1.索引
group by where having order by
https://blog.csdn.net/whathellll/article/details/80765020concat
https://baijiahao.baidu.com/s?id=1595349117525189591&wfr=spider&for=pc
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6.round
https://www.runoob.com/sql/sql-func-round.html
https://blog.csdn.net/chinabestchina/article/details/72799826
7.count
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8.distinct
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9.group by
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10.having
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11.执行顺序
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12.子查询
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13.自连接,一个题
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14.表和视图
https://blog.csdn.net/m0_37248802/article/details/80563691
