SQL语句在空表上返回结果集

Question

        String sql = "Select MAX(ORDERLINEID) From ORDERLINESTABLE";
        ResultSet rst;
        rst = stmt.executeQuery(sql);

        if(rst.next())
        {
            next = rst.getInt("ORDERLINEID");
            next++;
        }

I have a table called ORDERLINESTABLE in my database which is currently empty. I have run the above code with the aim is to get the highest integer stored in the ORDERLINEID column allowing me to increment it when adding items to the database.

I expected this query to return nothing as the table is empty but when debugging I noticed that the search is returning true for the rst.next() method.

Does anyone have any idea why this would be? I have looked at the resultset.next() documentation and as far as I can see it should return false.

Answer 1

When in doubt, look at your data. Here is a sample query from any db engine.

select max(field) maxValue
from table
where 1=3

It will yield

maxValue
Null

In other words, your query is returning one record with a value of null.

Answer 2

It is much better to fetch the ORDERLINEID filled in by the database after the INSERT statement. Make the column ORDERLINEID of type INT AUTOINCREMENT.

String sql = "INSERT INTO ORDERLINESTABLE(xxx, yyy) VALUES (?, ?)";
try (PreparedStatement stmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS)) {
    stmt.setString(1, xxx);
    stmt.setInt(2, yyy);
    int updateCount = stmt.executeUpdate(); // 1
    try (ResultSet id = stmt.getGeneratedKeys()) {
        if (id.next()) { // 'if' as just 1 row inserted.
            int orderLineId = id.getInt(1); // 1 key per row.
        }
    }
}

Java has a database independent way to fetch the generated keys of an INSERT. That is a lot safer than taking the MAX afterwards or before, in a multi-user environment.

Scenarios for wrong IDs are numerous in a multiuser environment:

• first SELECT
• second SELECT
• second increment for new ID
• first increment for new ID
• first INSERT
• second INSERT