jpql写case when
时间: 2023-11-03 12:00:05 浏览: 25
在JPQL中,CASE WHEN语句用于在查询中进行条件判断和返回不同的值。它的基本语法如下:
CASE WHEN condition1 THEN result1
WHEN condition2 THEN result2
...
ELSE result
END
其中,condition1、condition2等是条件表达式,result1、result2等是满足相应条件时返回的值,ELSE result是当没有条件满足时的默认返回值。
举个例子,假设你有一个实体类Person,其中有一个属性age。你可以使用JPQL的CASE WHEN语句来根据不同的age值返回不同的结果:
SELECT p.name,
CASE WHEN p.age < 18 THEN '未成年'
WHEN p.age >= 18 AND p.age < 60 THEN '成年人'
ELSE '老年人'
END
FROM Person p
这个查询将返回每个人的姓名和根据年龄判断的结果。
相关问题
jpql case when用in
jpql case when用in是指在JPQL中使用CASE WHEN语句来判断一个值是否在一个集合中。例如,你可以使用如下语法:
CASE WHEN value IN (value1, value2, ...) THEN result ELSE otherResult END
这个语句会判断value是否在指定的集合中,如果在,则返回result,否则返回otherResult。
jpql fetch
JPQL (Java Persistence Query Language) is a query language used in Java Persistence API (JPA) to perform database queries. It is similar to SQL but operates on the object model rather than the database schema.
In JPQL, the "fetch" keyword is used to indicate the eager loading of associated entities in a query result. Eager loading means that the associated entities will be loaded along with the main entity in a single query, reducing the need for additional queries to fetch the associated data later.
For example, consider a scenario where you have an entity called "Order" with a Many-to-One relationship with an entity called "Customer". If you want to retrieve all orders along with their associated customers, you can use the "fetch" keyword to eagerly load the customers in the query:
```java
SELECT o FROM Order o JOIN FETCH o.customer
```
In this query, the "JOIN FETCH" syntax ensures that the customer entity is eagerly loaded along with each order entity. Without using "fetch", the associated customer entity would be lazily loaded, resulting in additional queries being executed when accessing the customer information.
Note that using "fetch" should be done judiciously as it can have performance implications. Eagerly loading too many associations in a single query can result in excessive data retrieval and memory usage.