mysql statistical functions and group by

Five statistical functions (used alone, of little significance, often used in combination with group by)
max maximum select max(shop_price) from goods;
min minimum select min(shop_price) from goods;
sum Sum select sum(shop_price) from goods;
avg Find average select avg(shop_price) from goods;
count How many rows are there in all value rows
count (*) absolute row number null Also included
In addition to count (column name), calculate the number of non-null rows in this column

count uses

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 | 
+-------------+

Query inventory type 4

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

group by
Count the inventory under each type of group

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 | 
+--------+-------------------+

is not a standard sql statement and cannot be explained logically (each category cat_id contains Many goods_name)
It is not recommended to select goods_name,sum(goods_number) from goods group by cat_id;
Explanation: In select a/b, it must be in group by a/b/c to have no semantic problem

Tips: To understand the query statement, start with the subsequent conditional filtering. First understand the filtering conditions, and then look at the previous execution

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