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总结和Mysql相关的知识

时间:2021-07-01 10:21:17 帮助过:4人阅读

获得数据库和表的信息

一般正常的程序员或DBA都会在敲代码的时候突然想到这样的一系列问题:我是谁?我在哪?我在干什么? 我的数据库呢?我的表呢?我表怎么创建的?我该怎么办呢?你可能会想到SHOW DATABASES; 命令。But, 这个命令是列出由mysql管理的databases. 不是知道我再哪的命令。到底哪个命令是呢?

我左某人在翻阅上古的典籍的时候查到这样的一个命令:

SELECT DATABASE();

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test |
+------------+
1 row in set (0.00 sec)

mysql>

显而易见,这是一个告诉我我再哪个数据库的命令。然后肯定会有一群二五仔问:我要是没有进入任何数据库那会显示什么呢?

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)

mysql>

当然是NULL了,还能有什么?

现在,我们找到了正在使用的数据库(test) 。然后,该找要找的表了,比如说(pet)。 根据古籍上面的记载应该使用如下的命令:

SHOW TABLES;

mysql> SHOW TABLES;
+----------------+
| Tables_in_test |
+----------------+
| event |
| pet |
+----------------+
2 rows in set (0.00 sec)

mysql>

And then I want to know 表的结构。What should I do?

DESCRIBE pet;

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

mysql>

老司机一般都简写成

DESC pet;

Field 表示列名字

Type表示列的数据类型

Null表示能否为NULL

Key表示是否被索引

Default表示字段的默认值

如果表有索引,SHOW INDEX FROM tbl_name 显示索引的信息。

常用查询的例子

在搞事情之前,肯定要先建一个表:假定有一个表(shop)来存储某商人()的每件物品()的价格()。(物品、商人作为主键)

操作如下:

mysql> CREATE TABLE shop(
-> article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
-> dealer CHAR(20) DEFAULT '' NOT NULL,
-> price DOUBLE(16,2) DEFAULT '0.00' NOT NULL,
-> PRIMARY KEY(article, dealer));
Query OK, 0 rows affected (0.56 sec)

mysql>

然后插入一些数据:

mysql> INSERT INTO shop VALUES
-> (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
-> (3,'C',1.69),(3,'D',1.25),(4,'D',19.95);
Query OK, 7 rows affected (0.24 sec)
Records: 7 Duplicates: 0 Warnings: 0

mysql>

查看一下表:

mysql> SELECT * FROM shop;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | A | 3.45 |
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | B | 1.45 |
| 0003 | C | 1.69 |
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
7 rows in set (0.00 sec)

mysql>

然后我们就可以学习后面的内容了


列的最大值

举例:在shop中的最大的物品号?

操作如下:

SELECT MAX(article) FROM shop;

mysql> SELECT MAX(article) FROM shop;
+--------------+
| MAX(article) |
+--------------+
| 4 |
+--------------+
1 row in set (0.00 sec)

mysql>

举例:找最贵的商品

操作如下:

SELECT MAX(price) FROM shop;

mysql> SELECT MAX(price) FROM shop;
+------------+
| MAX(price) |
+------------+
| 19.95 |
+------------+
1 row in set (0.00 sec)

mysql>

知道MAX()函数是干啥的了吧。

拥有某个列的最大值的行

栗子:查询最贵的商品的信息

操作如下:

SELECT * FROM shop WHERE price = (SELECT MAX(price) FROM shop);

mysql> SELECT * FROM shop
-> WHERE price =
-> (SELECT MAX(price) FROM shop);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0004 | D | 19.95 |
+---------+--------+-------+
1 row in set (0.00 sec)

mysql>

还有一种操作:

SELECT * FROM shop ORDER BY price DESC LIMIT 1;

mysql> SELECT * FROM shop
-> ORDER BY price DESC
-> LIMIT 1;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0004 | D | 19.95 |
+---------+--------+-------+
1 row in set (0.00 sec)

mysql>

前者是一个嵌套查询,后者是根据价格排序只显示一个。

列的最大值:按组

栗子:每项物品(article)的最高价格是多少?

