SQL 基础

<span style="font-size: 15px"><strong>联合主键就是用2个或2个以上的字段组成主键。</strong></span><br><span style="font-size: 15px"><strong>用这个主键包含的字段作为主键，这个组合在数据表中是唯一，且加了主键索引。</strong> <br></span>

<span style="font-size: 15px"><strong>比如这种时候 </strong></span><br><span style="font-size: 15px"><strong>商品品牌 商品型号 </strong></span><br><span style="font-size: 15px"><strong>诺基亚        920 </strong></span><br><span style="font-size: 15px"><strong>三星         NOTE2 </strong></span><br><span style="font-size: 15px"><strong>诺基亚        8088  </strong> <br></span>

<strong><span style="font-size: 15px">就靠这样的联合主键来确定这条记录的唯一性。<br>语句：<br></span></strong>

<span style="font-size: 16px"><strong>create table product( pro_name varchar(<span style="color: #800080">20</span>), pro_type varchar(<span style="color: #800080">20</span>), primary key (pro_name,pro_type) );</strong><br></span>

<strong><span style="font-size: 15px"> <br>2.sql查询一张表中重复的记录：<br></span></strong>

<span style="font-size: 16px"><strong><span style="color: #0000ff">select</span> * <span style="color: #0000ff">from</span> xi a <span style="color: #0000ff">where</span> (a.username) <span style="color: #0000ff">in</span>  (<span style="color: #0000ff">select</span> username <span style="color: #0000ff">from</span> xi group by username  having count(*) > <span style="color: #800080">1</span>);</strong></span>

<strong><span style="font-size: 16px">3.查询出所有数据进行分组之后，和重复数据的重复次数的查询数据</span></strong>

<strong><span style="font-size: 16px"><span style="color: #0000ff">select</span>  count(username) <span style="color: #0000ff">as</span> <span style="color: #800000">‘</span><span style="color: #800000">重复次数</span><span style="color: #800000">‘</span>,username <span style="color: #0000ff">from</span> xi group by username  having count(*)><span style="color: #800080">1</span> order by username desc</span></strong>

<span style="font-size: 16px"><strong>4. </strong></span>

<span style="font-size: 15px"> </span>

<span style="font-size: 15px"> </span>

SQL 基础

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