mysql链接

inner join
from table_a a inner join table_b b on a.xxx = b.xxx
left join
左链接会读取左表中的数据，即便右边中没有数据
from table_a a left join table_b b on a.xxx = b.xxx
right join
右链接会读取右表中的数据，即便左表中没有数据
from table_a a right join table_b b on a.xxx = b.xxx