- 主键：
1）任意两行都不具有相同的主键值：
2）每个行都必须具有一个或多个主键值（主键列不允许NULL值）
3）主键列中的值不允许修改或更新；
4）主键值不能重用（如果某行从表中删除，它的主键不能赋给以后的新行） - 在指定一条ORDER BY 子句里，应保证它是SELECT语句中最后一条子句。
- 按多个列排序：
SELECT prod_id,prod_price,prod_name
FROM Products
ORDER BY prod_price, prod_name; - 指定排序方向：
数据库默认的排序方式为升序排序；
为了进行降序排序，可以指定DESC关键字。
SELECT prod_id, prod_price, prod_name
FROM Products
ORDER BY prod_price DESC;
如果想在多个列上进行降序排序，必须对列指定DESC关键字。 - 拼接字段：
Mysql用法：concat函数用于拼接，关键字AS赋予别名
mysql> select concat ( vend_name, ‘(‘, vend_country, ‘)‘) as vend_title-> from Vendors
-> order by vend_name;
+--------------------------------+
| vend_title |
+--------------------------------+
| Bear Emporium(USA) |
| Bears R Us(USA) |
| Doll House Inc.(USA) |
| Fun and Games(England) |
| Furball Inc.(USA) |
| Jouets et ours(France) |
+--------------------------------+
6 rows in set (0.00 sec)
- 执行算术运算
mysql> select prod_id, quantity, item_price, quantity*item_price as expanded_price
-> from OrderItems-> where order_num = 20008;
+---------+----------+------------+----------------+
| prod_id | quantity | item_price | expanded_price |
+---------+----------+------------+----------------+
| RGAN01 | 5 | 4.99 | 24.95 |
| BR03 | 5 | 11.99 | 59.95 |
| BNBG01 | 10 | 3.49 | 34.90 |
| BNBG02 | 10 | 3.49 | 34.90 |
| BNBG03 | 10 | 3.49 | 34.90 |
+---------+----------+------------+----------------+
5 rows in set (0.02 sec)
- 与SQL语句不一样，SQL函数是不可移植的。
- 聚集函数(aggregate function) 运行在行组上（行，或者记录）上，计算和返回单个值的函数。
函数 说明 AVG() 返回某列的平均值 COUNT() 返回某列的行数 MAX() 返回某列的最大值 MIN() 返回某列的最小值 SUM() 返回某列值之和 注：表格里虽然某列某列，实质是作用于行上的，“对于该列的所有行的平均值，行数，最大值，最小值，和”
mysql> SELECT prod_price-> FROM Products;
+------------+
| prod_price |
+------------+
| 5.99 |
| 8.99 |
| 11.99 |
| 3.49 |
| 3.49 |
| 3.49 |
| 4.99 |
| 9.49 |
| 9.49 |
+------------+
9 rows in set (0.00 sec)
mysql> select avg(prod_price) as avg_price
-> from Products;
+-----------+
| avg_price |
+-----------+
| 6.823333 |
+-----------+
1 row in set (0.00 sec)
mysql> SELECT AVG(prod_price) AS avg_price
-> from Products
-> where vend_id = ‘DLL01‘;
+-----------+
| avg_price |
+-----------+
| 3.865000 |
+-----------+
1 row in set (0.00 sec)
mysql> SELECT COUNT(*)
-> FROM Products;
+----------+
| COUNT(*) |
+----------+
| 9 |
+----------+
1 row in set (0.00 sec)
注：
1）使用COUNT(*)对表中行的数目进行计数，不管列表中包含的是空值还是非空值。
2）使用COUNT(column)对特定列中具有值的行进行计数，忽略NULL值。mysql> SELECT COUNT(cust_email) AS num_cus
-> FROM Customers;
+---------+
| num_cus |
+---------+
| 3 |
+---------+
1 row in set (0.02 sec)
mysql> SELECT MAX(prod_price) AS max_price
-> FROM Products;
+-----------+
| max_price |
+-----------+
| 11.99 |
+-----------+
1 row in set (0.00 sec)
注：可以观察发现，聚集函数通过对所有行或者所有非空行或者符合where条件的所有（非空）行进行操作，结果集为只有一行数据。如果跟结果集为多行的查询一起，则会很怪异。如下：
mysql> SELECT prod_name, MAX(prod_price) AS price
-> FROM Products;
+-------------------+-------+
| prod_name | price |
+-------------------+-------+
| 8 inch teddy bear | 11.99 |
+-------------------+-------+
1 row in set (0.00 sec)
（注：上面的是错误的用法，具体对比看下面的全表，就能发现错误）
