mysql 統(tǒng)計(jì)函數(shù)和group by

五個(gè)統(tǒng)計(jì)函數(shù)（單獨(dú)使用，意義不大，經(jīng)常和分組group by組合使用）
max 最大 select max(shop_price) from goods;
min 最小 select min(shop_price) from goods;
sum 求和 select sum(shop_price) from goods;
avg 求平均值 select avg(shop_price) from goods;
count 所有值得行數(shù)有多少行?
count（*）絕對(duì)行數(shù)null也計(jì)算在內(nèi)
除此之外count（列名），計(jì)算這一列非null的行數(shù)

count使用?

mysql>?select?*?from?test8;  +------+------+  |?id?|?name?|  +------+------+  |?1?|?lisi?|?  |?2?|?NULL?|?  +------+------+  mysql>?select?count(*)?from?test8;  +----------+  |?count(*)?|  +----------+  |?2?|?  +----------+  mysql>?select?count(name)?from?test8;  +-------------+  |?count(name)?|  +-------------+  |?1?|?  +-------------+

查詢類(lèi)型為4的庫(kù)存

select?sum(goods_number)?from?goods?where?cat_id=4;

group by
統(tǒng)計(jì)一下每個(gè)類(lèi)型分組下的庫(kù)存

mysql>?select?cat_id,sum(goods_number)?from?goods?group?by?cat_id;  +--------+-------------------+  |?cat_id?|?sum(goods_number)?|  +--------+-------------------+  |?2?|?0?|?  |?3?|?203?|?  |?4?|?4?|?  |?5?|?8?|?  |?8?|?61?|?  |?11?|?23?|?  |?13?|?4?|?  |?14?|?9?|?  |?15?|?2?|?  +--------+-------------------+

不是標(biāo)準(zhǔn)的sql語(yǔ)句，邏輯上解釋不通（每個(gè)類(lèi)別cat_id里有很多goods_name）
不推薦 select goods_name ,sum(goods_number) from goods group by cat_id;?
解釋?zhuān)涸趕elect?a/b中必須在group by?a/b/c語(yǔ)意上才沒(méi)有問(wèn)題

技巧：查詢語(yǔ)句理解上從后面的條件過(guò)濾開(kāi)始，先理解過(guò)濾條件，再看前面的執(zhí)行

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