JPA标准API顺序按NULL last

Question

我使用JPA criteria API从数据库中获取记录。我有一个实体记录，其字段dateTime可以为空。我会编码：

public List<Record> find(RecordFilter recordFilter, int page, int pageSize) {
    CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
    CriteriaQuery<Record> criteriaQuery = criteriaBuilder.createQuery(Record.class);
    Root<Record> recordRoot = criteriaQuery.from(Record.class);

    /*
     * JOINS. Left Joins are used for optional fields, or fields inside of the optional fields.
     */
    Join<Record, Agency> recordAgencyJoin = recordRoot.join(RecordTable.FIELD_AGENCY);
    //Some other joins

    //This is where I had the problem. 
    applyOrderBy(criteriaQuery, criteriaBuilder, recordRoot);

    /*
     * Specify which columns to select and their order.
     * criteriaQuery.multiselect(....);
     */              
    applyMultiSelect(recordRoot, recordAgencyJoin, /*other joins*/ criteriaQuery);

    /*
     * criteriaQuery.where(somePredicate);
     */
    applyFilter(recordFilter, criteriaQuery, criteriaBuilder,
            recordRoot, recordAgencyJoin /*, other joins*/);
    TypedQuery<Record> query = entityManager.<Record>createQuery(criteriaQuery);
    RepositoryUtils.applyPagination(query, page, pageSize);
    return query.getResultList();
}


private void applyOrderBy(CriteriaBuilder criteriaBuilder, Root<Record> recordRoot, CriteriaQuery<Record> criteriaQuery) {
    //Other fields to be added to the final sort.

    Order dateTimeDescOrder = criteriaBuilder.desc(recordRoot.get(RecordTable.FIELD_DATE_TIME));
    criteriaQuery.orderBy(dateTimeDescOrder /*, other orders by*/);
}

事实证明，dateTimeField为空的记录首先显示。我使用Postrgres数据库。我会回答这个问题，因为我找到了解决办法。这里有一个类似的帖子。JPA标准查询API和订单（按空）

Answer 1

JPA或Eclipselink中没有任何内容可以在CriteriaBuilder中使用。

例子：

Order order1 = cBuilder.desc(cBuilder.selectCase().when(cBuilder.isNotNull(from.get('ColumnName1')), from.get('ColumnName1')).otherwise(from.get('ColumnName1')));
CQuery.orderBy(order1);

它会给你最好的答案

Answer 2

JPA规范中没有任何内容可以控制如何处理空值（所有RDBMS都首选默认值）。这在JPQL（基于字符串的查询）或Criteria API中都不可用。有了JPQL，所有主要的JPA提供商都允许使用

NULLS [FIRST|LAST]

在订购子句中。对于标准，您受到标准应用编程接口的约束，所以什么都不可能。我知道数据核心JPA提供了JPA标准应用编程接口的自定义版本，允许规范

Order order = criteriaBuilder.asc(myExpression).nullsFirst();

但很明显，这是特定于JPA提供商的。

Answer 3

在这里，我给出了这个任务的答案。

首先，Postgres默认首先返回空值。

SELECT * FROM record ORDER BY date_time_field DESC;

https://stackoverflow.com/a/7621232/4587961

SELECT * FROM record ORDER BY date_time_field DESC NULLS LAST;

其次，我必须改变applyOrderBy方法

private void applyOrderBy(CriteriaBuilder criteriaBuilder, Root<Record> recordRoot, CriteriaQuery<Record> criteriaQuery) {
    //In the class code
    //private static final Date MIN_DATE = new Date(0L);
    final Date MIN_DATE = new Date(0L);

    //We treat records will NULL dateTimeField as if it was MIN_DATE.
    Order dateTimeDescOrder = criteriaBuilder.desc(
            //NULL values - last - WORKAROUND.
            criteriaBuilder.coalesce(recordRoot.get(RecordTable.FIELD_DATE_TIME), MIN_DATE));
    criteriaQuery.orderBy(dateTimeDescOrder);
}

请注意，CriteriaBuilder来自hibernate-jpa-2.1。

/**
 * Create an expression that returns null if all its arguments
 * evaluate to null, and the value of the first non-null argument
 * otherwise.
 *
 * @param x expression
 * @param y value
 *
 * @return coalesce expression
 */
<Y> Expression<Y> coalesce(Expression<? extends Y> x, Y y);