操作如下:

SELECT article, MAX(price) AS price FROM shop GROUP BY article;

mysql> SELECT article, MAX(price) AS price
-> FROM shop
-> GROUP BY article;
+---------+-------+
| article | price |
+---------+-------+
| 0001 | 3.99 |
| 0002 | 10.99 |
| 0003 | 1.69 |
| 0004 | 19.95 |
+---------+-------+
4 rows in set (0.00 sec)

mysql>

拥有某个字段的组间最大值的行

并不明白标题是啥意思。。。。

栗子:对每项物品,找出最贵价格的物品的经销商。

操作如下:

SELECT article, dealer, price
FROM shop s1
WHERE price = (SELECT MAX(price)
FROM shop s2
WHERE s1.article = s2.article);

mysql> SELECT article, dealer, price
-> FROM shop s1
-> WHERE price = (SELECT MAX(s2.price)
-> FROM shop s2
-> WHERE s1.article = s2.article);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | C | 1.69 |
| 0004 | D | 19.95 |
+---------+--------+-------+
4 rows in set (0.00 sec)

这里书上没写为什么,自己也不是十分理解。求懂的大佬在评论区讲解●﹏●。

使用用户变量

栗子:找出价格最高或最低的物品

操作如下:

SELECT @min_price:=MIN(price), @max_price:=MAX(price) FORM shop;

SELECT * FROM shop WHERE price = @min_price OR price = @max_price;

mysql> SELECT @min_price:=MIN(price), @max_price:=MAX(price) FROM shop;
+------------------------+------------------------+
| @min_price:=MIN(price) | @max_price:=MAX(price) |
+------------------------+------------------------+
| 1.25 | 19.95 |
+------------------------+------------------------+
1 row in set (0.13 sec)

mysql> SELECT * FROM shop WHERE price=@min_price OR price = @max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
2 rows in set (0.09 sec)

mysql>

关于用户变量后面会有的,好奇的鸨鸨可以百度。

使用外键

多说无意 直接上操作, 上面有一个传送门,讲的很不错了。

CREATE TABLE person (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    name CHAR(60) NOT NULL,
    PRIMARY KEY (id)
);
 
CREATE TABLE shirt (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
    color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
    owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
    PRIMARY KEY (id)
);
 
INSERT INTO person VALUES (NULL, 'Antonio Paz');
 
SELECT @last := LAST_INSERT_ID();
 
INSERT INTO shirt VALUES
(NULL, 'polo', 'blue', @last),
(NULL, 'dress', 'white', @last),
(NULL, 't-shirt', 'blue', @last);
 
INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');
 
SELECT @last := LAST_INSERT_ID();
 
INSERT INTO shirt VALUES
(NULL, 'dress', 'orange', @last),
(NULL, 'polo', 'red', @last),
(NULL, 'dress', 'blue', @last),
(NULL, 't-shirt', 'white', @last);
 
SELECT * FROM person;
+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Antonio Paz         |
|  2 | Lilliana Angelovska |
+----+---------------------+
 
SELECT * FROM shirt;
+----+---------+--------+-------+
| id | style   | color  | owner |
+----+---------+--------+-------+
|  1 | polo    | blue   |     1 |
|  2 | dress   | white  |     1 |
|  3 | t-shirt | blue   |     1 |
|  4 | dress   | orange |     2 |
|  5 | polo    | red    |     2 |
|  6 | dress   | blue   |     2 |
|  7 | t-shirt | white  |     2 |
+----+---------+--------+-------+
 
 
SELECT s.* FROM person p, shirt s
 WHERE p.name LIKE 'Lilliana%'
   AND s.owner = p.id
   AND s.color <> 'white';
 
+----+-------+--------+-------+
| id | style | color  | owner |
+----+-------+--------+-------+
|  4 | dress | orange |     2 |
|  5 | polo  | red    |     2 |
|  6 | dress | blue   |     2 |
+----+-------+--------+-------+



我错了,网断了。只好拷贝书上的代码了。

mysql> show create table shirt\G
*************************** 1. row ***************************
Table: shirt
Create Table: CREATE TABLE `shirt` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`style` enum('t-shirt','polo','dress') NOT NULL,
`color` enum('red','blue','orange','white','black') NOT NULL,
`owner` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=latin1
1 row in set (0.01 sec)

mysql>

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