兔八哥笔记14(3)：Hibernate 查询语言(HQL)

兔八哥笔记14(3)：Hibernate 查询语言(HQL)

10.11 子查询（Subqueries）

Hibernate支持子查询，子查询必须用圆括号包围。子查询通常被SQL聚集函数调用。子查询中引用一个外部查询的别名也是被允许的。from eg.Cat as fatcat where fatcat.weight > ( select avg(cat.weight) from eg.DomesticCat cat ) from eg.DomesticCat as cat where cat.name = some ( select name.nickName from eg.Name as name ) from eg.Cat as cat where not exists ( from eg.Cat as mate where mate.mate = cat ) from eg.DomesticCat as cat where cat.name not in ( select name.nickName from eg.Name as name)

10.12 例子（Examples）

Hibernate的查询功能相当强大，而且复杂。事实上，功能强大的查询语言是Hibernate的主要的卖点（selling points。原来这是一个外来词呀，今天才知道。^_^）。 下面是一些简单的查询例子，这是从我最近的项目中挑出来的，在大多数情况下，你写的查询要比这些更简单。 下面这个查询使用到的表：order, order_line, product, catalog 和price。有4个内连接和一个没有关系（uncorrelated）的子查询。

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

简直是怪物！（What a monster!）。现实生活中（in real life），我们对子查询并不是很关心，我们大多数的查询更像下面这样：

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

下面这个 查询翻译成SQL将需要2个内连接和一个相关联的子查询。它用到的表：payment, payment_status 和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

如果我映射tatusChanges为一个List代替set，那个我们的查询将更简单：

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

下面这个查询用到MS SQL的isnull（）函数，用到3个内连接和1个外联接，用大的表：account, payment, payment_status, account_type, organization和org_user。

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

对于其他的数据库，我们需要使用关联的子查询：

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

10.13 提示和窍门

你可以不用真正的返回查询结果，而知道查询结果的数量：( (Integer) session.iterate("select count(*) from ....").next() ).intValue() 按照返回的集合的大小排序，可以使用下面的语句：select usr.id, usr.name from User as usr left join usr.messages as msggroup by usr.id, usr.nameorder by count(msg)如果你的数据库支持子查询，那么你可以在where从句中指定一个查询返回结果尺寸的条件：from User usr where size(usr.messages) >= 1如果你的数据库不支持子出查询，那么你可以使用相面的查询：select usr.id, usr.namefrom User usr.name join usr.messages msggroup by usr.id, usr.name having count(msg) >= 1上面这个办法因为使用了内连接不能返回拥有0个消息的用户的信息，下面这个查询是有用的：select usr.id, usr.namefrom User as usr left join usr.messages as msggroup by usr.id, usr.namehaving count(msg) = 0JavaBean的属性可以作为查询的指定参数被绑定：Query q = s.createQuery("from foo in class Foo where foo.name=:name and foo.size=:size");q.setProperties(fooBean); // fooBean has getName() and getSize()List foos = q.list();集合可以通过使用带有过滤器的Query接口实现分页（pageable）功能：Query q = s.createFilter( collection, "" ); // the trivial filterq.setMaxResults(PAGE_SIZE);q.setFirstResult(PAGE_SIZE * pageNumber);List page = q.list();使用查询过滤器，可以实现集合元素的排序和分组：Collection orderedCollection = s.filter( collection, "order by this.amount" );Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );你可以不用对集合进行初始化，就可以得到它的大小：( (Integer) session.iterate("select count(*) from ....").next() ).intValue(); 兔八哥2004年1月4日星期日 17:57