against 是介词，意思是"对着，对比"。
database为先行词，其后的against which talented players may be compared in order to identify strengths and weaknesses是一个定语从句。
关系代词which代指先行词database，在该定语从句中充当介词against的宾语。
因为定语从句的语法需要，介词和关系词which一起提到从句句首。
正常句序为：
against which talented players may be compared in order to identify strengths and weaknesses
= talented players may be compared against which in order to identify strengths and weaknesses
= talented players may be compared against (the database) in order to identify strengths and weaknesses
