forked from docs/doc-exports
Su, Xiaomeng
04d4597cf3
Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com> Co-authored-by: Su, Xiaomeng <suxiaomeng1@huawei.com> Co-committed-by: Su, Xiaomeng <suxiaomeng1@huawei.com>
32 lines
4.3 KiB
HTML
32 lines
4.3 KiB
HTML
<a name="dli_08_0172"></a><a name="dli_08_0172"></a>
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<h1 class="topictitle1">LEFT SEMI JOIN</h1>
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<div id="body8662426"><div class="section" id="dli_08_0172__en-us_topic_0093946868_sdbe86f1d6233452e9d569664ce48b01c"><h4 class="sectiontitle">Function</h4><p id="dli_08_0172__en-us_topic_0093946868_a142cef1c6b0d48b2b56d4fc60d0da9bf">This statement is used to query the records that meet the JOIN condition from the left table.</p>
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</div>
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<div class="section" id="dli_08_0172__en-us_topic_0093946868_sec0be3cfa6be49218443413c9c0a32b4"><h4 class="sectiontitle">Syntax</h4><div class="codecoloring" codetype="Sql" id="dli_08_0172__screen1637711521349"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
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<span class="normal">2</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="n">attr_expr_list</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">table_reference</span>
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<span class="w"> </span><span class="k">LEFT</span><span class="w"> </span><span class="n">SEMI</span><span class="w"> </span><span class="k">JOIN</span><span class="w"> </span><span class="n">table_reference</span><span class="w"> </span><span class="k">ON</span><span class="w"> </span><span class="n">join_condition</span><span class="p">;</span>
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</pre></div></td></tr></table></div>
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</div>
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</div>
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<div class="section" id="dli_08_0172__en-us_topic_0093946868_s2e45a11f3ef940e1a2bf822cbacac79b"><h4 class="sectiontitle">Keyword</h4><p id="dli_08_0172__en-us_topic_0093946868_a7d695821105c484fbddf5714f36c3365">LEFT SEMI JOIN: Indicates to only return the records from the left table. LEFT SEMI JOIN can be achieved by nesting subqueries in LEFT SEMI JOIN, WHERE...IN, or WHERE EXISTS. LEFT SEMI JOIN returns the records that meet the JOIN condition from the left table, while LEFT OUTER JOIN returns all the records from the left table or NULL if no records that meet the JOIN condition are found.</p>
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<div class="section" id="dli_08_0172__en-us_topic_0093946868_sec14e88cae4a43ef971994ba82b9e840"><h4 class="sectiontitle">Precautions</h4><ul id="dli_08_0172__en-us_topic_0093946868_ud4189340dc60486f8c3d9866e9e0ea0b"><li id="dli_08_0172__en-us_topic_0093946868_l148f8c3ddbb245e88853d9facfe5ada5">The to-be-joined table must exist. Otherwise, an error is reported.</li><li id="dli_08_0172__en-us_topic_0093946868_ld575ad7aad6e487cb09ac4c36e87a3ee">he fields in attr_expr_list must be the fields in the left table. Otherwise, an error is reported.</li></ul>
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</div>
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<div class="section" id="dli_08_0172__en-us_topic_0093946868_s1d063ab342774e0a9c2c94b8bc3b56fb"><h4 class="sectiontitle">Example</h4><p id="dli_08_0172__en-us_topic_0093946868_ac8da23226ab24bfca4dacd8cb132b104">To return the names of students who select the courses and the course IDs, run the following statement:</p>
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<div class="codecoloring" codetype="Sql" id="dli_08_0172__screen394716454351"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
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<span class="normal">2</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="n">student_info</span><span class="p">.</span><span class="n">name</span><span class="p">,</span><span class="w"> </span><span class="n">student_info</span><span class="p">.</span><span class="n">courseId</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">student_info</span>
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<span class="w"> </span><span class="k">LEFT</span><span class="w"> </span><span class="n">SEMI</span><span class="w"> </span><span class="k">JOIN</span><span class="w"> </span><span class="n">course_info</span><span class="w"> </span><span class="k">ON</span><span class="w"> </span><span class="p">(</span><span class="n">student_info</span><span class="p">.</span><span class="n">courseId</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">course_info</span><span class="p">.</span><span class="n">courseId</span><span class="p">);</span>
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</pre></div></td></tr></table></div>
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</div>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dli_08_0165.html">JOIN</a></div>
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</div>
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</div>
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