mysql执行顺序
Mysql语法顺序，即当sql中存在下面的关键字时，它们要保持这样的顺序：
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select[distinct]
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from
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join（如left join）
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on
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where
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group by
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having
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union
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order by
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limit
Mysql执行顺序，即在执行时sql按照下面的顺序进行执行：
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from
-
on
-
join
-
where
-
group by
-
having
-
select
-
distinct
-
union
-
order by