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Lu, Huayi ef0ada5a59 DWS DEV 20240716 version 
Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com>
Co-authored-by: Lu, Huayi <luhuayi@huawei.com>
Co-committed-by: Lu, Huayi <luhuayi@huawei.com>
2024-11-02 09:07:47 +00:00

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<a name="EN-US_TOPIC_0000002040291766"></a><a name="EN-US_TOPIC_0000002040291766"></a>
<h1 class="topictitle1">Usage of UNION</h1>
<div id="body8662426"><p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p93141350194819"><strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b75114218399">UNION</strong> is a powerful SQL operator that combines the result sets of two or more SELECT statements into one. During combination, the number of columns and data types in the two tables must be the same and correspond to each other. Use the <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b729652983911">UNION</strong> or <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b195635324399">UNION</strong> ALL keyword between SELECT statements.</p>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p980816145488">UNION removes duplicate rows, while UNION ALL keeps them. Deduplication is time-consuming, so UNION ALL can be faster than UNION if the data sets are already distinct by logic.</p>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p1382514481643"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image13270437756" src="figure/en-us_image_0000001775288212.png" width="518.7" height="188.6206" title="Click to enlarge" class="imgResize"></span></p>
<div class="section" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_section762418103526"><h4 class="sectiontitle">Syntax</h4><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen13560253135120"><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="k">column</span><span class="p">,...</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">table1</span><span class="w"> </span><span class="k">UNION</span><span class="w"> </span><span class="p">[</span><span class="k">ALL</span><span class="p">]</span><span class="k">SELECT</span><span class="w"> </span><span class="k">column</span><span class="p">,...</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">table2</span>
</pre></div></td></tr></table></div>
</div>
</div>
<div class="section" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_section978619715519"><h4 class="sectiontitle">Example</h4><ol id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_ol36741126171219"><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li18674426121212"><span>Create the student information table <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b14682144413476">student</strong> (ID, name, gender, and school).</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen194420101418"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SET</span><span class="w"> </span><span class="n">current_schema</span><span class="o">=</span><span class="k">public</span><span class="p">;</span>
<span class="k">DROP</span><span class="w"> </span><span class="k">TABLE</span><span class="w"> </span><span class="k">IF</span><span class="w"> </span><span class="k">EXISTS</span><span class="w"> </span><span class="n">student</span><span class="p">;</span>
<span class="k">CREATE</span><span class="w"> </span><span class="k">table</span><span class="w"> </span><span class="n">student</span><span class="p">(</span><span class="w"> </span>
<span class="n">sId</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">,</span>
<span class="n">sname</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">,</span>
<span class="n">ssex</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULl</span><span class="w"> </span><span class="p">,</span>
<span class="n">sschool</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULl</span><span class="w"> </span><span class="p">);</span>
</pre></div></td></tr></table></div>
</div>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li162121181314"><span>Insert data into the <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b5181257194712">student</strong> table.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen161791414151512"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span>
<span class="normal">8</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s01'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'ZhaoLei'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'NENU'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s02'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'QianDian'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'SJTU'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s03'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'SunFenng'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'Tongji'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s04'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'LIYun'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'CCOM'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s05'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'ZhouMei'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'female'</span><span class="p">,</span><span class="w"> </span><span class="s1">'FuDan'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s06'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'WuLan'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'female'</span><span class="p">,</span><span class="w"> </span><span class="s1">'WHU'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s07'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'ZhengZhu'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'female'</span><span class="p">,</span><span class="w"> </span><span class="s1">'NWAFU'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">student</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'s08'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'ZhangShan'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'female'</span><span class="p">,</span><span class="w"> </span><span class="s1">'Tongji'</span><span class="p">);</span>
</pre></div></td></tr></table></div>
</div>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li17275144711718"><span>View the student table.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen964921702"><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="o">*</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">student</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p820013581937">Information similar to the following is displayed.</p>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p1742549837"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image18231144815504" src="figure/en-us_image_0000001861127377.png"></span></p>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li141226961810"><span>Create the teacher information table <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b186371188483">teacher</strong> (ID, name, gender, and school).</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen1611453782119"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">DROP</span><span class="w"> </span><span class="k">TABLE</span><span class="w"> </span><span class="k">IF</span><span class="w"> </span><span class="k">EXISTS</span><span class="w"> </span><span class="n">teacher</span><span class="p">;</span>
<span class="k">CREATE</span><span class="w"> </span><span class="k">table</span><span class="w"> </span><span class="n">teacher</span><span class="p">(</span><span class="w"> </span>
<span class="n">tid</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">,</span>
<span class="n">tname</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">,</span>
<span class="n">tsex</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">,</span>
<span class="n">tschool</span><span class="w"> </span><span class="nb">VARCHAR</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="k">NOT</span><span class="w"> </span><span class="k">NULL</span><span class="w"> </span><span class="p">);</span>
</pre></div></td></tr></table></div>
</div>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li13846145412114"><span>Insert data to the <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b0528122720488">teacher</strong> table.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen96201226225"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">teacher</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'t01'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'ZhangLei'</span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'FuDan'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">teacher</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'t02'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'LiLiang'</span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'WHU'</span><span class="p">);</span><span class="w"> </span>
