CriteriaQuery中的条件左连接 [英] Condition left join in CriteriaQuery

问题描述

大家好，我正在CriteriaQuery中尝试执行此操作，我已经搜索了很长时间，但是找不到任何可以执行的操作，有人可以帮助我吗?

Hi everybody I am trying to do this in CriteriaQuery, I was searching so long but I can't found anything to do it, someone can help me?

SELECT b.name
FROM Empl a 
LEFT OUTER JOIN Deplo b ON (a.id_depl = b.id_depl) AND b.id_place = 2;

我只是想在left join子句中做一个条件，我看到了".on"函数，但是我不知道它是否可以工作以及它如何工作，因为我试图做这样的事情:

I'm just trying to do a condition in left join clause, I saw ".on" function but I don't know if it will work and how it work because I tried to do something like this:

Join Table1, Table2j1 = root.join(Table1_.table2, JoinType.LEFT).on(cb.and(cb.equal(table2_.someid, someId)));

但是它需要一个布尔表达式.

But it need a boolean expresion.

推荐答案

JPA 2.1中引入了on子句.解决方案示例:

The on clause has been introduced in JPA 2.1. An example solution:

    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<String> cq = cb.createQuery(String.class);
    Root<Empl> a = cq.from(Empl.class);
    Join<Empl,Deplo> b = a.join("b", JoinType.LEFT); //left outer join
    b.on(
        cb.and(
            cb.equal(b.get("id_place"), cb.parameter(Integer.class, "idPlace")),
            cb.equal(b.get("id_depl"), a.get("id_depl"))
        )   
    );
    cq.select(b.<String>get("name"));

    List<String> results = em.createQuery(cq)
                             .setParameter("idPlace", 2)
                             .getResultList();

这篇关于CriteriaQuery中的条件左连接的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！