Very Strange error when trying to INSERT into a database with more than 1 entry - mysql

Question

HOPEFULLY.....the title makes sense. But basically this is what is going on. I'm creating a form where a user will be able to View, Update, Insert and Delete records from tables within a database.
I'm using a combo box to display the table data:
Now If I try to Insert a new record, it works fine, until I try and more than one.
This is me placing just the personID record in. and it works fine:
But if i try and add a personID AND a firstName this happens:
as you can see, it ignores the personID entry, and just goes for the firstName.
I cant go further in my code without figuring out what is causing me to not be able to put in 2 things at once.
Here is my INSERT code:
string myInsertSQL;
myInsertSQL = "INSERT INTO person(";
myInsertSQL += "personID)";
myInsertSQL += " VALUES ('";
myInsertSQL += personID.Text + "')";
myInsertSQL = "INSERT INTO person(";
myInsertSQL += "firstName)";
myInsertSQL += " VALUES ('";
myInsertSQL += firstName.Text + "')";
MySqlConnection conn = new MySqlConnection(connstring);
MySqlCommand cmd = new MySqlCommand(myInsertSQL, conn);
With the above, it will only insert the firstName.
I have no idea why this would happen, and I would love some input. Thanks!

Answer 1

By doing this, the DB would insert the ID and the firstname on 2 distinct rows, if you didn't overwrite the first string (myInsertSQL = "INSERT INTO person(";). Here, it only performs the second query (i.e. inserting first name).
Try:
myInsertSQL = "INSERT INTO person(";
myInsertSQL += "personID, FirstName)";
myInsertSQL += " VALUES ('";
myInsertSQL += personID.Text + "', '"+ firstName.Text +"')";
Please note that this is wide open to SQL injections. Try to use prepared statements instead of directly injecting plain text into queries:
string myInsertSQL = "INSERT INTO person(personID, firstName) VALUES (#personID, #firstName)";
MySqlCommand cmd = new MySqlCommand(myInsertSQL, conn);
cmd.Parameters.AddWithValue("#personID", personID.Text);
cmd.Parameters.AddWithValue("#firstName", firstName.Text);
EDIT :
According to your comment, you try to insert more than those 2 values. This work the same.
The error says that you did specify a different number of values and fields. You speak of 6 values, but only list 5 (personID, firstName, address, phoneNumber, postCode). I suspect you forgot the lastName:
string myInsertSQL = "INSERT INTO person(personID, lastName, firstName, address, phoneNumber, postCode) ";
myInsertSQL += "VALUES (#personID, #lastName, #firstName, #address, #phoneNumber, #postCode)";
MySqlCommand cmd = new MySqlCommand(myInsertSQL, conn);
cmd.Parameters.AddWithValue("#personID", personID.Text);
cmd.Parameters.AddWithValue("#lastName", lastName.Text);
cmd.Parameters.AddWithValue("#firstName", firstName.Text);
cmd.Parameters.AddWithValue("#address", address.Text);
cmd.Parameters.AddWithValue("#phoneNumber", phoneNumber.Text);
cmd.Parameters.AddWithValue("#postCode", postCode.Text);
// and so on...

Answer 2

myInsertSQL = "INSERT INTO person(";
here you completely overwrite the myInsertSQL String, so whatever you put in there before is gone before you even send it to the database.

Answer 3

You are overwriting variable value
myInsertSQL += personID.Text + "')";
myInsertSQL = "INSERT INTO person(";