(any body can answer this)

Question

st.executeUpdate("select * from trainee where id = " + num );is not working. its taking num as string.....I tried alot...
HELP ME

Answer 1

st.executeUpdate("select * from trainee where id = " + num );Java 101: the "+" operator in the context where one operand is a String will always convert the other operand to a String, and concatenate the two Strings. In other words, yes, you've taken "num" and converted it to a String, an concatenated that onto the end of your select statement.
BUT your database doesn't execute Java objects, it executes the SQL you've sent it. So it will take the SQL you send it:
select * from trainee where id = [some numeric value written as a character string]and PARSE it; in so doing, it will convert that numberic value written as a character string back into a number. However, it will do so using it's rules, which may or may not be Java's rules. If the number is an integer, the conversion will usually work - the same number will be used in Java and inside the database; if the number is not an integer (or a tiny subset of the real numbers that happen to convert back and forth exactly), rounding will probably occur, and other conversion problems may also bite you.
The solution to all this (and many many other problems) is to use a PreparedStatement instead of a Statement.
PreparedStatement pstmt = conn.prepareStatement("select * from trainee where id = ?" );
// use the right data type - long, int, whatever
pstmt.setLong(1,num);
// don't use executeUpdate when you're not doing an update
pstmt.executeQuery();