sql行列互换

有一个SQL题在面试中出现的概率极高，最近有学生出去面试仍然会遇到这样的题目，在这里跟大家分享一下。

题目：数据库中有一张如下所示的表，表名为sales。

年	季度	销售量
1991	1	11
1991	2	12
1991	3	13
1991	4	14
1992	1	21
1992	2	22
1992	3	23
1992	4	24

要求：写一个SQL语句查询出如下所示的结果。

年	一季度	二季度	三季度	四季度
1991	11	12	13	14
1992	21	22	23	24

我给出的答案是这样的：

<code class="language-SQL hljs sql has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-operator" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">select</span> 年, 
<span class="hljs-aggregate" style="box-sizing: border-box;">sum</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">when</span> 季度=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span> 销售量 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 一季度, 
<span class="hljs-aggregate" style="box-sizing: border-box;">sum</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">when</span> 季度=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span> 销售量 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 二季度, 
<span class="hljs-aggregate" style="box-sizing: border-box;">sum</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">when</span> 季度=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span> 销售量 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 三季度, 
<span class="hljs-aggregate" style="box-sizing: border-box;">sum</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">when</span> 季度=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span> 销售量 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">end</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 四季度 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> sales <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">group</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> 年</span></code>