Hibernate tips 2
Formula
(from : http://weishuwei.iteye.com/blog/68030)
1,formula ="()",里面的是sql语句,字段和表名都应该和数据库相应,而不是字段,若带有参数如cur.id= currencyID,这个currencyID才是对象的东东.
2,formula ="( sql )",这个括号不能少,不然会报错,我试了几次,没括号就报错,添上就没问题
3,操作字段一定要用别名
(from:http://www.cnblogs.com/iloveu/archive/2009/01/20/1378810.html)
* formula公式里的字段都是数据库里的字段名，除了查询条件参数是要和配置文件里的字段匹配。
(from:http://zhiweiv.iteye.com/blog/361525)
这下子对于formula里的sql语句为什么必须放在括号里应该明白了，因为formula里面的值会作为一条sql语句select和from之间的内容，我们把sql语句放到括号里实际上就是做为一条子查询执行！！！ 例如最开始示例的的rank对应的sql语句就应如下样子
dynamic-update
- when true, dynamic generate sql, if any changes for the property, it will appear in update sql, otherwise, wont be included.
dynamic-insert
- when true, insert will dynamic generate sql, if the property is NOT null, it will be included in SQL, otherwise, wont.
lazy-loading
1. If no record in database, still lazy-load uninitialized object.
2. only throw exception when call method of the proxy object
3. when access ID: getID(). wont trigger any database query
Hibernate performance
1. lazy-loading
2. batch-size: the size of elements to retrieve each time
3. one-to-one & many-to-one, better use outer-join to fetch data, it will reduce query times.
4. max-fetch-depth: outer join tables depths...
Join Query
1. hibernate configure: outer-join="true", sql will be left outer join
2. HQL e.g. Person has items, "from Person p inner join p.items" No "on" like the one in SQL
3. SQL, we have "select xxx inner join xxx ON...."
4. HQL join will return a list of array which contains joined objects. e.g. "from customer c left outer join c.order"; if we only want customer, we can use "Select c from customer c left outer join c.order"
5. HQL join fetch will get first object and load second fetch object as reference
6. inner-join & outer-join , result is all arrays which contains joined objects
HQL & Criteria
* hql will check whether passed object has map or not
* criteria will load all implementor of given class.
* check field null using "is null", not "=null" which wont get any result (might has grammar error)
Implict Join applys to "Many-to-one" and "one-to-one" relationship. Not "One-to-Many"
left/right (outer) join : outer can be omited
Hibernate configure
use_query_cache=true, is not enought, need set Query.setCachable("true")
Query or Criteria result
1. If more than one result return, need use setMaxResult() to restrict output and then use uniqueResult
Criterial #Expression #Order