mysql> SELECT prod_name, prod_price
-> FROM Products;
+---------------------+------------+
| prod_name | prod_price |
+---------------------+------------+
| 8 inch teddy bear | 5.99 |
| 12 inch teddy bear | 8.99 |
| 18 inch teddy bear | 11.99 |
| Fish bean bag toy | 3.49 |
| Bird bean bag toy | 3.49 |
| Rabbit bean bag toy | 3.49 |
| Raggedy Ann | 4.99 |
| King doll | 9.49 |
| Queen doll | 9.49 |
+---------------------+------------+
9 rows in set (0.01 sec)
又一例子
mysql> select vend_id, count(*) as num_prods-> from Products;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| BRS01 | 9 |
+---------+-----------+
1 row in set (0.00 sec)
mysql> select vend_id
-> from Products;
+---------+
| vend_id |
+---------+
| BRS01 |
| BRS01 |
| BRS01 |
| DLL01 |
| DLL01 |
| DLL01 |
| DLL01 |
| FNG01 |
| FNG01 |
+---------+
9 rows in set (0.00 sec)
接上注又注：但是结果集同为一行的聚集函数作为查询则不会出错（即组合不同的聚集函数）
接上注又再注：又一个错误的用法，集合==行？mysql> SELECT prod_name, prod_price
-> FROM Products
-> WHERE prod_price = MAX(prod_price);
ERROR 1111 (HY000): Invalid use of group function
mysql> SELECT prod_name, prod_price
-> FROM Products
-> WHERE MAX(prod_price);
ERROR 1111 (HY000): Invalid use of group function
WHY？
- 聚集不同值：
对所有的行执行计算，指定ALL参数或不给参数（ALL为默认）
只不含不同的值，指定DISTINCT参数 - 分组：
mysql> SELECT cust_id, COUNT(cust_name) AS COUNT
-> FROM Customers
-> Where cust_name = ‘Fun4All‘;
+------------+-------+
| cust_id | COUNT |
+------------+-------+
| 1000000003 | 2 |
+------------+-------+
1 row in set (0.01 sec)
mysql> SELECT cust_id, COUNT(cust_name) AS count
-> FROM Customers
-> WHERE cust_name = ‘Fun4All‘
-> GROUP BY cust_id;
+------------+-------+
| cust_id | count |
+------------+-------+
| 1000000003 | 1 |
| 1000000004 | 1 |
+------------+-------+
2 rows in set (0.01 sec)
(注：GROUP BY对COUNT函数起到很重要的作用，COUNT根据GOUP BY的分组来计算）
- GROUP BY 子句指示DBMS分组数据，然后对每个组而不是整个结果集进行聚集。
- 过滤分组
WHERE 过滤行，而HAVING过滤分组
- 使用HAVING和WHERE
HAVING与WHERE非常类似，如果不指定GROUP BY，则大多数DBMS将把它们作为相同的东西对待。应该仅在GROUP BY 子句结合才使用HAVING，而WHERE子句用于标准的行级过滤。 - SELECT 子句顺序
子句 说明 是否必须使用 SELECT 要返回的列或表达式 是 FROM 从中检索数据的表 仅在从表选择数据时使用 WHERE 行级过滤 否 GROUP BY 分组说明 仅在按组计算聚集时使用 HAVING 组级过滤 否 ORDER BY 输出排序顺序 否 - (SELECT COUNT(*)
- 作为计算字段使用子查询
看一个经典段子，好好品味一下：
SELECT cust_name,
cust_state,
(SELECT COUNT(*)
FROM Orders
WHERE Orders.cust_id = Customers.cust_id) AS
orders
FROM Customers
ORDER BY cust_name;
结果：
cust_name cust_state orders
------------------------------ --------------- -----------
Fun4All IN 1
Fun4All AZ 1
Kids Place OH 0
The Toy Store IL 1
Village Toys MI 2
该子查询对检索出的每个客户执行一次。在此例中，该子查询执行了5次，因为检索出了5个客户。 - 笛卡儿积（cartesian product）由没有联结条件的表关系返回的结果为笛卡儿积。检索出的行的数目将是第一个表中的行数乘以第二个表中的行数。
mysql> SELECT vend_name
-> FROM Vendors;
+-----------------+
| vend_name |
+-----------------+
| Bears R Us |
| Bear Emporium |
| Doll House Inc. |
| Furball Inc. |
| Fun and Games |
| Jouets et ours |
+-----------------+
6 rows in set (0.00 sec)
mysql> SELECT prod_name
-> FROM Products;
+---------------------+
| prod_name |
+---------------------+
