日期：2011-05-02 15:44:00 来源：本站整理
MySQL常用查询的例子[MySQL防范]
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下面是一些学习若何用MySQL办理一些常见问题的例子.一些例子利用数据库表"shop",包含某个商人的每篇文章(物品号)的价钱.假定每个商人的每篇文章有一个单独的固定价钱,那么(物品,商人)是记录的主键.
你能这样成立例子数据库表:
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、商人和价钱"
在ANSI-SQL中这很简单用一个子查询做到:
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.
利用该值编出实际的查询:
SELECT article, dealer, price
FROM shop
WHERE price=19.95
另一个办理筹划是按价钱降序排序全部行并用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:假若有多个最贵的文章( 比方每个19.95),LIMIT办理筹划仅仅显示他们之一!
3.3 列的最大值:按组:只有值
"每篇文章的最高的价钱是什么?"
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,找出有最贵的价钱的交易者."
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钱的行.
这可以很简单用一个暂时表做到:
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;
假如你不利用一个TEMPORARY表,你也必须锁定"tmp"表.
"它能一个单个查询做到吗?"
是的,但是只有利用我称之为"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 利用外键
不需求外键联合2个表.
MySQL唯一不做的事情是CHECK以保证你利用的键确切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.假如你象普通那样利用你的键值,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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