检查是否存在行，否则相应插入

Question

I'm trying to check if the database has the row Comment matches according from getComments2.

If there exists such row, proceed with the next data checking else it will execute the insert statement

After I ran this code, it gave me:

1. java.sql.SQLException: Illegal operation on empty result set (When database is empty) or
2. java.sql.SQLException: After end of result set (When database is filled).

Code:

String url = "test";
String getComments2 = "test1";
String getTime1 = "test2";
String sqlSelect = "Select comment from predata";
PreparedStatement ps1 = conn.prepareStatement(sqlSelect);
ResultSet rs =  ps1.executeQuery();

boolean exists = false;
while(!rs.last()||(!exists)) {
    rs.next();
    if(rs.getString("Comment").compareTo(getComments2)==0) {
        exists = true;
    }
    if(!exists||rs ==null) {
        PreparedStatement ps = conn.prepareStatement("INSERT into predata (topic,comment,date) VALUES (?,?,?)");
        ps.setString(1, url);
        ps.setString(2, getComments2);
        ps.setString(3, getTime1);
        ps.executeUpdate();
    }
}

Answer 1

rs.last() moves the cursor to the last row. Then rs.next() moves it past the last row, so rs.getString("Comment") throws an exception.

The correct logic should be:

boolean exists = false;
while (rs.next() && !exists) {
    if(rs.getString("Comment").equals(getComments2)) {
        exists = true;
    }
}
if (!exists) {
    PreparedStatement ps = conn.prepareStatement("INSERT into predata (topic,comment,date) VALUES (?,?,?)");
    ps.setString(1, url);
    ps.setString(2, getComments2);
    ps.setString(3, getTime1);
    ps.executeUpdate();
}

Answer 2

You can use Merge in your query itself instead of checking condition in code. Else you can user Insert or Replace in query itself.