mysql statistical functions:
1. check prices highest price:
select max(shop_price) from goods;
2. Query commodity prices, the lowest price:
select min(shop_price) from goods;
3. The total inventory requirements:
select sum(goods_number) from goods;
4. Check the average price of all commodities:
select avg(shop_price) from goods;
There are many kinds of goods within 5. Statistical Mall (not a number):
select count(*) from goods;
count () function is a statistical line number, reason to use count (*) instead of count (column name) because if there is a behavior that column empty, then count (column name) does not take it into account.
Look at a small example:
Even if there is a row full of empty, with the count (*) It can also be taken into account:
Statistical functions used alone has little significance, and to have the group meet up with:
mysql the group by grouping queries:
Calculate the amount of inventory of all items at the third and columns :
select cat_id,sum(goods_number) from goods where cat_id=3 group by cat_id;
一次计算完每一个栏目下的库存量之和:
select cat_id,sum(goods_number) from goods group by cat_id;
严格的讲，以group by a,b,c为列，select的列只能在a,b,c或者是统计函数中选择，这样在语义上才没有矛盾。
做个小练习：有以下成绩表，求出挂科数等于或高于2门的学生的平均成绩：
select name,avg(score),sum(score<60)as fail from test5 group by name having fail >=2;
排序：order by
取出第四个栏目下的商品，把价格由高到低进行排序：
select *from goods where cat_id=4 order by shop_price desc; #desc 代表降序排列
按栏目升序排列，同一个栏目下的商品按价格降序排列：
select * from goods order by cat_id asc,shop_price desc;
限制输出条目limit：
取出商品价格前十的商品：
select goods_id,goods_name,cat_id,shop_price from goods order by shop_price desc limit 10;
最后强调一点：Where,group by,having,order by,limit 五个子句顺序不能乱！