sql - Sql 多重嵌套选择

Question

如您所见，使用此代码选择一周的开始和结束。我是 sql 新手，我理解这里一般发生了什么，但我无法详细解释。例如 ;

(select lookupweekid-1 from lookupday where dateoftransaction = date) t2 (lookupweekid)

这让我很困惑。(...) t2 (...), <-我不明白最后一部分的括号在说什么

select start_of_week,end_of_week from
(select dateoftransaction
 from lookupday t1,
      (select lookupweekid-1 from lookupday 
       where dateoftransaction = date) t2 (lookupweekid)
 where t1.lookupweekid=t2.lookupweekid
   and t1.weekdaynumber=1) t3 (start_of_week)
,(select dateoftransaction
  from lookupday t1,
       (select lookupweekid-1 from lookupday 
        where dateoftransaction = date) t2 (lookupweekid)
  where t1.lookupweekid=t2.lookupweekid
  and t1.weekdaynumber=7) t4 (end_of_week)
;

Answer 1

t4 (end_of_week)是一个表别名，它也定义了一个列别名。这避免了必须在派生表中指定列别名。

所以这：

(select dateoftransaction
 from lookupday t1 ... 
 where t1.lookupweekid=t2.lookupweekid
 and t1.weekdaynumber=7) t4 (end_of_week)

相当于：

(select dateoftransaction as end_of_week
 from lookupday t1 ... 
 where t1.lookupweekid=t2.lookupweekid
 and t1.weekdaynumber=7) t4

这在Postgres 手册中有解释