是否可以与自身连接表

Question

这个查询的结果是否能够在不相乘的情况下加入。

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”是否有可能与 9 列连接，指的是第三年的前 3 列，第二年的后 3 列和第一年的后 3 列。 谢谢你的好意回答。

结果我想到的是这样的

学生证 |学生姓名 |学生年 ----------------------------------------- 1 | foo1 | 1 ----------------------------------------- 2 | foo2 | 1 ----------------------------------------- 3 | foo3 | 2 ----------------------------------------- 4 | foo4 | 2 ----------------------------------------- 5 | foo5 | 2 ----------------------------------------- 6 | foo6 | 3 ----------------------------------------- 学生 ID-1 |学生姓名-1 | StudentYear-1 |学生ID-2 |学生姓名-2 |学生二年级 |学生ID-3 |学生姓名-3 | StudentYear-3 -------------------------------------------------- -------------------------------------------------- ------------------------------------------------ 1 | foo1 | 1 | 3 | foo3 | 2 |6 | foo6 | 3 -------------------------------------------------- -------------------------------------------------- ------------------------------------------------ 2 | foo2 | 1 | 4 | foo4 | 2 | | | -------------------------------------------------- -------------------------------------------------- ------------------------------------------------ | | | 5 | foo5 | 2 | | | -------------------------------------------------- -------------------------------------------------- ------------------------------------------------

Answer 1

好的，将我的答案更改为使用子查询并将每个子查询视为具有自己别名列的表。这不会产生您想要的确切结果，这将在其他 6 个未填充的列中产生空值。话虽如此，我相信这与您将在查询中做您想做的事情差不多。

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))

这是查询的结果

我会说我不推荐这种方法。 MySQL 在处理子查询方面已经做得更好了，但是 5.5 之前的任何东西都会有可怕的性能，一旦你达到一定的数据水平，即使 5.5 也不会很好。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，我会检索记录集，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

要达到你的要求，你必须有一些 id 类型的东西加入，所以使用行号，否则你将一对多对多结果

在 SQL Server 中工作。所以在MYSQL中试一试。

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表示它是一个临时表。

Answer 3

是的，您可以将这些查询合并为

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

如果那是你真正想做的。

祝你好运。