| 8 inch teddy bear |
| 12 inch teddy bear |
| 18 inch teddy bear |
| Fish bean bag toy |
| Bird bean bag toy |
| Rabbit bean bag toy |
| Raggedy Ann |
| King doll |
| Queen doll |
+---------------------+
9 rows in set (0.01 sec)
mysql> SELECT vend_name, prod_name, prod_price
-> FROM Vendors, Products;
+-----------------+---------------------+------------+
| vend_name | prod_name | prod_price |
+-----------------+---------------------+------------+
| Bears R Us | 8 inch teddy bear | 5.99 |
| Bear Emporium | 8 inch teddy bear | 5.99 |
| Doll House Inc. | 8 inch teddy bear | 5.99 |
| Furball Inc. | 8 inch teddy bear | 5.99 |
| Fun and Games | 8 inch teddy bear | 5.99 |
| Jouets et ours | 8 inch teddy bear | 5.99 |
| Bears R Us | 12 inch teddy bear | 8.99 |
| Bear Emporium | 12 inch teddy bear | 8.99 |
| Doll House Inc. | 12 inch teddy bear | 8.99 |
| Furball Inc. | 12 inch teddy bear | 8.99 |
| Fun and Games | 12 inch teddy bear | 8.99 |
| Jouets et ours | 12 inch teddy bear | 8.99 |
| Bears R Us | 18 inch teddy bear | 11.99 |
| Bear Emporium | 18 inch teddy bear | 11.99 |
| Doll House Inc. | 18 inch teddy bear | 11.99 |
| Furball Inc. | 18 inch teddy bear | 11.99 |
| Fun and Games | 18 inch teddy bear | 11.99 |
| Jouets et ours | 18 inch teddy bear | 11.99 |
| Bears R Us | Fish bean bag toy | 3.49 |
| Bear Emporium | Fish bean bag toy | 3.49 |
| Doll House Inc. | Fish bean bag toy | 3.49 |
| Furball Inc. | Fish bean bag toy | 3.49 |
| Fun and Games | Fish bean bag toy | 3.49 |
| Jouets et ours | Fish bean bag toy | 3.49 |
| Bears R Us | Bird bean bag toy | 3.49 |
| Bear Emporium | Bird bean bag toy | 3.49 |
| Doll House Inc. | Bird bean bag toy | 3.49 |
| Furball Inc. | Bird bean bag toy | 3.49 |
| Fun and Games | Bird bean bag toy | 3.49 |
| Jouets et ours | Bird bean bag toy | 3.49 |
| Bears R Us | Rabbit bean bag toy | 3.49 |
| Bear Emporium | Rabbit bean bag toy | 3.49 |
| Doll House Inc. | Rabbit bean bag toy | 3.49 |
| Furball Inc. | Rabbit bean bag toy | 3.49 |
| Fun and Games | Rabbit bean bag toy | 3.49 |
| Jouets et ours | Rabbit bean bag toy | 3.49 |
| Bears R Us | Raggedy Ann | 4.99 |
| Bear Emporium | Raggedy Ann | 4.99 |
| Doll House Inc. | Raggedy Ann | 4.99 |
| Furball Inc. | Raggedy Ann | 4.99 |
| Fun and Games | Raggedy Ann | 4.99 |
| Jouets et ours | Raggedy Ann | 4.99 |
| Bears R Us | King doll | 9.49 |
| Bear Emporium | King doll | 9.49 |
| Doll House Inc. | King doll | 9.49 |
| Furball Inc. | King doll | 9.49 |
| Fun and Games | King doll | 9.49 |
| Jouets et ours | King doll | 9.49 |
| Bears R Us | Queen doll | 9.49 |
| Bear Emporium | Queen doll | 9.49 |
| Doll House Inc. | Queen doll | 9.49 |
| Furball Inc. | Queen doll | 9.49 |
| Fun and Games | Queen doll | 9.49 |
| Jouets et ours | Queen doll | 9.49 |
+-----------------+---------------------+------------+
54 rows in set (0.01 sec)
mysql> SELECT vend_name, Vendors.vend_id, Products.vend_id, prod_name
-> FROM Vendors, Products
