sql中as用法

<strong><span style="color: rgb(51, 51, 51);">as   可理解为：用作、当成，作为；</span><span style="color: rgb(255, 0, 0);">一般是重命名列名或者表名</span><span style="color: rgb(51, 51, 51);">。</span></strong><span style="color: rgb(51, 51, 51);">1.例如有表table，  列 column_1,column_2  你可以写成 select  column_1  as  列1,column_2 as  列2   from  table  as  表上面的语句就可以解释为，选择 column_1  作为  列1,column_2 作为   列2  从 table  当成 表</span>

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2.<pre id="answer-content-800959615" class="answer-text mb-10" name="code" style="white-space: pre-wrap; word-wrap: break-word; margin-top: 0px; margin-bottom: 10px; padding: 0px; font-family: arial, 'courier new', courier, 宋体, monospace; color: rgb(51, 51, 51); background-color: rgb(255, 255, 255);">SELECT * FROM Employee AS emp这句意思是查找所有Employee 表里面的数据，并把Employee表格命名为 emp。当你命名一个表之后，你可以在下面用 emp 代替 Employee.例如 SELECT * FROM emp.

</pre><pre id="answer-content-800959615" class="answer-text mb-10" name="code" style="white-space: pre-wrap; word-wrap: break-word; margin-top: 0px; margin-bottom: 10px; padding: 0px; font-family: arial, 'courier new', courier, 宋体, monospace; color: rgb(51, 51, 51); background-color: rgb(255, 255, 255);">3.<pre id="answer-content-800972086" class="answer-text mb-10" name="code" style="white-space: pre-wrap; word-wrap: break-word; margin-top: 0px; margin-bottom: 10px; padding: 0px; font-family: arial, 'courier new', courier, 宋体, monospace; background-color: rgb(255, 255, 255);">把查询对像起个别名的作用。select ID as 用户ID，Name as 用户名 from Table_user查出结果就以中文显示select * from tb_user as mytableA join select * from Tb_UserGroup as mytableBon mytableA.ID=mytableB.ID。这样就可以把查询结果起别名