假设我们有以下数据库条目:
@Entity
data class Dog(
@ColumnInfo(name = "name") val name: String,
@ColumnInfo(name = "breed") val breed: String?
)
如您所见,并非每只狗都定义了品种。现在我们要查询以根据品种搜索所有狗。
@Dao
interface DogDao {
@Query("SELECT * FROM dogs WHERE breed = :breed")
fun getByBreed(breed: String?): List<Dog>
}
有时我们想搜索具有特定品种的狗,有时我们想搜索没有定义品种的狗。问题是在第二种情况下,上面的查询将不起作用。为什么?当 getByBreed(breed:)
方法的 breed
参数为空时,Room 会将此查询转换为如下内容:
SELECT * FROM dogs WHERE breed = NULL
不幸的是,SQLite 查询空值应该是这样的:
SELECT * FROM dogs WHERE breed IS NULL
问题是,如何定义接受可选参数的查询?
最佳答案
您应该查看 SQLite 运算符。直接取自此link .
The IS and IS NOT operators work like = and != except when one or both of the operands are NULL. In this case, if both operands are NULL, then the IS operator evaluates to 1 (true) and the IS NOT operator evaluates to 0 (false). If one operand is NULL and the other is not, then the IS operator evaluates to 0 (false) and the IS NOT operator is 1 (true). It is not possible for an IS or IS NOT expression to evaluate to NULL. Operators IS and IS NOT have the same precedence as =.
所以你应该拥有并且将等同于你拥有的是:
@Dao
interface DogDao {
@Query("SELECT * FROM dogs WHERE breed IS :breed")
fun getByBreed(breed: String?): List<Dog>
}
关于android - 可选查询参数不适用于 Room,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/58992321/