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<pre accuse="aContent" class="best-text mb-10" id="best-content-439209165">
delete dede_addron where char_length(trim(contant))<100
delete dede_addron where char_length(contant)<100</pre>
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<p>如果<span> USING</span>已经产生，而结果字符串不符合给出的字符集，则会发出警告。<span> </span>同样，如果严格的<span>SQL</span>模式被激活，则<span>CHAR()</span>的结果会成为<span> NULL</span>。</p>
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<p></p>
<ul type="disc">
<li><span>CHAR_LENGTH(<i>str</i>) </span></li>
</ul>
<p><span>返回值为字符串</span><span><i><span>str</span></i></span><span> </span><span>的长度，长度的单位为字符。一个多字节字符算作一个单字符。<i>对于一个</i>包含五个二字节字符集<span>, LENGTH()</span>返回值为<span> 10, </span>而<span>CHAR_LENGTH()</span>的返回值为<span>5</span>。 </span></p>
<ul type="disc">
<li><span>CHARACTER_LENGTH(<i>str</i>) </span></li>
</ul>
<p><span>CHARACTER_LENGTH()</span><span>是<span>CHAR_LENGTH()</span>的同义词。</span></p>
<ul type="disc">
<li><span>COMPRESS(<i>string_to_compress</i>) </span></li>
</ul>
<p><span>压缩一个字符串。这个函数要求<span> MySQL</span>已经用一个诸如<span>zlib</span>的压缩库压缩过。<span> </span>否则，返回值始终是<span>NULL</span>。<span>UNCOMPRESS() </span>可将压缩过的字符串进行解压缩。 </span></p>
<p><span>mysql> <b>SELECT LENGTH(COMPRESS(REPEAT('a',1000)));</b></span></p>
<p><span> -> 21</span></p>
<p><span>mysql> <b>SELECT LENGTH(COMPRESS(''));</b></span></p>
<p><span> -> 0</span></p>
<p><span>mysql> <b>SELECT LENGTH(COMPRESS('a'));</b></span></p>
<p><span> -> 13</span></p>
<p><span>mysql> <b>SELECT LENGTH(COMPRESS(REPEAT('a',16)));</b></span></p>
<p><span> -> 15</span></p>
<p><span>压缩后的字符串的内容按照以下方式存储：</span></p>
<ul type="disc">
<li>
<ul type="circle">
<li><span>空字符串按照空字符串存储。</span></li>
<li><span>非空字符串未压缩字符串的四字节长度进行存储<span>(</span>首先为低字节<span>),</span>后面是压缩字符串。如果字符串以空格结尾，就会在后加一</span><span>个<span>"."</span>号</span><span>,</span><span>以防止当结果值是存储在</span><span>CHAR</span><span>或<span>VARCHAR</span>类型</span><span>的字段列时</span><span>,</span><span>出现自动把结尾空格去掉的现象。</span><span>(</span><span>不推荐使用<span> CHAR </span>或<span>VARCHAR </span>来存储压缩字符串。最好使用一个<span> BLOB </span>列代替）<span>。 </span></span></li>
</ul>
</li>
<li><span>CONCAT(<i>str1</i>,<i>str2</i>,...) </span></li>
</ul>
<p><span>返回结果为连接参数产生的字符串。如有任何一个参数为<span>NULL </span>，则返回值为<span> NULL</span>。或许有一个或多个参数。 如果所有参数均为非二进制字符串，则结果为非二进制字符串。 如果自变量中含有任一二进制字符串，则结果为一个二进制字符串。一个数字参数被转化为与之相等的二进制字符串格式；若要避免这种情况，可使用显式类型<span> cast, </span>例如：<span> SELECT CONCAT(CAST(int_col AS CHAR), char_col) </span></span></p>
<p><span>mysql> <b>SELECT CONCAT('My', 'S', 'QL');</b></span></p>
<p><span> -> 'MySQL'</span></p>
<p><span>mysql> <b>SELECT CONCAT('My', NULL, 'QL');</b></span></p>
<p><span> -> NULL</span></p>
<p><span>mysql> <b>SELECT CONCAT(14.3);</b></span></p>
<p><span> -> '14.3'</span></p>
<ul type="disc">
<li><span>CONCAT_WS(<i>separator</i>,<i>str1</i>,<i>str2</i>,...) </span></li>
</ul>
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