如何通过单次ObjectBox数据库查询实现两列求和？

Absolutely! You can absolutely compute the sum of both field1 and field2 in one database round-trip—this is way more efficient than running two separate queries, as it cuts down on database communication overhead. Since your original code uses QueryDSL-style syntax (the boxFor method is a dead giveaway), here's how to adjust it:

Single Query Implementation with Tuple #

The simplest way is to fetch both sums as a Tuple (QueryDSL's built-in container for query results):

// Initialize your base query
QQuery<A> query = controller.getStore().boxFor(A.class).query();

// Define the sum expressions for both fields
Expression<Long> sumField1Expr = query.property(A_.field1).sum();
Expression<Long> sumField2Expr = query.property(A_.field2).sum();

// Fetch both sums in one go
Tuple sumTuple = query.select(sumField1Expr, sumField2Expr).fetchOne();

// Extract the values (handle nulls if no records exist!)
Long sum1 = Objects.requireNonNullElse(sumTuple.get(sumField1Expr), 0L);
Long sum2 = Objects.requireNonNullElse(sumTuple.get(sumField2Expr), 0L);

// Calculate total if needed
Long totalSum = sum1 + sum2;

Alternative: Use a Custom DTO for Cleaner Results #

If you prefer a more type-safe approach over Tuple, create a simple DTO class to hold the sums, then use QueryDSL's projections to map the result directly:

Step 1: Create the DTO #

public class FieldSums {
    private final Long sumField1;
    private final Long sumField2;

    // Constructor must match the order of selected expressions
    public FieldSums(Long sumField1, Long sumField2) {
        this.sumField1 = sumField1;
        this.sumField2 = sumField2;
    }

    // Getters
    public Long getSumField1() { return sumField1; }
    public Long getSumField2() { return sumField2; }
}

Step 2: Fetch the Result into the DTO #

QQuery<A> query = controller.getStore().boxFor(A.class).query();

Expression<Long> sumField1Expr = query.property(A_.field1).sum();
Expression<Long> sumField2Expr = query.property(A_.field2).sum();

FieldSums sums = query.select(Projections.constructor(
    FieldSums.class,
    sumField1Expr,
    sumField2Expr
)).fetchOne();

// Access sums safely (again, handle nulls if needed)
Long totalSum = Objects.requireNonNullElse(sums.getSumField1(), 0L) 
              + Objects.requireNonNullElse(sums.getSumField2(), 0L);

Key Notes #

• This approach generates a single SQL query that looks roughly like:

  SELECT SUM(a.field1), SUM(a.field2) FROM A a
  

• If your fields are of type Integer instead of Long, just swap the type in the expressions and DTO.
• Always handle null values! If there are no records in the table, sum() will return null—using Objects.requireNonNullElse ensures you get a default 0 instead of a NullPointerException.

内容的提问来源于stack exchange，提问作者Сергей Кавказов