Two things, starting with the one you haven't noticed yet because it's a lot more critical.
Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
Sort that throughout your whole app urgently first, then read on.
The other is simple: spaces are not allowed in column names and neither are punctuation characters, unless the name is escaped each time you use it in an SQL command:
insert into UserData (School ID, Firstname, Lastname, Age, Address, Mobile no., Gender) values (@School ID, @fname, @lname, @age, @Address, @phone, @Gender)
are both illegal and would need either changing to
or escaping with square brackets:
insert into UserData ([School ID], Firstname, Lastname, Age, Address, [Mobile no.], Gender) values (@School ID, @fname, @lname, @age, @Address, @phone, @Gender)
Spaces are not allowed at all in variable names and cannot be escaped, so
needs the space removed everyu time you try to use it.