SQL中Having与Where的区别
<p>含义</p>
<p>
<strong>where</strong></p>
<p>
在分组之前就进行选择操作</p>
<p>
<strong>having</strong></p>
<p>
having在分组操作之后才进行选择操作，所以having可以选择聚合函数<br>
eg：</p>
<div class="jb51code">
<div>
<div class="syntaxhighlightersql" id="highlighter_158123">
<div class="toolbar">
<span>?</span>
</div>
<table border="0" cellpadding="0" cellspacing="0"><tbody><tr>
<td class="gutter">
<div class="line number1 index0 alt2">
1</div>
<div class="line number2 index1 alt1">
2</div>
<div class="line number3 index2 alt2">
3</div>
<div class="line number4 index3 alt1">
4</div>
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2">
<code class="sql plain">// 下面这句是可以的</code>
</div>
<div class="line number2 index1 alt1">
<code class="sql keyword">SELECT</code> <code class="sql color2">COUNT</code><code class="sql plain">(*) </code><code class="sql keyword">FROM</code> <code class="sql plain">A</code>
</div>
<div class="line number3 index2 alt2">
<code class="sql keyword">WHERE</code> <code class="sql plain">A.a1>0</code>
</div>
<div class="line number4 index3 alt1">
<code class="sql keyword">GROUP</code> <code class="sql keyword">BY</code> <code class="sql plain">a2；</code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
<div class="jb51code">
<div>
<div class="syntaxhighlightersql" id="highlighter_961059">
<div class="toolbar">
<span>?</span>
</div>
<table border="0" cellpadding="0" cellspacing="0"><tbody><tr>
<td class="gutter">
<div class="line number1 index0 alt2">
1</div>
<div class="line number2 index1 alt1">
2</div>
<div class="line number3 index2 alt2">
3</div>
<div class="line number4 index3 alt1">
4</div>
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2">
<code class="sql plain">//但是下面就不行</code>
</div>
<div class="line number2 index1 alt1">
<code class="sql keyword">SELECT</code> <code class="sql color2">COUNT</code><code class="sql plain">(*) </code><code class="sql keyword">FROM</code> <code class="sql plain">A</code>
</div>
<div class="line number3 index2 alt2">
<code class="sql keyword">WHERE</code> <code class="sql plain">A.a1>0 </code><code class="sql color1">and</code> <code class="sql color2">count</code><code class="sql plain">(*)>1</code>
</div>
<div class="line number4 index3 alt1">
<code class="sql keyword">GROUP</code> <code class="sql keyword">BY</code> <code class="sql plain">a2；</code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
<div class="jb51code">
<div>
<div class="syntaxhighlightersql" id="highlighter_121695">
<div class="toolbar">
<span>?</span>
</div>
<table border="0" cellpadding="0" cellspacing="0"><tbody><tr>
<td class="gutter">
<div class="line number1 index0 alt2">
1</div>
<div class="line number2 index1 alt1">
2</div>
<div class="line number3 index2 alt2">
3</div>
<div class="line number4 index3 alt1">
4</div>
<div class="line number5 index4 alt2">
5</div>
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2">
<code class="sql plain">//必须要改为</code><code class="sql keyword">having</code>
</div>
<div class="line number2 index1 alt1">
<code class="sql keyword">SELECT</code> <code class="sql color2">COUNT</code><code class="sql plain">(*) </code><code class="sql keyword">FROM</code> <code class="sql plain">A</code>
</div>
<div class="line number3 index2 alt2">
<code class="sql keyword">WHERE</code> <code class="sql plain">A.a1>0</code>
</div>
<div class="line number4 index3 alt1">
<code class="sql keyword">GROUP</code> <code class="sql keyword">BY</code> <code class="sql plain">a2</code>
</div>
<div class="line number5 index4 alt2">
<code class="sql keyword">HAVING</code> <code class="sql color2">COUNT</code><code class="sql plain">(*)>1</code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
頁:
[1]