2018/08/17--mysql数据库

SELECT AVG(prod_price) AS avg_price FROM products;

SELECT COUNT(*) FROM customers;

SELECT COUNT(cust_email) FROM customers;

SELECT MAX(prod_price) AS max_price FROM products;

SELECT MIN(prod_price) AS min_price FROM products;

SELECT SUM(quantity) AS items_ordered FROM orderitems WHERE order_num=20005;

SELECT COUNT(*) AS num_items,MAX(prod_price) AS max_price,MIN(prod_price) AS min_price,AVG(prod_price) AS avg_price FROM products;

SELECT COUNT(*) AS num_prods FROM products WHERE vend_id=1003;

SELECT vend_id,COUNT(*) AS num_prod FROM products GROUP BY vend_id;

SELECT vend_id,COUNT(*) AS num_prod FROM products GROUP BY vend_id WITH ROLLUP;

SELECT cust_id,cust_name,cust_contact,cust_email FROM customers;

cust_id	cust_name	cust_contact	cust_email
10001	Coyote Inc.	Y Lee	[email protected]
10002	Mouse House	Jerry Mouse	NULL
10003	Wascals	Jim Jones	[email protected]
10004	Yosemite Place	Y Sam	[email protected]
10005	The Fudds	E Fudd	[email protected]

SELECT COUNT(*) AS cust_num FROM customers;

cust_num

5

SELECT COUNT(cust_email) AS cust_num FROM customers;

cust_num

4

关于COUNT(*)和COUNT(列名)

COUNT(*) ：对表中所有行进行计数，不管值是空值还是非空值

COUNT( 列名) ：对表中具有值的行进行计数，忽略NULL值

SELECT * FROM orderitems;

order_num	order_item	prod_id	quantity	item_price
20005	1	ANV01	10	5.99
20005	2	ANV02	3	9.99
20005	3	TNT2	5	10.00
20005	4	FB	1	10.00
20006	1	JP2000	1	55.00
20007	1	TNT2	100	10.00

SELECT SUM(quantity) AS items_ordered FROM orderitems WHERE order_num=20005;

items_ordered

19

SELECT prod_id,vend_id,prod_name,prod_price FROM products;

prod_id	vend_id	prod_name	prod_price
AVN01	1001	.5 ton anvil	5.99
AVN02	1001	1 ton anvil	9.99
AVN03	1001	2 ton anvil	14.99
DTNTR	1003	Detonator	13.00
FB	1003	Bird seed	10.00
FC	1003	Carrots	2.50
FU1	1002	Fuses	3.42
JP1000	1005	JetPack 1000	35.00
JP2000	1005	JetPack 2000	55.00
OL1	1002	Oil can	8.99
SAFE	1003	Safe	50.00
SLING	1003	Sling	4.49
TNT1	1003	TNT (1 stick)	2.50
TNT2	1003	TNT(2 sticks)	10.00

SELECT vend_id,COUNT(*) AS num_prods FROM products GROUP BY vend_id;

vend_id	num_prods
1001	3
1002	2
1003	7
1005	2

关于WITH ROLLUP

WITH ROLLUP，在分组统计的基础上再进行统计汇总

SELECT vend_id,COUNT(*) AS num_prod FROM products GROUP BY vend_id;

vend_id	num_prod
1001	3
1002	2
1003	7
1005	2

SELECT vend_id,COUNT(*) AS num_prod FROM products GROUP BY vend_id WITH ROLLUP;

vend_id	num_prod
1001	3
1002	2
1003	7
1005	2
NULL	14