Question

我有应查询表达式并显示它的SQL查询

这是查询

SELECT (classes.amount_of_students - (SELECT COUNT(id_class) FROM classes_students WHERE id_class = 1)) AS free_playces FROM classes INNER JOIN classes_students ON classes.id_class = classes_students.id_class;

以下是表格：

CREATE TABLE classes_students ( id_class SERIAL REFERENCES classes (id_class), id_student SERIAL REFERENCES student (id_student), PRIMARY KEY (id_class, id_student) );

CREATE TABLE classes ( id_class SERIAL PRIMARY KEY, amount_of_hours VARCHAR NOT NULL, amount_of_students VARCHAR NOT NULL, id_subject SERIAL REFERENCES subject (id_subject), id_teacher SERIAL REFERENCES teacher (id_teacher) );

classes表中的数据：

id_class | amount_of_hours | amount_of_students | id_subject | id_teacher ----------+-----------------+--------------------+------------+------------ 1 | 180 | 9 | 1 | 2 2 | 12 | 9 | 2 | 1

还有classes_students：

id_class | id_student ----------+------------ 1 | 1 2 | 2 1 | 3

结果是：

`

free_places

        7
        7
        7

`

我不明白为什么要播三遍。我只需要展示一次。我该如何解决这个问题以及为什么会发生呢？我添加了与众不同的内容，但我认为这不是最佳解决方案。

Answer 1

外部查询中不需要JOIN。同样，您应该将内部查询中的类与外部查询中的类进行匹配。这称为相关子查询：

SELECT (c.amount_of_students -
        (SELECT COUNT(id_class)
         FROM classes_students cs
         WHERE cs.id_class = c.id_class
        ) 
       ) as free_places
FROM classes c
WHERE c.id_class = 1

SQL查询显示几次而不是一次

free_places

1 个答案: