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>
30 lines
4.8 KiB
HTML
30 lines
4.8 KiB
HTML
<a name="dli_08_0176"></a><a name="dli_08_0176"></a>
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<h1 class="topictitle1">Subquery Nested by FROM</h1>
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<div id="body8662426"><div class="section" id="dli_08_0176__en-us_topic_0093946817_s40f7ca4a52f04b1484bb9e295afb8b90"><h4 class="sectiontitle">Function</h4><p id="dli_08_0176__en-us_topic_0093946817_a94fb55735fc84e76b06b89e3de690535">This statement is used to nest subquery by FROM and use the subquery results as the data source of the external SELECT statement. </p>
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<div class="section" id="dli_08_0176__en-us_topic_0093946817_s39a9c309e3634aa1b1909be6fd9e058e"><h4 class="sectiontitle">Syntax</h4><div class="codecoloring" codetype="Sql" id="dli_08_0176__screen564313535381"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="p">[</span><span class="k">ALL</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="k">DISTINCT</span><span class="p">]</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="p">(</span><span class="n">sub_query</span><span class="p">)</span><span class="w"> </span><span class="p">[</span><span class="k">alias</span><span class="p">];</span>
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</pre></div></td></tr></table></div>
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<div class="section" id="dli_08_0176__en-us_topic_0093946817_s058d384f091848a0a4385db11ea3eeee"><h4 class="sectiontitle">Keyword</h4><ul id="dli_08_0176__en-us_topic_0093946817_ubf380dd75c454e388bb69bec29937839"><li id="dli_08_0176__en-us_topic_0093946817_l3b1c724fa7e3474eb13b8b2cdb328fcf">All is used to return repeated rows. By default, all repeated rows are returned. It is followed by asterisks (*) only. Otherwise, an error will occur.</li><li id="dli_08_0176__en-us_topic_0093946817_l0813e2c822d94eb3882a704430b294f5">DISTINCT is used to remove the repeated line from the result.</li></ul>
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<div class="section" id="dli_08_0176__en-us_topic_0093946817_s29d0fdf33f50469a8bdc168d19a07753"><h4 class="sectiontitle">Precautions</h4><ul id="dli_08_0176__en-us_topic_0093946817_u296025eaf9cb4b84bc41df355e0b5386"><li id="dli_08_0176__en-us_topic_0093946817_l136a8673c4a542d59a1627f2b2142be2">The to-be-queried table must exist. If this statement is used to query a table that does not exist, an error is reported. </li><li id="dli_08_0176__en-us_topic_0093946817_l17fc5716287b4342be5469469c621c4a">The subquery nested in FROM must have an alias. The alias must be specified before the running of the statement. Otherwise, an error is reported. It is advised to specify a unique alias. </li><li id="dli_08_0176__en-us_topic_0093946817_ld4435bbda0614b21a22ede9c1a80f92c">The subquery results sequent to FROM must be followed by the specified alias. Otherwise, an error is reported. </li></ul>
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<div class="section" id="dli_08_0176__en-us_topic_0093946817_s5aca6e0a1f4640bcb0a6909e666fc602"><h4 class="sectiontitle">Example</h4><p id="dli_08_0176__en-us_topic_0093946817_a38ad165bd55146338bbae2fb2e66cfb5">To return the names of students who select the courses in the <strong id="dli_08_0176__b14118114143120">course_info</strong> table and remove the repeated records using DISTINCT, run the following statement: </p>
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<div class="codecoloring" codetype="Sql" id="dli_08_0176__screen1334721418393"><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="k">DISTINCT</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="p">(</span><span class="k">SELECT</span><span class="w"> </span><span class="n">name</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">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="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><span class="w"> </span><span class="n">temp</span><span class="p">;</span>
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</pre></div></td></tr></table></div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dli_08_0174.html">Subquery</a></div>
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