sql語句UNION

SQL语句中的UNION操作符用于合并两个或多个SELECT语句的结果。UNION要求每个SELECT语句具有相同数量和顺序的列，并且这些列的数据类型也必须相似。UNION会去除重复的行，而UNION ALL不会去重，保留所有行。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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