<span class="k">INSERT</span><span class="w"> </span><span class="k">INTO</span><span class="w"> </span><span class="n">teacher</span><span class="w"> </span><span class="k">VALUES</span><span class="p">(</span><span class="s1">'t03'</span><span class="w"> </span><span class="p">,</span><span class="w"> </span><span class="s1">'WangGang'</span><span class="p">,</span><span class="w"> </span><span class="s1">'male'</span><span class="p">,</span><span class="w"> </span><span class="s1">'Tongji'</span><span class="p">);</span>
</pre></div></td></tr></table></div>
</div>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li951416179714"><span>Query the <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b2558133354811">teacher</strong> table.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen861013401077"><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="o">*</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">teacher</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p129231432372"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image1270820543514" src="figure/en-us_image_0000001814327924.png"></span></p>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li13500340152311"><span>Use <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b61911369499">UNION</strong> (combine and deduplicate) to obtain the schools of students and teachers and sort the schools in ascending order by initial letter of the school name.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen107661327283"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span>
<span class="normal">8</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">school</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="p">(</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">sschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">student</span>
<span class="w"> </span><span class="k">UNION</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">tschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">teacher</span>
<span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">t</span>
<span class="w"> </span><span class="k">ORDER</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">school</span><span class="w"> </span><span class="k">ASC</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p135519131435">Information similar to the following is displayed.</p>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p528111192313"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image7897111314819" src="figure/en-us_image_0000001820529137.png"></span></p>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li3248210202917"><span>Use <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b14964183894912">UNION ALL</strong> (combine without deduplication) to obtain the schools of all students and teachers and sort the schools by initial letter of the school name in ascending order.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen182178443111"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span>
<span class="normal">8</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">school</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="p">(</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">sschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">student</span>
<span class="w"> </span><span class="k">UNION</span><span class="w"> </span><span class="k">ALL</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">tschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">teacher</span>
<span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">t</span>
<span class="w"> </span><span class="k">ORDER</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">school</span><span class="w"> </span><span class="k">ASC</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p8269134919817"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image96853501384" src="figure/en-us_image_0000001773729626.png"></span></p>
</p></li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li26993413114"><span>Use <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b13429142124917">UNION ALL</strong> (combine the result sets of SQL statements with <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b37044415503">WHERE</strong> clause) to get all information about students and teachers from "Tongji' and sort by student and teacher number in ascending order.</span><p><div class="codecoloring" codetype="Sql" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_screen639071217327"><div class="highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span></pre></div></td><td class="code"><div><pre><span></span><span class="k">SELECT</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="o">*</span><span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="p">(</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">Sid</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="p">,</span><span class="n">Sname</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="p">,</span><span class="n">Ssex</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">sex</span><span class="w"> </span><span class="p">,</span><span class="n">Sschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">student</span>
<span class="w"> </span><span class="k">WHERE</span><span class="w"> </span><span class="n">Sschool</span><span class="o">=</span><span class="s1">'Tongji'</span>
<span class="w"> </span><span class="k">UNION</span><span class="w"> </span><span class="k">ALL</span>
<span class="w"> </span><span class="k">SELECT</span><span class="w"> </span><span class="n">Tid</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="p">,</span><span class="n">Tname</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="p">,</span><span class="n">Tsex</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">sex</span><span class="w"> </span><span class="p">,</span><span class="n">Tschool</span><span class="w"> </span><span class="k">AS</span><span class="w"> </span><span class="n">school</span>
<span class="w"> </span><span class="k">FROM</span><span class="w"> </span><span class="n">teacher</span>
<span class="w"> </span><span class="k">WHERE</span><span class="w"> </span><span class="n">Tschool</span><span class="o">=</span><span class="s1">'Tongji'</span>
<span class="p">)</span><span class="w"> </span><span class="n">t</span>
<span class="w"> </span><span class="k">ORDER</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">id</span><span class="w"> </span><span class="k">ASC</span><span class="p">;</span>
</pre></div></td></tr></table></div>
</div>
<p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p8769152821117"><span><img id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_image846116561526" src="figure/en-us_image_0000001814328680.png"></span></p>
</p></li></ol>
</div>
<div class="section" id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_section1421411466617"><h4 class="sectiontitle">Summary</h4><p id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_p1782819482618">In actual service scenarios, pay attention to the following points when using <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b166193775420">UNION</strong> and <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b41101015125411">UNION ALL</strong>:</p>
<ul id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_ul191561568710"><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li1115676676">The number of SQL fields and field types on the left and right sides must be the same.</li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li96510201775">Check whether data deduplication (deduplication before combination or during combination) is needed based on service requirements.</li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li89092318720">Based on the data volume, valuate the SQL execution efficiency and determine whether to use temporary tables.</li><li id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_li1212619258720">Select <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b14576979112">UNION</strong> or <strong id="EN-US_TOPIC_0000002040291766__en-us_topic_0000001820463205_b17576972117">UNION ALL</strong> wisely and consider the complexity when writing SQL statements.</li></ul>
</div>
</div>
<div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dws_04_1001.html">Data Read</a></div>
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