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113 lines
3.6 KiB
HTML
113 lines
3.6 KiB
HTML
<p>学生表: <code>Students</code></p>
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<pre>
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+---------------+---------+
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| Column Name | Type |
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+---------------+---------+
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| student_id | int |
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| student_name | varchar |
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+---------------+---------+
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在 SQL 中，主键为 student_id（学生ID）。
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该表内的每一行都记录有学校一名学生的信息。
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</pre>
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<p> </p>
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<p>科目表: <code>Subjects</code></p>
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<pre>
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+--------------+---------+
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| Column Name | Type |
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+--------------+---------+
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| subject_name | varchar |
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+--------------+---------+
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在 SQL 中，主键为 subject_name（科目名称）。
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每一行记录学校的一门科目名称。
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</pre>
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<p> </p>
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<p>考试表: <code>Examinations</code></p>
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<pre>
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+--------------+---------+
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| Column Name | Type |
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+--------------+---------+
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| student_id | int |
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| subject_name | varchar |
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+--------------+---------+
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这个表可能包含重复数据（换句话说，在 SQL 中，这个表没有主键）。
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学生表里的一个学生修读科目表里的每一门科目。
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这张考试表的每一行记录就表示学生表里的某个学生参加了一次科目表里某门科目的测试。
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</pre>
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<p> </p>
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<p>查询出每个学生参加每一门科目测试的次数，结果按 <code>student_id</code> 和 <code>subject_name</code> 排序。</p>
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<p>查询结构格式如下所示。</p>
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<p> </p>
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<p><strong>示例 1：</strong></p>
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<pre>
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<strong>输入：</strong>
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Students table:
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+------------+--------------+
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| student_id | student_name |
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+------------+--------------+
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| 1 | Alice |
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| 2 | Bob |
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| 13 | John |
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| 6 | Alex |
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+------------+--------------+
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Subjects table:
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+--------------+
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| subject_name |
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+--------------+
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| Math |
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| Physics |
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| Programming |
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+--------------+
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Examinations table:
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+------------+--------------+
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| student_id | subject_name |
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+------------+--------------+
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| 1 | Math |
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| 1 | Physics |
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| 1 | Programming |
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| 2 | Programming |
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| 1 | Physics |
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| 1 | Math |
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| 13 | Math |
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| 13 | Programming |
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| 13 | Physics |
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| 2 | Math |
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| 1 | Math |
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+------------+--------------+
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<strong>输出：</strong>
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+------------+--------------+--------------+----------------+
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| student_id | student_name | subject_name | attended_exams |
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+------------+--------------+--------------+----------------+
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| 1 | Alice | Math | 3 |
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| 1 | Alice | Physics | 2 |
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| 1 | Alice | Programming | 1 |
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| 2 | Bob | Math | 1 |
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| 2 | Bob | Physics | 0 |
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| 2 | Bob | Programming | 1 |
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| 6 | Alex | Math | 0 |
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| 6 | Alex | Physics | 0 |
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| 6 | Alex | Programming | 0 |
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| 13 | John | Math | 1 |
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| 13 | John | Physics | 1 |
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| 13 | John | Programming | 1 |
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+------------+--------------+--------------+----------------+
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<strong>解释：</strong>
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结果表需包含所有学生和所有科目（即便测试次数为0）：
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Alice 参加了 3 次数学测试, 2 次物理测试，以及 1 次编程测试；
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Bob 参加了 1 次数学测试, 1 次编程测试，没有参加物理测试；
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Alex 啥测试都没参加；
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John 参加了数学、物理、编程测试各 1 次。
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</pre>
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