sql语句优化

正常sql语句顺序：select-from- join on- where-group by-having-order by -limit join语句：inner join 取公共有部分   left join 取左表的独有和两表的共有  right join 取右表的独有和两表的共有 select *from a left join b on a.key=b.key where b.key i

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