Parameters for an example of parameterized SQL.
Basically, the idea is that your SQL includes references to parameters, e.g.
INSERT INTO SomeTable(Foo, Bar) VALUES (#Foo, #Bar)
and then you specify the values for #Foo and #Bar separately. The values then aren't part of the SQL itself, so it doesn't matter whether or not they contain characters which would have special meaning within the SQL.

DateTime.Now to mysql datetime

i got problem with a query, got something like this
command.CommandText = "SELECT " +
"COUNT(a.`id`) " +
"FROM " +
"`messageaccess` a " +
"WHERE " +
"a.`Users_LOGIN` = '" + Settings.UserLogin + "' " +
"AND a.`Status` = '" + Enums.MessageStatus.New + "' " +
"AND a.`FOLDER` = '" + Enums.MessageFolder.INBOX + "'" +
"AND a.`ShowAlert` = '" + Enums.YesNo.No + "'" +
"AND a.`Postponed` <= " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss") + "";
but sql throws me exception
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '14:40:37' at line 1
tried diffrent combinantions but nothing works :(

The simple answer is not to embed values directly into the SQL to start with.
Use a parameterized SQL statement, specify the parameter value as DateTime.Now, and all will be well:
Your SQL will be easier to read (as it'll just be the code, not the data)
You won't need to worry about formatting of things like numbers and dates
You won't be vulnerable to SQL injection attacks

You forgot the quotation marks around the date/time thing.

try using this line instead:
"AND a.`Postponed` <= NOW()"
and it should work with the native MySql function for the current time.

Have a look at named parameterized queries. They take care of these formatting issues for you.

You shouldn't build your query appending strings. This is not very safe (sql injection) and you're not taking advantage of the ADO .NET capabilities to set the correct format according the parameter type.
You should use parametrized queries.