4.8.4 Aggregate Functions in the SELECT Clause The result of a query
may be the result of an aggregate
function applied to a path expression.
The following aggregate functions can
be used in the SELECT clause of a
query: AVG, COUNT, MAX, MIN, SUM.
For all aggregate functions except
COUNT, the path expression that is the
argument to the aggregate function
must terminate in a state-field. The
path expression argument to COUNT may
terminate in either a state-field or a
association-field, or the argument to
COUNT may be an identification
variable.
Arguments to the functions SUM and AVG
must be numeric. Arguments to the
functions MAX and MIN must correspond
to orderable state-field types (i.e.,
numeric types, string types, character
types, or date types).
The Java type that is contained in the
result of a query using an aggregate
function is as follows:
COUNT returns Long.
MAX, MIN return the type of the state-field to which they are applied.
AVG returns Double.
SUM returns Long when applied to state-fields of integral types (other
than BigInteger); Double when applied
to state-fields of floating point
types; BigInteger when applied to
state-fields of type BigInteger; and
BigDecimal when applied to
state-fields of type BigDecimal.
If SUM, AVG, MAX, or MIN is used, and
there are no values to which the
aggregate function can be applied, the
result of the aggregate function is
NULL.
If COUNT is used, and there are no
values to which COUNT can be applied,
the result of the aggregate function
is 0.
The argument to an aggregate function
may be preceded by the keyword
DISTINCT to specify that duplicate
values are to be eliminated before the
aggregate function is applied.
Null values are eliminated before the aggregate function is applied, regardless of whether the keyword DISTINCT is specified.
4.8.4 Aggregate Functions in the SELECT Clause The result of a querymay be the result of an aggregatefunction applied to a path expression.The following aggregate functions canbe used in the SELECT cl...
кількістьстудентів
адресауніверситету
Spring Data
JPA
到Dao层在
JPA
Repository和CrudRepository之间
Створити服务层якийматимеописаніCRUDоперації
解决方案服务层位于Univercity。
import lombok.extern.slf4j.Slf4j;
import org.apache.commons.lang.StringUtils;
import org.apache.poi.hpsf.Decimal;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.tran.
在使用EJB3.0的
JPA
2.0技术的时候，遇到了一个很蛋疼的问题。
很久以前一直用
EntityManager
下get
Result
List()
查询
数据，即便一个数据亦是如此，所以要频繁的List.get(0)。新街口get
Single
Result
()出现了，然后就开始频繁报异常。“No entity found for query
在get
Single
Result
的源...
private
EntityManager
entityManager
;
public Page<RecordMerchantVO> queryMerchantRecord(QueryRecordReq req) {
long start = System.currentTimeMillis();
String sor...
getSing
Result
查询
不到Object不会
返回
null，而是报错----
JPA
什么设计
解决方案：
用自己弄的类 get
Single
Result
OrNull替代
public static T> T get
Single
Result
OrNull(TypedQueryT> query) {
query.s
今天做一个
查询
，因为确定只
返回
一条记录，所以用的get
Single
Result
()这个方法，但是这个方法
返回
的是Object类型，那到底Object类型里面是什么内容呢？
先看一下直接在工具中
查询
的结果：
只有一条记录，是id为160105的记录的统计数据，后面三个字段是数字。
如果用System.out.println查看输出的话，结果是：[L
java
.lang.Object;@2
String sql = "select * from T_
order
where ...."
EntityManager
emread;
Query qr = emread.createNativeQuery(sqlres);
return List lis
使用get
Single
Result
()方法时如果没有结果（也就是
返回
null）时习惯思维是应该
返回
null，而get
Single
Result
()方法中有这么一条语句： @throws EntityNotFoundException if there is no
result
也就是说，当他
查询
不到结果时，将会抛出EntityNotFoundException异常，而不是
返回
null。
