15.14. HQL 示例

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2023-12-01

Hibernate 查询可以非常的强大与复杂。实际上，Hibernate 的一个主要卖点就是查询语句的威力。这里有一些例子，它们与我在最近的一个项目中使用的查询非常相似。注意你能用到的大多数查询比这些要简单的多。

下面的查询对于某个特定的客户的所有未支付的账单，在给定给最小总价值的情况下，返回订单的 id，条目的数量和总价值，返回值按照总价值的结果进行排序。为了决定价格，查询使用了当前目录。作为转换结果的 SQL 查询，使用了ORDER，ORDER_LINE，PRODUCT，CATALOG 和 PRICE 库表。

select order.id, sum(price.amount), count(item)
from Order as order
    join order.lineItems as item
    join item.product as product,
    Catalog as catalog
    join catalog.prices as price
where order.paid = false
    and order.customer = :customer
    and price.product = product
    and catalog.effectiveDate < sysdate
    and catalog.effectiveDate 
>= all (
        select cat.effectiveDate
        from Catalog as cat
        where cat.effectiveDate < sysdate
    )
group by order
having sum(price.amount) 
> :minAmount
order by sum(price.amount) desc

这简直是一个怪物！实际上，在现实生活中，我并不热衷于子查询，所以我的查询语句看起来更像这个：

select order.id, sum(price.amount), count(item)
from Order as order
    join order.lineItems as item
    join item.product as product,
    Catalog as catalog
    join catalog.prices as price
where order.paid = false
    and order.customer = :customer
    and price.product = product
    and catalog = :currentCatalog
group by order
having sum(price.amount) 
> :minAmount
order by sum(price.amount) desc

下面一个查询计算每一种状态下的支付的数目，除去所有处于 AWAITING_APPROVAL 状态的支付，因为在该状态下当前的用户作出了状态的最新改变。该查询被转换成含有两个内连接以及一个相关联的子选择的 SQL 查询，该查询使用了表 PAYMENT，PAYMENT_STATUS 以及 PAYMENT_STATUS_CHANGE。

select count(payment), status.name
from Payment as payment
    join payment.currentStatus as status
    join payment.statusChanges as statusChange
where payment.status.name <
> PaymentStatus.AWAITING_APPROVAL
    or (
        statusChange.timeStamp = (
            select max(change.timeStamp)
            from PaymentStatusChange change
            where change.payment = payment
        )
        and statusChange.user <
> :currentUser
    )
group by status.name, status.sortOrder
order by status.sortOrder

如果我把 statusChanges 实例集映射为一个列表（list）而不是一个集合（set），书写查询语句将更加简单。

select count(payment), status.name
from Payment as payment
    join payment.currentStatus as status
where payment.status.name <
> PaymentStatus.AWAITING_APPROVAL
    or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <
> :currentUser
group by status.name, status.sortOrder
order by status.sortOrder

下面一个查询使用了 MS SQL Server 的 isNull() 函数用以返回当前用户所属组织的组织帐号及组织未支付的账。它被转换成一个对表 ACCOUNT，PAYMENT，PAYMENT_STATUS，ACCOUNT_TYPE，ORGANIZATION 以及 ORG_USER 进行的三个内连接，一个外连接和一个子选择的 SQL 查询。

select account, payment
from Account as account
    left outer join account.payments as payment
where :currentUser in elements(account.holder.users)
    and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate

对于一些数据库，我们需要弃用（相关的）子选择。

select account, payment
from Account as account
    join account.holder.users as user
    left outer join account.payments as payment
where :currentUser = user
    and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate