SELECT FirstName FROM Persons;

SELECT * FROM Persons;

SELECT * FROM Persons WHERE FirstName='Peter';

SELECT * FROM Persons WHERE FirstName LIKE 'a%';

SELECT * FROM Persons WHERE FirstName='Thomas' AND LastName='Carter';

SELECT * FROM Persons WHERE LastName BETWEEN 'Adams' AND 'Carter';

SELECT DISTINCT

ORDER BY

SELECT * FROM Persons ORDER BY FirstName DESC;

DELETE FROM Persons WHERE FirstName = 'Fred';

SELECT COUNT(*) FROM Persons;

select b.department_name as 部门 ,avg(a.salary) as 平均工资
from employees a inner join departments b
on   a.department_id = b.department_id
group by a.department_id;

select a.first_name,a.salary,b.department_name
from employees a inner join departments b
on   a.department_id = b.department_id
where a.first_name = 'steven' and b.department_name = 'executive';

-- (1)这张表有多少行数据？
select count(*)
from   player;
-- (2)id最大的前十位的name是什么？
select id,name
from   player
order by id desc
limit 10;

-- （1）周次
select 日期, date_format(日期,'%u') as 周次
from tb;
--  (2)订单总和、日均订单、极大值订单、极小值订单
select sum(订单) as 订单总和,sum(订单)/count(distinct 日期) as 日均订单,max(订单),min(订单)
from   tb;

select 姓名,max(case 科目 when '语文' then 分数 else 0 end) as 语文,max(case 科目 when '数学' then 分数 else 0 end) as 数学,max(case 科目 when '外语' then 分数 else 0 end) as 外语,
sum(分数) as 总分,avg(分数) as 平均分
from   table
group by 姓名;

select class
from   group
group by class
having count(id) < 1000;

select deptid as 部门,
sum(case subjmonth when 1 then sales else null end) as 一月销售额,
sum(case subjmonth when 2 then sales else null end) as 二月销售额,
sum(case subjmonth when 3 then sales else null end) as 三月销售额,
sum(case subjmonth when 4 then sales else null end) as 四月销售额,
from   deptsales
group by deptid;