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MySQL常用查询的例子_MySQL

时间:2021-07-01 10:21:17 帮助过:20人阅读

下面是一些学习如何用MySQL解决一些常见问题的例子。一些例子使用数据库表“shop”,包含某个商人的每篇文章(物品号)的价格。假定每个商人的每篇文章有一个单独的固定价格,那么(物品,商人)是记录的主键。

你能这样创建例子数据库表:

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));

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);

好了,例子数据是这样的:

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 |

+---------+--------+-------+

3.1 列的最大值

“最大的物品号是什么?”

SELECT MAX(article) AS article FROM shop

+---------+

| article |

+---------+

|  4 |

+---------+

3.2 拥有某个列的最大值的行

“找出最贵的文章的编号、商人和价格”

在ANSI-SQL中这很容易用一个子查询做到:

SELECT article, dealer, price

FROM  shop

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

在MySQL中(还没有子查询)就用2步做到:

用一个SELECT语句从表中得到最大值。

使用该值编出实际的查询:

SELECT article, dealer, price

FROM  shop

WHERE price=19.95

另一个解决方案是按价格降序排序所有行并用MySQL特定LIMIT子句只得到的第一行:

SELECT article, dealer, price

FROM  shop

ORDER BY price DESC

LIMIT 1

注意:如果有多个最贵的文章( 例如每个19.95),LIMIT解决方案仅仅显示他们之一!

3.3 列的最大值:按组:只有值

“每篇文章的最高的价格是什么?”

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 |

+---------+-------+

3.4 拥有某个字段的组间最大值的行

“对每篇文章,找出有最贵的价格的交易者。”

在ANSI SQL中,我可以用这样一个子查询做到:

SELECT article, dealer, price

FROM  shop s1

WHERE price=(SELECT MAX(s2.price)

 FROM shop s2

 WHERE s1.article = s2.article)

在MySQL中,最好是分几步做到:

得到一个表(文章,maxprice)。见3.4 拥有某个域的组间最大值的行。

对每篇文章,得到对应于存储最大价格的行。

这可以很容易用一个临时表做到:

CREATE TEMPORARY TABLE tmp (

article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,

price  DOUBLE(16,2) DEFAULT '0.00' NOT NULL);

LOCK TABLES article read;

INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

SELECT article, dealer, price FROM shop, tmp

WHERE shop.article=tmp.articel AND shop.price=tmp.price;

UNLOCK TABLES;

DROP TABLE tmp;

如果你不使用一个TEMPORARY表,你也必须锁定“tmp”表。

“它能一个单个查询做到吗?”

是的,但是只有使用我称之为“MAX-CONCAT诡计”的一个相当低效的诡计:

SELECT article,

  SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,

 0.00+LEFT( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price

FROM  shop

GROUP BY article;

+---------+--------+-------+

| article | dealer | price |

+---------+--------+-------+

|0001 | B | 3.99 |

|0002 | A | 10.99 |

|0003 | C | 1.69 |

|0004 | D | 19.95 |

+---------+--------+-------+

最后例子当然能通过在客户程序中分割连结的列使它更有效一点。

3.5 使用外键

不需要外键联结2个表。

MySQL唯一不做的事情是CHECK以保证你使用的键确实在你正在引用表中存在,并且它不自动从有一个外键定义的表中删除行。如果你象平常那样使用你的键值,它将工作得很好!

CREATE TABLE persons (

id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,

name CHAR(60) NOT NULL,

PRIMARY KEY (id)

);

CREATE TABLE shirts (

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 persons,

PRIMARY KEY (id)

);

INSERT INTO persons VALUES (NULL, 'Antonio Paz');

INSERT INTO shirts VALUES

(NULL, 'polo', 'blue', LAST_INSERT_ID()),

(NULL, 'dress', 'white', LAST_INSERT_ID()),

(NULL, 't-shirt', 'blue', LAST_INSERT_ID());

INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska');

INSERT INTO shirts VALUES

(NULL, 'dress', 'orange', LAST_INSERT_ID()),

(NULL, 'polo', 'red', LAST_INSERT_ID()),

(NULL, 'dress', 'blue', LAST_INSERT_ID()),

(NULL, 't-shirt', 'white', LAST_INSERT_ID());

SELECT * FROM persons;

+----+---------------------+

| id | name|

+----+---------------------+

| 1 | Antonio Paz |

| 2 | Lilliana Angelovska |

+----+---------------------+

SELECT * FROM shirts;

+----+---------+--------+-------+

| 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 persons p, shirts 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 |

+----+-------+--------+-------+

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