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语法:order by order_col [asc|desc]
order by colum asc 时,null默认被放在最后
order by colum desc 时,null默认被放在最前
语法:order by order_col [asc|desc] nulls [first|last]
nulls first 时,强制null放在最前,不为null的按声明顺序[asc|desc]进行排序
nulls last 时,强制null放在最后,不为null的按声明顺序[asc|desc]进行排序