Java SQL - Insert into table only new entries

Question

I am trying to insert new entries into my database, but only the new entries. If a class with the crn that I am adding already exists in the database then I would like to skip it to not have duplicates.

Below is the code I have right now. I have tried a few different methods but I keep getting exception:

java.sql.SQLSyntaxErrorException: 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 'EXCEPT crn' at line 1

The database entry works fine without the "EXCEPT crn", but again it adds duplicates.

try {
        String query = null;
        try {
            query = "INSERT INTO Classes (crn, subject, creditHours, title, capacity, instructor, schedule) "
                    + "VALUES (?, ?, ?, ?, ?, ?, ?) EXCEPT crn";
        } catch(Exception e) {
            conn.close();
        }

        PreparedStatement preparedStmt = conn.prepareStatement(query);
        preparedStmt.setInt(1, crn);
        preparedStmt.setString(2, subject);
        preparedStmt.setInt(3, creditHours);
        preparedStmt.setString(4, title);
        preparedStmt.setString(5, capacity);
        preparedStmt.setString(6, instructor);
        preparedStmt.setString(7, schedule);
        preparedStmt.executeUpdate();
    } catch (SQLException e) {
        e.printStackTrace();
    }

Answer 1

If a class with the crn that I am adding already exists in the database then I would like to skip it to not have duplicates.

In MySQL, I would recommend the insert... on duplicate key syntax :

INSERT INTO Classes (crn, subject, creditHours, title, capacity, instructor, schedule)
VALUES (?, ?, ?, ?, ?, ?, ?)
ON DUPLICATE KEY UPDATE crn = VALUES(crn);

For this to work, you need a unique constraint (or the like) on column crn :

ALTER TABLE Classes ADD CONSTRAINT cs_classes_uniq_crn UNIQUE(crn);

Then, when an INSERT occurs that would generate a duplicate crn , the query goes to the UPDATE clause, that actually performs a no-op.

Answer 2

Can you alter the row or rows you want? You could just put a unique constraint on them so they can't accept columns that have the same value.

ALTER TABLE table_name ADD UNIQUE (column_name);

If you need multiple columns you can add the constraint to the table like this: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column1, column2, etc);

Answer 3

The SQL statement that you try to execute is invalid because MySql does not support EXCEPT .
Since you want to skip the insertion of rows that already exist you can use INSERT IGNORE :

query = "INSERT IGNORE INTO Classes (crn, subject, creditHours, title, capacity, instructor, schedule) "
        + "VALUES (?, ?, ?, ?, ?, ?, ?)";

but for this to work there should be a unique constraint for the column crn .
It seems like it is the PRIMARY KEY of the table so it is already unique.
If there isn't a unique constraint for the column crn you can use NOT EXISTS like this:

query = "INSERT INTO Classes (crn, subject, creditHours, title, capacity, instructor, schedule) "
        + "SELECT ?, ?, ?, ?, ?, ?, ? "
        + "WHERE NOT EXISTS (SELECT 1 FROM Classes WHERE crn = ?)";

so you will have to pass as the 8th parameter of the Prepared Statement crn again:

preparedStmt.setInt(8, crn);