DB4O學習（三）對象的查詢

夜的影子2009

1.三種查詢方式
  （1）QBE--只能精確查找
  （2）Simple Object Data Access（SODA）--DB4O的一套查詢API
  （3）Native Queries（NQ）--調整查詢以達到更高的查詢速度

2.三種查詢方式的對比


3.QBE的幾種方式：
（1）通過構造器指定參數(shù)
Person template = new Person();
template.setName("Ben");
ObjectSet res = db.get(template);
（2）通過setter指定參數(shù)
Person template = new Person();
template.setName("Ben");
template.setAge(42);
ObjectSet res = db.get(template);
（3）其他方法
Person template = new Person(null, 82);   // match age only Person template = new Person("Ben", 0);   // match name only
Person template = new Person(null, 0);    // ensure an empty template

（4）指定類型
ObjectSet res = db.get(Person.class);   // JAVA
（5）獲取全部類型
ObjectSet res = db.get(null);   // JAVA

4.QBE的幾個問題
• You can’t use the values that are defined to mean “empty” as constraints. Fields specified
as null, 0, or "" (empty string) are treated as unconstrained, which could be a problem
in some cases. For example, what if you actually want to find Person objects with _age
equal to 0? You can’t when using QBE.
如果想查找age為0的person則不能用QBE.
• You might run into problems if your classes initialize fields when they are declared. For
example, what if the Person class initializes the attribute string _name = "Joe"? In that
case, a query that uses the default constructor to create an “empty” template will only
return “Joe” objects. To get a truly empty template, you would have to explicitly set the
_name attribute to null.
對于類內部初始化的情況要特別注意初始化的值。


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