JPA条件查询API，最后按null排序

Question

我的问题是空值必须是按语句的最后顺序。 我的代码片段如下。 我使用javax持久性条件构建器。 我的查询很复杂。

import javax.persistence.criteria.CriteriaBuilder;

public Predicate getSomePredicate() {
    Predicate predicate = cb.conjunction();....

    ...predicate.getExpressions().add(cb.and(cb.or(cb.and(v1, v2), cb.and(s1, s2))));

    EOrderByType orderType = EOrderByType.values()[orderBy]
            ;
    switch (orderType) {
    case PRICE: cq.where(predicate).orderBy(cb.asc(root.get("price")));
        break;
    case PRICE_HIGH_TO_LOW: cq.where(predicate).orderBy(cb.desc(root.get("price")));
        break;
    case CONSUPTION: cq.where(predicate).orderBy(cb.desc(root.get("consume")));
        break;
    default:
        break;
    }

    return cq.getRestriction();
}

如何使用条件生成器按价格为空的顺序实现订单？

Answer 1

嗨，我几乎搜索了所有互联网页面，然后找到了解决方案，您可以按部分编写开关案例的顺序。 如下所示：如果price为空，则按desc排序，价格值为1000000，如果price为空，则按asc排序，价格值为0。如果需要这些，则可以编写如下表达式。

                EOrderByType orderType = EOrderByType.values()[orderBy];                    
                Expression<Object> queryCase = cb.selectCase().when(cb.isNull(root.get("price")), 100000000).otherwise(root.get("price"));
                Direction dir = Direction.ASC;

                switch (orderType) {
                    case UCUZDAN_PAHALIYA:
                        queryCase = cb.selectCase().when(cb.isNull(root.get("price")), 100000000).otherwise(root.get("price"));
                        break;
                    case PAHALIDAN_UCUZA:
                        queryCase = cb.selectCase().when(cb.isNull(root.get("price")), 0).otherwise(root.get("price"));
                        dir = Direction.DESC;
                        break;
                }

                  cq.where(predicate).orderBy(direction( cb, queryCase, dir));

Answer 2

这是katsu对自己问题的回答的扩展。 我试图找到一种解决方案，能够对表的大多数列进行排序，其中某些列允许具有空值。 我想按升序排序时，将空值放在最低的非空值前面，而当按降序排序时，要在最低的非空值之后。 换句话说，这与（Oracle的）默认行为截然相反。

我找到了其他方法可以做到这一点，但是这种方法不需要我脱离Hibernate和JPA 2持久性，但仍然可以获得我想要的结果。 这是摘录自我的实际代码的一小段代码，但合并在一个位置，并且更改了一些名称。 您看到的任何语法，编译类型错误都可能是由于该原因。

// sortByColumn is a String containing the Hibernate version of the column name, which had 
// been assigned as the ID of the table header column of the column by which we are sorting.

// sortAscending is a Boolean object containing Boolean.TRUE if we are to sort in ascending 
// order or Boolean.FALSE or null if we are to sort in descending order. This may seem a 
// bit odd, but in the case we need this for, the default sort column is a release date and 
// reverse chronological order is the most useful in that case.

// Also defined are: CriteriaQuery<SoftwareVersion> criteriaQuery and 
// CriteriaBuilder criteriaBuilder by the typical means.

final Root<SoftwareVersion> softwareVersionRoot = 
    criteriaQuery.from(SoftwareVersion.class);

private static final String EMPTY_STRING = "";

if (sortByColumn != null && sortByColumn.trim().length() > 0) {
  Order sortOrder;
  Expression<String> sortColumnExpression;
  if (sortByColumn.equalsIgnoreCase(SoftwareVersion_.installationFileLength.getName()) || 
      sortByColumn.equalsIgnoreCase(SoftwareVersion_.releaseTimestamp.getName())) {
    // The two non-String fields (exposed to the user) that we don't need to have the
    // lower() function operate upon.
    sortColumnExpression = oemSoftwareVersionRoot.get(sortByColumn);
  } else {
    // We use the lower() function to enforce case insensitive sorting on the columns we
    // show to the user, which are all Strings except as noted above.
    Expression<String> rootExpression = oemSoftwareVersionRoot.get(sortByColumn);
    sortColumnExpression = criteriaBuilder.lower(rootExpression);
  }

  // The columns for installation file name, installation file length and release timestamp
  // are just three of the columns that we allow the user to sort by. However, these three
  // may have null values in the database, and require some special handling.
  if (sortByColumn.equalsIgnoreCase(SoftwareVersion_.installationFileLength.getName()) || 
    sortByColumn.equalsIgnoreCase(SoftwareVersion_.installationFileName.getName()) ||
    sortByColumn.equalsIgnoreCase(SoftwareVersion_.releaseTimestamp.getName())
    ) {
    Expression<Object> queryCase;
    if (sortByColumn.equalsIgnoreCase(SoftwareVersion_.installationFileName.getName())) {
      // Installation file name is a (case insensitive) String
      queryCase = criteriaBuilder.selectCase().when(
        criteriaBuilder.isNull(sortColumnExpression), 
        StringUtil.EMPTY_STRING).otherwise(sortColumnExpression);
    } else if (sortByColumn.equalsIgnoreCase(SoftwareVersion_.releaseTimestamp.getName())) {
      // Release timestamp is a database timestamp
      LocalDateTime dateTime = LocalDateTime.of(1970,1,1,0,0); 
      // Equivalent to Unix epoch time. Note month is 1-12, not 0-11
      queryCase = criteriaBuilder.selectCase().when(
        criteriaBuilder.isNull(sortColumnExpression), 
          Timestamp.valueOf(dateTime)).otherwise(sortColumnExpression);
    } else {
      // Installation file length is a Long (or BigDecimal) computed when the file is uploaded.
      // The user can't set or change it, but can sort by it.
      queryCase = criteriaBuilder.selectCase().when(
        criteriaBuilder.isNull(sortColumnExpression), 
          Long.valueOf(0)).otherwise(sortColumnExpression);
    }

    if (asc != null && asc.booleanValue()) {
      sortOrder = criteriaBuilder.asc(queryCase);
    } else {
      sortOrder = criteriaBuilder.desc(queryCase);
    }
  } else {
    if (asc != null && asc.booleanValue()) {
      sortOrder = criteriaBuilder.asc(sortColumnExpression);
    } else {
      sortOrder = criteriaBuilder.desc(sortColumnExpression);
    }
  }
  criteriaQuery.orderBy(sortOrder);
}