Question

此查询的结果是否能够在不相互相乘的情况下加入。

Select StudID, StudName, StudYear from tbl_students where StudYear='Third Year';
Select StudID, StudName, StudYear from tbl_students where StudYear='Second Year';
Select StudID, StudName, StudYear from tbl_students where StudYear='First Year';

或者这些查询的“结果”是否可能与9列连接，引用第三年的前3列，第二年的第3列和第1年的最后3列。谢谢你的回答。

我想到的结果是这样的

StudentID   | StudentName   | StudentYear
-----------------------------------------
1           | foo1           |  1
-----------------------------------------
2           | foo2           |  1
-----------------------------------------
3           | foo3           |  2
-----------------------------------------
4           | foo4           |  2
-----------------------------------------
5           | foo5           |  2
-----------------------------------------
6           | foo6           |  3
-----------------------------------------

StudentID-1   | StudentName-1   | StudentYear-1  |StudentID-2   | StudentName-2   | StudentYear-2 | StudentID-3   | StudentName-3   | StudentYear-3
----------------------------------------------------------------------------------------------------------------------------------------------------
1             | foo1            |  1             | 3            | foo3            |  2             |6             | foo6           | 3       
----------------------------------------------------------------------------------------------------------------------------------------------------
2             | foo2            |  1             | 4            | foo4            |  2             |              |                |      
----------------------------------------------------------------------------------------------------------------------------------------------------
              |                 |                | 5            | foo5            |  2             |              |                |       
----------------------------------------------------------------------------------------------------------------------------------------------------

Answer 1

确定改变了我使用子查询的答案，并将每个子查询视为具有自己的别名列的表。这不会产生您想要的确切结果，这将在未填充的其他6列中产生空值。这就是说我相信这一点非常接近您将在查询中执行您想要的操作。 / p>

SELECT StudentFirst.*, StudentSecond.*, StudentThird.*
FROM (((tbl_students ts 
left join (SELECT StudID AS StudIDFirst, StudName AS StudNameFirst, StudYear AS StudYearFirst FROM tbl_students WHERE StudYear = 'First Year') AS StudentFirst on (ts.StudID = StudentFirst.StudIDFirst))
left join (SELECT StudID AS StudIDSecond, StudName AS StudNameSecond, StudYear AS StudYearSecond FROM tbl_students WHERE StudYear = 'Second Year') AS StudentSecond on (ts.StudID = StudentSecond.StudIDSecond))
left join (SELECT StudID AS StudIDThird, StudName AS StudNameThird, StudYear AS StudYearThird FROM tbl_students WHERE StudYear ='Third Year') AS StudentThird on (ts.StudID = StudentThird.StudIDThird))

以下是查询的结果

我会说我不推荐这种方法。 MySQL在处理子查询方面已经有了很多好处，但5.5之前的任何事情都会有糟糕的表现，一旦你达到一定的数据水平，甚至5.5不会很好。同样使用你的mysql语句来格式化结果也不是一个好主意。如果您需要以某种方式查看结果，我将检索记录集，然后将它们以该格式放在应用程序的前端。我希望这会有所帮助。

Answer 2

是的，您可以将这些查询一起加入

SELECT * FROM
  (Select StudentID, StudentName, StudentYear as Third_StudYear
     from Students
     where StudentYear=3) y3
  CROSS JOIN (Select StudentID, StudentName, StudentYear as Second_StudYear
                from Students
                where StudentYear=2) y2
  CROSS JOIN (Select StudentID, StudentName, StudentYear as First_StudYear
                from Students
                where StudentYear=1) y1

如果这是你真正想做的事情。

祝你好运。

Answer 3

为了达到你的要求，你必须有一些id类的东西才能加入，所以使用了行号，否则你会有一对多到多个resutls

在SQL Server中工作。所以在MYSQL中尝试一下。

CREATE TABLE #STUDENT(ID INT, NAME VARCHAR(10), STUDENT_YEAR INT);
INSERT INTO #STUDENT VALUES(1,'FOO1',1);
INSERT INTO #STUDENT VALUES(2,'FOO2',1);
INSERT INTO #STUDENT VALUES(3,'FOO3',2);
INSERT INTO #STUDENT VALUES(4,'FOO4',2);
INSERT INTO #STUDENT VALUES(5,'FOO5',2);
INSERT INTO #STUDENT VALUES(6,'FOO6',3);


SELECT * FROM (
    SELECT 
        ROW_NUMBER() OVER (ORDER BY S1.ID ASC) RN,
        S1.ID AS [ID_1], S1.NAME AS [NAME_1],S1.STUDENT_YEAR AS [YEAR_1]  
    FROM #STUDENT S1
    where s1.STUDENT_YEAR=1
) T1    
FULL JOIN (
    SELECT 
        ROW_NUMBER() OVER (ORDER BY S2.ID ASC) RN,
        S2.ID AS [ID_2], S2.NAME AS [NAME_2],S2.STUDENT_YEAR AS [YEAR_2]  
    FROM #STUDENT S2
    where s2.STUDENT_YEAR=2
) T2 ON T1.RN = T2.RN
FULL JOIN (
    SELECT 
        ROW_NUMBER() OVER (ORDER BY S3.ID ASC) RN,
        S3.ID AS [ID_3], S3.NAME AS [NAME_3],S3.STUDENT_YEAR AS [YEAR_3]  
    FROM #STUDENT S3
    where s3.STUDENT_YEAR=3
) T3 ON T1.RN = T3.RN

注意：#Student表示它是临时表。

是否可以将表与自己联系起来

3 个答案: