mysql将表a中查询的数据插入到表b中
如果表b存在
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<code class=sql><code class=sql><code class=sql>insert into b select * from a;</code></code></code>
如果表b不存在
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<code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql>create table b as select * from a;</code></code></code></code></code></code>
扩展：
将b表中的某写字段值插入到a表中
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<code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql>insert into a (userid,username) select b.userid,b.username from tr_ajax_chat_messages;</code></code></code></code></code></code></code></code></code></code></code>
将a表和b表userid相等的值保存到a表
1<code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql><code class=sql>update a set a.username=(select b.username from b where a.userid = b.userid);</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>，
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