函数在做计算时，会将空值列排出//count(*)时不会排出空值
count()
sum
avg
max
min
group by
select
from
where
group by
having
order by
关键字执行顺序排列
from--where--group by --having--order by--select
like %%或like %test 走的是全盘扫描
like test% 走的是索引 相对效率高
多表联合查询
内链接 只查询两表相关联列
select xx.xx ,xx.xx from table1,table2 where ..
select xx.xx,xx.xx from table1 join table1 on ....
外连接 不匹配列也会进行显示
分左外连接和右外连接
左外连接
select e.last_name, d.department_name
from employees e, departments d
where e.department_id=d.department_id(+);
select e.last_name, d.department_name
from employees e, left outer join departments d
on e.department_id=d.department_id
右外连接
select e.last_name, d.department_name
from employees e, departments d
where e.department_id（+）=d.department_id;
select e.last_name, d.department_name
from employees e right outer join departments d
on e.department_id=d.department_id;
完全外连接
select e.last_name, d.department_name
from employees e full outer join departments d
on e.department_id=d.department_id
多个表链接则
select e.last_name, d.department_name
from employees e right outer join departments d
on e.department_id=d.department_id
right outer join XXXX
on XXX=XXX
自然连接 //自动链接查询相同列
select xx.xx,xx.xx from table1 natural join tableb