-> WHERE Vendors.vend_id = Products.vend_id;
+-----------------+---------+---------+---------------------+
| vend_name | vend_id | vend_id | prod_name |
+-----------------+---------+---------+---------------------+
| Bears R Us | BRS01 | BRS01 | 8 inch teddy bear |
| Bears R Us | BRS01 | BRS01 | 12 inch teddy bear |
| Bears R Us | BRS01 | BRS01 | 18 inch teddy bear |
| Doll House Inc. | DLL01 | DLL01 | Fish bean bag toy |
| Doll House Inc. | DLL01 | DLL01 | Bird bean bag toy |
| Doll House Inc. | DLL01 | DLL01 | Rabbit bean bag toy |
| Doll House Inc. | DLL01 | DLL01 | Raggedy Ann |
| Fun and Games | FNG01 | FNG01 | King doll |
| Fun and Games | FNG01 | FNG01 | Queen doll |
+-----------------+---------+---------+---------------------+
9 rows in set (0.00 sec)
mysql> SELECT cust_id, cust_name, cust_contact
-> FROM Customers;
+------------+---------------+--------------------+
| cust_id | cust_name | cust_contact |
+------------+---------------+--------------------+
| 1000000001 | Village Toys | John Smith |
| 1000000002 | Kids Place | Michelle Green |
| 1000000003 | Fun4All | Jim Jones |
| 1000000004 | Fun4All | Denise L. Stephens |
| 1000000005 | The Toy Store | Kim Howard |
+------------+---------------+--------------------+
5 rows in set (0.01 sec)
- 联结多个表
对于多表联结，外键相等作为WHERE条件是最基本的过滤条件，（因为除外键相等的这部分“行”外，都是联结所带来的无效的“行”）——自然联结 - 自联结
看一个很屌的例子，充分说明了自联结的好处。
说明：这个例子找到cust_contact为‘Jim Jones’的客户的cust_name。妙在cust_name是可以重名的，即两个客户可以叫同一个名字。这个例子就是在查询某个cust_name的信息，而cust_name是从cust_contact处得来。如果不用自联结，where cust_contact = ‘Jim Jones‘得到的结果集是完全不一样的。 - UNION
利用UNION，可给出多条SELECT语句，将它们的结果组合成单个结果集。
UNION的使用：只要在每条SELECT语句之间放上关键字UNION。mysql> SELECT cust_name, cust_contact, cust_email
-> FROM Customers
-> WHERE cust_state IN(‘IL‘, ‘IN‘,‘MI‘)
-> UNION
-> SELECT cust_name, cust_contact, cust_email
-> FROM Customers
-> WHERE cust_name = ‘Fun4All‘;
+---------------+--------------------+-----------------------+
| cust_name | cust_contact | cust_email |
+---------------+--------------------+-----------------------+
| Village Toys | John Smith | [email protected] |
| Fun4All | Jim Jones | [email protected] |
| The Toy Store | Kim Howard | NULL |
| Fun4All | Denise L. Stephens | [email protected] |
+---------------+--------------------+-----------------------+
4 rows in set (0.01 sec)
UNION中的每个查询必须包含相同的列、表达式或聚集函数（不过各个列不需要以相同的次序列出）
- 插入更新删除
mysql> INSERT INTO Customers(cust_id,
-> cust_name,
-> cust_address,
-> cust_city,
-> cust_state,
-> cust_zip,
-> cust_country,
-> cust_contact,
-> cust_email)
-> VALUES(‘1000000006‘,‘Toy Land‘,‘123 Any Street‘,
-> ‘New York‘,
-> ‘NY‘,
-> ‘11111‘,
-> ‘USA‘,
-> NULL,
-> NULL);
Query OK, 1 row affected (0.03 sec)
mysql> UPDATE Customers
-> SET cust_email = ‘[email protected]‘
-> WHERE cust_id = ‘1000000005‘;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> DELETE FROM Customers
-> WHERE cust_id = ‘1000000006‘;
Query OK, 1 row affected (0.00 sec)
mysql> SELECT *
-> FROM Customers
-> WHERE cust_id = ‘1000000006‘;
Empty set (0.00 sec)
