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我们可以在返回一行结果的 SELECT 中创建一个子查询吗？

例如，我必须使用如下查询：

SELECT columnA as aliasA, columnB as aliasB,
(SELECT columnC FROM table2
WHERE clause2) as aliasC,
(SELECT  columnD FROM table2
WHERE clause2) as  aliasD
FROM table1
WHERE clause1

我想改进它，因为它需要太多时间来执行。有什么方法可以创建查询，例如：

SELECT columnA as aliasA, columnB as aliasB,
(SELECT columnC, columnD FROM table2
WHERE clause2) as (aliasC, aliasD)
FROM table1
WHERE clause1

谢谢

注意：更具体地说，我举一个例子：我需要在第一季度的每个月获得学生的最佳和最差成绩。如果我加入表格，我将为同一个学生提供不止一行（每个月一个）但我需要：

SELECT firstname, lastname, 
(SELECT MAX(value) FROM results WHERE results.student_id = student.id AND results.date
BETWEEN 'JAN-01' AND 'JAN-31') AS best_result_of_jan,
(SELECT MIN(value) FROM results WHERE results.student_id = student.id AND results.date
BETWEEN 'JAN-01' AND 'JAN-31') AS worst_result_of_jan,
 ..., 
(SELECT MIN(value) FROM results WHERE results.student_id = student.id 
AND results.date BETWEEN 'MAR-01' AND 'MAR-31') AS worst_result_of_mar 
FROM student 
WHERE student.has_maths = 1
4

3 回答 3

3

编辑

SELECT s.firstname, s.lastname,
       t.best_result_of_jan, t.worst_result_of_mar
FROM student s 
LEFT JOIN (
    (SELECT student_id,
            MAX(CASE WHEN results.date BETWEEN 'JAN-01' AND 'JAN-31'
                     THEN value ELSE NULL END) best_result_of_jan,
            MIN(CASE WHEN results.date BETWEEN 'MAR-01' AND 'MAR-31'
                     THEN value ELSE NULL END) worst_result_of_mar
     FROM results
     GROUP BY student_id
    ) t ON t.student_id = s.id
WHERE s.has_maths = 1

子查询将数据透视到每个学生所需的列中。然后，外部查询将这些列添加到学生表列中。

首先，只有当子查询返回标量结果（即单行或无）时，您的原始查询才会起作用。这样可以安全地转换为 LEFT JOIN 表单，这也可能导致单行（无笛卡尔积）或无行（fizzed join）。

   SELECT a.columnA as aliasA, a.columnB as aliasB,
          b.columnC as aliasC, b.columnD as aliasD
     FROM table1 a
LEFT JOIN table2 b ON clause2
    WHERE clause1;

确保为第 1 和 2 条中的列设置别名。

注意：此类查询的计划将导致 table2 被处理一次，并且 columnC/D 被解析。您的原始查询可能会导致子查询逐行执行，并导致两次启动，因此缺乏性能。

于 2013-04-17T13:48:49.823 回答
2

您想要加入，例如：

SELECT table1.columnA as aliasA, table1.columnB as aliasB, 
       table2.columnC as aliasC,
       table2.columnD as aliasD
FROM table1 left outer join
     table2
     on clause2
WHERE clause1
于 2013-04-17T13:48:20.730 回答
0

在您的查询中，如果数据范围参数相同，那么您可以编写一个带有最小值和最大值字段的单个选择语句。在您当前的情况下，除了其他解决方案之外，您可以尝试使用 UNION ALL 语句并组合两组查询。这可能比子查询方法更快。例子：

Select A.FirstName,
   A.LastName,
   Sum(A.Best_JAN) as BEST_JAN,
   sum(A.Worst_MAR) as WORST_MAR
From (

    Select student.FirstName,
            student.LastName,
            MAX(result.value) as Best_JAN,
            0 as Worst_MAR
      From Student, Results
     Where Student.Studentid = Results.studentid
       and results.date BETWEEN 'JAN-01' AND 'JAN-31'
     Group By student.FirstName, student.LastName

    Union ALL

    Select student.FirstName,
           student.LastName,
           0 as Best_JAN,
           MIN(result.value) as Worst_MAR
      From Student, Results
     Where Student.Studentid = Results.studentid
       And results.date BETWEEN 'MAR-01' AND 'MAR-31'
     Group By student.FirstName, student.LastName

  ) as A

  Group By A.FirstName, A.LastName
于 2013-04-17T14:17:18.043 回答