Question

我如何在表中两次显示同一列，但使用sql显示不同的数据？

我有3张桌子tbstu, tbexm, tbscr

tbstu = stucod int PK, stunam var(100)

tbexm = exmcod int PK, exmnam var(100)

tbscr = scrcod int PK, scrsub var(100), scrach int 
scrstucod int FK (to table tbstu)
screxmcod int FK (to table tbexm)

qry = 
  select scrcod, screxmcod, scrstucod, scrsub, scrach, scrach 
  from tbscr, tbstu, tbexm
  where scrstucod = stucod 
  and screxmcod = exmcod 
  and stucod = 101 
  and exmcod = 1001

现在，我希望选择的第二个scrach显示关于exmcod=1002和总计列的数据，以对和and进行求和

Answer 1

如果我理解正确，

使用左联接：

    select
      tbscr1.scrcod,
      tbscr2.scrcod ,
      tbscr1.screxmcod,
      tbscr2.screxmcod,
      tbscr1.scrstucod
      tbscr2.scrstucod,
      tbscr1.scrsub,
      tbscr2.scrsub
      tbscr1.scrach,
      tbscr2.scrach,
      tbscr1.scrach+tbscr2.scrach AS Total
from tbstu,tbexm
    left join
      tbscr as tbscr1 on tbscr1.scrstucod =tbstu.stucod and tbscr1.screxmcod = tbexm.exmcod and tbstu.stucod = 101 and tbexm.exmcod =1001
    left join
      tbscr as tbscr2 on tbscr2 .scrstucod =tbstu.stucod and tbscr2 .screxmcod = tbexm.exmcod and tbstu.stucod = 101 and tbexm.exmcod =1002

您也可以尝试：

select scrcod ,
      screxmcod,
      scrstucod,
      scrsub,
      scrach,
      (select scrach from tbscr ,tbstu,tbexm  
        where scrstucod =stucod and screxmcod = exmcod and stucod = 101 and exmcod =1002) AS scrach2
from tbscr ,tbstu,tbexm  
where scrstucod =stucod and screxmcod = exmcod and stucod = 101 and exmcod =1001

Answer 2

根据我的理解，我认为您希望为特定学生获得以下知识：

两次考试（1001和1002）的考试信息
两次考试的总分

如果正确，您可以执行以下操作：

SELECT
  sc1.scrcod, sc1.screxmcod, sc1.scrsub, sc1.scrach,
  sc2.scrcod, sc2.screxmcod, sc2.scrsub, sc2.scrach,
  COALESCE(sc1.scrach, 0) + COALESCE(sc2.scrach, 0) AS TotalScore
FROM tbstu st
LEFT JOIN tbscr sc1 ON st.stucod = sc1.scrstucod AND sc1.screxmcod = 1001 -- Exam 1 info
LEFT JOIN tbscr sc2 ON st.stucod = sc2.scrstucod AND sc2.screxmcod = 1002 -- Exam 2 info
WHERE st.stucod = 101 -- Only get info for student 101

更新
根据您的评论，您似乎还希望根据科目（scrsub来划分考试成绩。您可以将此字段添加到您的JOIN条件中：

SELECT
  st.stucod, 
  sc1.scrsub, 
  sc1.scrach, 
  sc2.scrach, 
  COALESCE(sc1.scrach,0) + COALESCE(sc2.scrach,0) AS TotalScore
FROM tbstu st
LEFT JOIN tbscr sc1 -- Exam 1 info
  ON st.stucod = sc1.scrstucod AND sc1.screxmcod = 1001
LEFT JOIN tbscr sc2 -- Exam 2 info
  ON st.stucod = sc2.scrstucod AND sc2.screxmcod = 1002 AND sc1.scrsub = sc2.scrsub
WHERE st.stucod = 101 -- Only get info for student 101

按照编码，此查询有一些假设：

这仅适用于考试1001和1002
考试1001和1002具有相同的scrsub条目（即每项考试有4行）

让我知道它的工作原理。

我被这个查询卡住了

2 个答案: