select e.last_name, e.first_name, d.dept_name
from employees as e
left join dept_emp as de
on e.emp_no = de.emp_no
left join departments as d
on de.dept_no = d.dept_no
create table
create table if not exists ...
ccomment ''
添加备注
create table select from ....
索引
create (unique) index nameofindex on table(column);
创建索引
强制索引查询
create index idx_emp_no on salaries(emp_no);
select * from salaries FORCE INDEX (idx_emp_no) where emp_no = 10005;
能够根据索引定位到列，然后根据条件查询。能够增加检索效率
创建视图
create view table as select....
create view table (新列名1，2，3) as
select ....from ....
alter table
用于在已有的表中添加、修改或删除列。
ALTER TABLE table_name
ADD column_name datatype
构造触发器
CREATE <触发器名> < BEFORE | AFTER >
<INSERT | UPDATE | DELETE >
ON <表名> FOR EACH Row<触发器主体>
