jpql按尺寸排序錯誤

Question

任何人都可以解釋爲什麼會拋出JPQL異常嗎？

Query q = em.createQuery("select object(o) from SiteMember as o order by size(o.memberThread) desc"); 

memberThread refer a list. 

Answer 1

很可能是因爲您使用的JPA實現（也是數據庫供應商可能很重要），它沒有針對此查詢的供應商擴展。根據JPA 2.0以下類型的值可以在ORDER BY：

1.A state_field_path_expression that evaluates to an orderable state field of an 
    entity or embeddable class abstract schema type designated in the SELECT clause 
    by one of the following: 
• a general_identification_variable 
• a single_valued_object_path_expression 

2.A state_field_path_expression that evaluates to the same state field of the same 
    entity or embeddable abstract schema type as a state_field_path_expression in the 
    SELECT clause 

3.A result_variable that refers to an orderable item in the SELECT clause for which 
the same result_variable has been specified. This may be the result of an 
aggregate_expression, a scalar_expression, or a state_field_path_expression in the 
SELECT clause. 

您的查詢工作，例如與EclipseLink的（2.3.2）和MySQL。