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45 lines
1.7 KiB
HTML
45 lines
1.7 KiB
HTML
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<p>部门表 <code>Department</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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| id | int |
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| revenue | int |
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| month | varchar |
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+---------------+---------+
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(id, month) 是表的联合主键。
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这个表格有关于每个部门每月收入的信息。
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月份（month）可以取下列值 ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]。
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</pre>
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<p> </p>
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<p>编写一个 SQL 查询来重新格式化表，使得新的表中有一个部门 id 列和一些对应 <strong>每个月 </strong>的收入（revenue）列。</p>
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<p>查询结果格式如下面的示例所示：</p>
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<pre>
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Department 表：
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+------+---------+-------+
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| id | revenue | month |
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+------+---------+-------+
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| 1 | 8000 | Jan |
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| 2 | 9000 | Jan |
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| 3 | 10000 | Feb |
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| 1 | 7000 | Feb |
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| 1 | 6000 | Mar |
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+------+---------+-------+
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查询得到的结果表：
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+------+-------------+-------------+-------------+-----+-------------+
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| id | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
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+------+-------------+-------------+-------------+-----+-------------+
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| 1 | 8000 | 7000 | 6000 | ... | null |
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| 2 | 9000 | null | null | ... | null |
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| 3 | null | 10000 | null | ... | null |
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+------+-------------+-------------+-------------+-----+-------------+
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注意，结果表有 13 列 (1个部门 id 列 + 12个月份的收入列)。
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</pre>
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