以下的文章主要讲述的是MySQL select技巧使用大全,主要对一些select的实际使用技巧的记录,比如正确使用IN、LIMIT以及CONCAT与DISTINCT等实际操作,以下就是文章的对其具体操作步骤的描述。 记录一些MySQL select的技巧: 1、select语句可以用回车分隔 $ sq
以下的文章主要讲述的是MySQL select技巧使用大全,主要对一些select的实际使用技巧的记录,比如正确使用IN、LIMIT以及CONCAT与DISTINCT等实际操作,以下就是文章的对其具体操作步骤的描述。
记录一些MySQL select的技巧:
1、select语句可以用回车分隔
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select * from article where id=1"</span><span> </span></span></li></ol>
和 $sql="select * from article
where id=1",都可以得到正确的结果,但有时分开写或许能更明了一点,特别是当sql语句比较长时
2、批量查询数据
可以用in来实现
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select * from article where id in(1,3,5)"</span><span> </span></span></li></ol>
3、使用concat连接查询的结果
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select concat(id,"</span><span>-",con) as res from article where </span><span class="attribute">id</span><span>=</span><span class="attribute-value">1</span><span>" </span></span></li></ol>
返回"1-article content"
4、使用locate
用法:select locate("hello","hello baby");返回1
不存在返回0
5、MySQL select的技巧:使用group by
以前一直没怎么搞明group by 和 order by,其实也满简单的,group by 是把相同的结果编为一组
exam:
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select city ,count(*) from customer group by city"</span><span>; </span></span></li></ol>
这句话的意思就是从customer表里列出所有不重复的城市,及其数量(有点类似distinct)
group by 经常与AVG(),MIN(),MAX(),SUM(),COUNT()一起使用
6、使用having
having 允许有条件地聚合数据为组
<ol class="dp-xml"> <li class="alt"><span><span>$</span><span class="attribute">sql</span><span>="select city,count(*),min(birth_day) from customer </span></span></li> <li> <span>group by city having count(*)</span><span class="tag">></span><span>10"; </span> </li> </ol>
这句话是先按city归组,然后找出city地数量大于10的城市
btw:使用group by + having 速度有点慢
同时having子句包含的表达式必须在之前出现过
7、组合子句
where、group by、having、order by(如果这四个都要使用的话,一般按这个顺序排列)
8、使用distinct
distinct是去掉重复值用的
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select distinct city from customer order by id desc"</span><span>; </span></span></li></ol>
这句话的意思就是从customer表中查询所有的不重复的city
9、使用limit
如果要显示某条记录之后的所有记录
<ol class="dp-xml"><li class="alt"><span><span>$</span><span class="attribute">sql</span><span>=</span><span class="attribute-value">"select * from article limit 100,-1"</span><span>; </span></span></li></ol>
10、多表查询
<ol class="dp-xml"> <li class="alt"><span><span>$</span><span class="attribute">sql</span><span>="select user_name from user u,member m </span></span></li> <li> <span>where </span><span class="attribute">u.id</span><span>=</span><span class="attribute-value">m</span><span>.id and </span> </li> <li class="alt"> <span>m.reg_date</span><span class="tag">></span><span>=2006-12-28 </span> </li> <li><span>order by u.id desc" </span></li> </ol>
注意:如果user和member两个标同时有user_name字段,会出现mysql错误(因为mysql不知道你到底要查询哪个表里的user_name),必须指明是哪个表的
以上的相关内容就是对MySQL select的技巧的介绍,望你能有所收获。

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