MYSQL 學習筆記2
阿新 • • 發佈:2018-12-17
5、資料彙總
聚集函式:執行在組上,計算和返回單個值的函式
avg count max min sum
select avg(prod_price) as avg_price from products;
好處:不再顯示所有的資料,節省傳輸頻寬
max min也可用於文字,表示最後(前)行
select avg(distinct prod_price) as avg_price from products where vend_id = 1003;
使用disctinct,只計算其中不同值的平均值
組合使用:
select count(*) as num_items,
min(prod_price) as price_min,
max(prod_price) as price_max,
from products;
6、資料分組
select vend_id,count(*) as num_prods from products group by vend_id;
以vend_id,分組求和
group by 位置在where之後 order by之前
select vend_id,count(*) as num_prods
from products
where prod_price >= 10
group by vend_id
having count(*) >= 2;
統計price 大於等於10的,並按id計數,最終顯示計數大於等於2的
where用於前過濾,having用於後過濾