Mysql：核心的数据库操作

2021-08-04 00:51:01

Mysql核心点

对于每一位研发同学，Mysql都是必须掌握的技能啦。基本的Mysql的操作，还是得很好的掌握的。

一、Mysql

学习一个技术，一定要先去官网学习。
Mysql官网

二、基本的查询

1.创建表并插入数据

创建表

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));

插入数据

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);

查看这张表

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

2.查询一列中的最大值


SELECT MAX(article) AS article FROM shop;

+---------+
| article |
+---------+
|       4 |
+---------+

3.查询这一列中最大值所在的行

通过子查询的方式，能够轻松操作。

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

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0004 | D      | 19.95 |
+---------+--------+-------+

其他的解决方案，例如使用LEFT JOIN，或者根据指定的元素进行排序，并且使用特定的Mysql的LIMIT语句进行操作。

方案1：
SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.price < s2.price
WHERE s2.article IS NULL;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0004 | D      | 19.95 |
+---------+--------+-------+

方案2：
SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1;
这里使用了Limit值为1，意为只取一个值。

4.找到每篇文章的最大值，如果一篇文章有多个值，只取最大值

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

5.求出每篇文章，价格最高对应的经销商

方法1：用自查询

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

但是使用自查询是效率比较低下的一种方式。
那么另外的解决方案，是在FROM中使用子查询或者LEFT JOIN。

SELECT s1.article, dealer, s1.price
FROM shop s1
JOIN (
  SELECT article, MAX(price) AS price
  FROM shop
  GROUP BY article) AS s2
  ON s1.article = s2.article AND s1.price = s2.price;

LEFT JOIN：

SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price
WHERE s2.article IS NULL;

6.使用用户自定义的变量查询

您可以使用MySQL用户变量来记住结果，而无需将它们存储在客户端的临时变量中。
例如，要查找价格最高和最低的文章，您可以执行以下操作：

mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

码农公寓