今天给大家介绍六条比较有用的MySQL的SQL语句，可能很多人都通过PHP来实现这些功能。 1. 计算年数 你想通过生日来计算这个人有几岁了。 SELECT DATE_FORMAT(FROM_DAYS(TO_DAYS(now())-TO_DAYS(@dateofbirth)), '%Y' )+0; 2. 两个时间的差 取得两个 datetime
今天给大家介绍六条比较有用的MySQL的SQL语句，可能很多人都通过PHP来实现这些功能。
1. 计算年数
你想通过生日来计算这个人有几岁了。
<ol class="dp-sql"><li class="alt"><span><span class="keyword">SELECT</span><span> DATE_FORMAT(FROM_DAYS(TO_DAYS(now()) - TO_DAYS(@dateofbirth)), </span><span class="string">'%Y'</span><span>) + 0; </span></span></li></ol>
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2. 两个时间的差 取得两个 datetime 值的差。
假设 dt1 和 dt2 是 datetime 类型，其格式为 ‘yyyy-mm-dd hh:mm:ss’，那么它们之间所差的秒数为：
<ol class="dp-sql"><li class="alt"><span><span>UNIX_TIMESTAMP( dt2 ) - UNIX_TIMESTAMP( dt1 ) </span></span></li></ol>
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除以60就是所差的分钟数，除以3600就是所差的小时数，再除以24就是所差的天数。
3. 显示某一列出现过N次的值
<ol class="dp-sql"><li class="alt"><span><span class="keyword">SELECT</span><span> id </span><span class="keyword">FROM</span><span> tbl </span><span class="keyword">GROUP</span><span> </span><span class="keyword">BY</span><span> id </span><span class="keyword">HAVING</span><span> </span><span class="func">COUNT</span><span>(*) = N; </span></span></li></ol>
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4. 计算两个日子间的工作日 所谓工作日就是除出周六周日和节假日。
<ol class="dp-sql"><li class="alt"><span><span class="keyword">SELECT</span><span> </span><span class="func">COUNT</span><span>(*) </span><span class="keyword">FROM</span><span> calendar </span><span class="keyword">WHERE</span><span> d </span><span class="op">BETWEEN</span><span> Start </span><span class="op">AND</span><span> Stop </span><span class="op">AND</span><span> DAYOFWEEK(d) </span><span class="op">NOT</span><span> </span><span class="op">IN</span><span>(1,7) </span><span class="op">AND</span><span> holiday=0; </span></span></li></ol>
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5. 查找表中的主键
<ol class="dp-sql">
<li class="alt"><span><span class="keyword">SELECT</span><span> k.column_name </span><span class="keyword">FROM</span><span> information_schema.table_constraints t </span></span></li>
<li><span> </span></li>
<li class="alt"><span><span class="op">JOIN</span><span> information_schema.key_column_usage k USING (constraint_name,table_schema,table_name) </span></span></li>
<li><span> </span></li>
<li class="alt"><span><span class="keyword">WHERE</span><span> t.constraint_type=</span><span class="string">'PRIMARY KEY'</span><span> </span><span class="op">AND</span><span> t.table_schema=</span><span class="string">'db'</span><span> </span><span class="op">AND</span><span> t.table_name=tbl' </span></span></li>
</ol>
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6. 查看你的数库有多大
<ol class="dp-sql">
<li class="alt"><span><span class="keyword">SELECT</span><span> </span></span></li>
<li><span> </span></li>
<li class="alt"><span>table_schema <span class="keyword">AS</span><span> </span><span class="string">'Db Name'</span><span>, </span></span></li>
<li><span> </span></li>
<li class="alt"><span>Round( <span class="func">Sum</span><span>( data_length + index_length ) / 1024 / 1024, 3 ) </span><span class="keyword">AS</span><span> </span><span class="string">'Db Size (MB)'</span><span>, </span></span></li>
<li><span> </span></li>
<li class="alt"><span>Round( <span class="func">Sum</span><span>( data_free ) / 1024 / 1024, 3 ) </span><span class="keyword">AS</span><span> </span><span class="string">'Free Space (MB)'</span><span> </span></span></li>
<li><span> </span></li>
<li class="alt"><span><span class="keyword">FROM</span><span> information_schema.tables </span><span class="keyword">GROUP</span><span> </span><span class="keyword">BY</span><span> table_schema ; </span></span></li>
</ol>
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希望对你有帮助。
原文链接：http://www.cnblogs.com/mingxuan/archive/2011/10/09/six-usefull-mysql-statement.html
