on与where的区别

select b.town_id ,a.school_name from school a left join town b on (a.town_id = b.town_id); --416
select b.town_id ,a.school_name from school a left join town b on (a.town_id = 110108 and a.town_id = b.town_id); --416
select b.town_id ,a.school_name from school a left join town b on (b.town_id = 110108 and a.town_id = b.town_id); --416
select b.town_id ,a.school_name from school a right join town b on (a.town_id = b.town_id); --416
select b.town_id ,a.school_name from school a right join town b on (a.town_id = 110108 and a.town_id = b.town_id); --91
select b.town_id ,a.school_name from school a right join town b on (b.town_id = 110108 and a.town_id = b.town_id); --91

1、join中on与where
join取得是内连接条件写在on和where后边的结果是一样的。写在on后边是在进行表关联之前过滤数据然后进行关联生成临时表，这样关联的数据量可能少一些，可能会快一些，尤其是表的数据量比较大的时候会明显一些；写在where后边是两张表先进行关联在生成临时表在临时表的基础上进行where条件过滤筛选。
2、left、right中on与where
left、right就是进行左或右连接，无论on后边的条件是否为真都会返回left或right表中的全部数据。
on负责两张表关联时进行数据过滤，但是会返回left或right表中的全部记录（无论是否匹配上）
where负责对生成的临时表进行过滤
以上就是对left、right、join后on与where的理解。水平有限，如果理解的不对望指导。