通常，如果您有一个正确的连接查询：

SELECT select_statement
FROM table1 
RIGHT JOIN table2 
ON join_condition;

您只需将其更改为left join query，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left join时，您更改了select\语句，它会出错。
只需将上述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;

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;

table r 正在通过此子查询创建：

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` 以及 `class2` 无歧义地解决。 `t1.Sid` 可用作 `r.Sid` 在连接中 `Student.SId = r.Sid` . 因此，查询是有效的。
引起问题的第二个查询是：

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;

有两个问题需要解决：
问题1： `Student.*` 未解析，因为from节没有任何引用。
问题2：表格 `r` 构造不正确。
解决这两个问题后，将生成第二个修改后的查询，该查询可以工作：

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;