Question

先谢谢。

我有正确的加入工作正常，但当我尝试将其更改为左连接时，我收到了错误。

问题是从表格中获得有关学生的所有信息，该学生在第1课中的成绩高于第2课，而不是来自名为SC的成绩 - 学生表。

学生（SID，SNAME，鼠尾草，SSEX）

SC（SID，CID，得分）

这是我正确的加入：

SELECT * FROM Student RIGHT JOIN (
    SELECT t1.SId, class1, class2 FROM
      (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1,
      (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2
    WHERE t1.SId = t2.SId
    AND t1.class1 > t2.class2
    )r 
ON Student.SId = r.SId;

然后我尝试了类似的事情：

SELECT t1.SId, class1, class2, Student.* FROM
    (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1,
    (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2
    WHERE t1.SId = t2.SId
    AND t1.class1 > t2.class2
)r
LEFT JOIN Student
ON Student.SId = r.SId;

但它没有用。

我对SQL很新，希望你能解释一下。

我正在使用MySQL5.7

Answer 1

通常，如果您有正确的联接查询：

SELECT select_statement
FROM table1 
RIGHT JOIN table2 
ON join_condition;

您只需将其更改为左连接查询，如下所示：

SELECT select_statement
FROM table2 
LEFT JOIN table1
ON join_condition;

对于您的情况，当您将查询更改为左连接时，您更改了select_statement，这会导致错误。

只需将以上内容应用于您的查询：

SELECT * 
FROM  (
    SELECT t1.SId, class1, class2 
    FROM
        (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01') AS t1, 
        (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02') AS t2
    WHERE t1.SId = t2.SId
        AND t1.class1 > t2.class2
) r 
LEFT JOIN Student
ON Student.SId = r.SId;

Answer 2

第一个有效的查询是：

SELECT * FROM Student RIGHT JOIN (
    SELECT t1.SId, class1, class2 FROM
      (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1, 
      (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2
      WHERE t1.SId = t2.SId
      AND t1.class1 > t2.class2
    )r 
ON Student.SId = r.SId;

正在通过此子查询创建表r：

SELECT t1.SId, class1, class2 FROM
  (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1, 
  (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2
WHERE t1.SId = t2.SId
AND t1.class1 > t2.class2

t1.Sid，class1和class2的解析没有歧义。 t1.Sid在r.Sid加入Student.SId = r.Sid时可用SELECT t1.SId, class1, class2, Student.* FROM (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1, (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2 WHERE t1.SId = t2.SId AND t1.class1 > t2.class2 )r LEFT JOIN Student ON Student.SId = r.SId;。因此，查询有效。

导致问题的第二个查询在此处：

Student.*

有两个问题需要解决：

问题1：r未解决，因为FROM部分没有任何引用。
问题2：表SELECT * from (SELECT t1.SId, class1, class2 FROM (SELECT SId, score as class1 FROM sc WHERE sc.CId = '01')AS t1, (SELECT SId, score AS class2 FROM sc WHERE sc.CId = '02')AS t2 WHERE t1.SId = t2.SId AND t1.class1 > t2.class2 )r LEFT JOIN Student ON Student.SId = r.SId;未正确构建。

解决后的两个问题会导致此修改后的第二个查询生效：

IsIncetiveAllowed

什么是左连接等于此右连接？

2 个答案: