相关子查询

from_now 发表于 2011-12-23 03:44

<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 10px; padding-left: 0px; width: 993px; min-height: 200px; float: left; text-overflow: ellipsis; overflow-x: hidden; overflow-y: hidden; word-break: break-all; " id="topics"><div class="post" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; "><div class="postBody" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 5px; padding-right: 2px; padding-bottom: 5px; padding-left: 5px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: black; "><div style="line-height: 1.5em; color: rgb(57, 57, 57); font-family: verdana, 'ms song', Arial, Helvetica, sans-serif; font-size: 14px; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; " id="cnblogs_post_body"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; ">SQL子查询可以分为 <span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; background-color: rgb(255, 255, 255); color: rgb(128, 0, 128); ">相关子查询 和 嵌套子查询 两类。前提，假设Books表如下：类编号  图书名         出版社               价格 -------------------------------------------------------- 2      c#高级应用    圣通出版            23.00 2      Jsp开发应用   机械出版社          45.00 3      高等数学       济南出版社          25.00 3      疯狂英语       清华大学出版社     32.00 　　嵌套子查询的执行不依赖与外部的查询。执行过程：（1）执行子查询，其结果不被显示，而是传递给外部查询，作为外部查询的条件使用。（2）执行外部查询，并显示整个结果。　　嵌套子查询一般可以分为：返回单值的子查询和返回一个列表的子查询，下面举例说明：1.返回单值：  --查询所有价格高于平均价格的图书名，作者，出版社和价格。 <div class="cnblogs_code" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 5px; padding-right: 5px; padding-bottom: 5px; padding-left: 5px; background-color: rgb(245, 245, 245); font-family: 'Courier New'; font-size: 12px; border-top-width: 1px; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-top-style: solid; border-right-style: solid; border-bottom-style: solid; border-left-style: solid; border-top-color: rgb(204, 204, 204); border-right-color: rgb(204, 204, 204); border-bottom-color: rgb(204, 204, 204); border-left-color: rgb(204, 204, 204); word-break: break-all; overflow-x: auto; overflow-y: auto; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; "><span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">USE tempdb GO SElECT 图书名，作者，出版社，价格 FROM Books WHERE 价格 <span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">><span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); "> ( SELECT AVG(价格) FROM Books ) GO</div></div>2.返回值列表--查询所有借阅图书的读者信息<div class="cnblogs_code" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 5px; padding-right: 5px; padding-bottom: 5px; padding-left: 5px; background-color: rgb(245, 245, 245); font-family: 'Courier New'; font-size: 12px; border-top-width: 1px; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-top-style: solid; border-right-style: solid; border-bottom-style: solid; border-left-style: solid; border-top-color: rgb(204, 204, 204); border-right-color: rgb(204, 204, 204); border-bottom-color: rgb(204, 204, 204); border-left-color: rgb(204, 204, 204); word-break: break-all; overflow-x: auto; overflow-y: auto; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; "><span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">SElECT <span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">*<span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); "> FROM Readers WHERE 读者编号 IN ( SELECT 读者编号 FROM ) GO</div></div>　　相关子查询的执行依赖于外部查询。多数情况下是子查询的WHERE子句中引用了外部查询的表。执行过程：（1）从外层查询中取出一个元组，将元组相关列的值传给内层查询。（2）执行内层查询，得到子查询操作的值。（3）外查询根据子查询返回的结果或结果集得到满足条件的行。（4）然后外层查询取出下一个元组重复做步骤1-3，直到外层的元组全部处理完毕。 　　下面举例说明：--查询Booka表中大于该类图书价格平均值的图书信息<div class="cnblogs_code" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 5px; padding-right: 5px; padding-bottom: 5px; padding-left: 5px; background-color: rgb(245, 245, 245); font-family: 'Courier New'; font-size: 12px; border-top-width: 1px; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-top-style: solid; border-right-style: solid; border-bottom-style: solid; border-left-style: solid; border-top-color: rgb(204, 204, 204); border-right-color: rgb(204, 204, 204); border-bottom-color: rgb(204, 204, 204); border-left-color: rgb(204, 204, 204); word-break: break-all; overflow-x: auto; overflow-y: auto; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; "><span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">SElECT 图书名,出版社,类编号,价格 FROM Books As a WHERE 价格 <span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">><span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); "> ( SELECT AVG(价格) FROM Books AS b WHERE a.类编号<span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">=<span style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; font-family: 'Courier New'; font-size: 12px; line-height: 1.5; color: rgb(0, 0, 0); ">b.类编号 ) GO</div></div></div>与前面介绍过的子查询不同，相关子查询无法独立于外部查询而得到解决。该子查询需要一个“类编号”的值。而这个值是个变量，随SQLSever检索Books表中的不同行而改变。下面详细说明该查询执行过程： 　　先将Books表中的第一条记录的“类编号”的值“2”代入子查询中，子查询变为：　　　　　　SELECT AVG(价格)    　　　　　　FROM Books AS b     　　　　 WHERE b.类编号=2　　子查询的结果为该类图书的平均价格，所以外部查询变为：　　　　　　SElECT 图书名,出版社,类编号,价格   　　　　　　 FROM Books As a   　　　　　WHERE 价格 > 34 如果WHERE条件为True，则第一条结果包括在结果集中，则否不包括。对Books表中的所有行运行相同的过程，最后形成的结果集及最后返回结果。</div><div id="blog_post_info_block" style="margin-top: 20px; 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相关子查询