How to determine the number of records in a table that have no links to records in another table?

To determine the number of records in one table that have no relations with records in another table, a SQL query with a LEFT JOIN operator and a NULL check in the related table is usually used. For example, if there is a table A and a related table B, the query would look like this:

SELECT COUNT(*)
FROM A
LEFT JOIN B ON A.id = B.a_id
WHERE B.a_id IS NULL;

This query selects all records from A, tries to find corresponding records in B, and then counts only those that have no related record (i.e., B.a_id is NULL).

In Java, to execute such a query, you can use JDBC or ORM (e.g., JPA/Hibernate). An example with JDBC:

String sql = "SELECT COUNT(*) FROM A LEFT JOIN B ON A.id = B.a_id WHERE B.a_id IS NULL";
try (PreparedStatement stmt = connection.prepareStatement(sql)) {
    ResultSet rs = stmt.executeQuery();
    if (rs.next()) {
        int count = rs.getInt(1);
        System.out.println("Number of records without relations: " + count);
    }
}

Thus, using an SQL query and Java, you can obtain the required number of records without relations.