拼接

select concat(vend_name,’(’,vend_country,’)’)
as vend_title
from vendors;

组合聚集函数

select count(*) as num_items,
	min(prod_price) as price_min,
	max(prod_price) as price_max,
	avg(prod_price) as price_avg
from products;

分组与排序

1.group by ,having（接在group by 后），order by(必须最后)
2.group by 后必须接实际列或表达式列，不可以接表达式列的别名

-- # 按照vend_id对count分组
select vend_id,count(*) as num_prod
from products
group by vend_id;

select vend_id, count(*) as num_prod
from products
where prod_price >=4 -- 过滤行级：一行一行过滤
group by vend_id
having count(*) >= 2  -- 过滤组级：一整组过滤
order by vend_id