在rhce考试题中，第21、22题为数据库查询题
题目：
在system1上创建一个Maria DB数据库，名为Contacts,要求：
- 数据库应该包含来自数据库users.mdb的内容，数据库只能被localhost访问
- 除了root用户，此数据库只能被用户Marry查询，此用户密码为redhat
- root用户的数据库密码为redhat，同时不允许空密码登录
- 密码是fadora的人的名字是什么?
- 有多少人姓名是John，同时居住在Santa Clara?
为了完成这个实验，我已将user.mdb放到本文最下方，复制保存为user.mdb，在数据库中source 该文件即可执行
该文章主要看第四和第五个问题
1、密码是fadora的人的名字
SELECT u_name.firstname FROM u_name, u_passwd WHERE u_name.userid = u_passwd.uid AND u_passwd.PASSWORD = 'fadora';
2、有多少名为John并且居住在Santa Clara的
SELECT COUNT(*) FROM u_name, u_loc WHERE u_name.userid = u_loc.uid AND u_name.firstname = 'John' AND u_loc.location = 'Santa Clara'
COUNT(*) 统计排除值为null的行的个数
COUNT(1) 统计第一个字段不会null的行的个数
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讲解：
SELECT 执行顺序为最后 ，显示以下条件符合之后需要的值 u_name.firstname FROM 执行顺序为第一，在from下的表中查询 u_name, u_passwd WHERE 执行顺序为第二 ，查询from下的表符合条件的 u_name.userid = u_passwd.uid AND u_passwd.PASSWORD = 'fadora';
参考2得出的结论
SELECT 你想要的东西 FROM 这些东西有关的所有的表 WHERE 这些表中的相同条件 AND 某张表中单独的条件 AND 某张表中单独的条件
连表查询：
找到将多张表关联起来的字段，然后查询条件中写使关联的字段相等，最后写条件
练习例子：
有a,b,c三张表
a(uid,name)
b(uid.password)
c(name.local)
三张表关联为a和b通过uid关联，a和c通过name关联，b和c无关联
查询name为jerry,password为sun并且local是four road的人名字
SELECT res.a_name FROM ( SELECT name.a_name FROM name, passwd WHERE name.a_uid = passwd.u_uid AND passwd.u_passwd = 'sun' ) res ,local where res.a_name = local.a_name AND local.a_local = 'four road'
res为新表别名
或者可以写为
SELECT res.a_name FROM ( SELECT name.a_name FROM name, passwd WHERE name.a_uid = passwd.u_uid AND passwd.u_passwd = 'sun' ) res left join local ON res.a_name = local.a_name AND local.a_local = 'four road'
user.mdb文件内容
-- MySQL dump 10.14 Distrib 5.5.35-MariaDB, for Linux (x86_64) -- -- Host: localhost Database: Contacts -- ------------------------------------------------------ -- Server version 5.5.35-MariaDB /*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */; /*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */; /*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */; /*!40101 SET NAMES utf8 */; /*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */; /*!40103 SET TIME_ZONE='+00:00' */; /*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */; /*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */; /*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */; /*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */; -- -- Table structure for table `u_name` -- DROP TABLE IF EXISTS `u_name`; /*!40101 SET @saved_cs_client = @@character_set_client */; /*!40101 SET character_set_client = utf8 */; CREATE TABLE `u_name` ( `userid` int(11) NOT NULL AUTO_INCREMENT, `firstname` varchar(50) NOT NULL, `lastname` varchar(50) NOT NULL, PRIMARY KEY (`userid`) ) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=latin1; /*!40101 SET character_set_client = @saved_cs_client */; -- -- Dumping data for table `u_name` -- LOCK TABLES `u_name` WRITE; /*!40000 ALTER TABLE `u_name` DISABLE KEYS */; INSERT INTO `u_name` VALUES (1,'san','zhang'),(2,'si','li'),(3,'wu','wang'),(4,'Barack','Obama'),(5,'George','Walker Bush'),(6,'Bill','Clinton'),(7,'Hillary','Clinton'),(8,'John','Clinton'),(9,'George','wang'),(10,'John','li'),(11,'Bill','wang'),(12,'George','Obama'),(13,'George','wang'),(14,'Michael','Jackson'),(15,'John','Clinton'),(16,'Michael','Walker Bush'),(17,'Michael','Obama'),(18,'Hillary','Jackson'),(19,'Georgexx','Jackson'),(20,'Barack','Jackson'),(21,'John','Jackson'),(22,'John','Obama'),(23,'John','Clinton'),(24,'John','Walker Bush'),(25,'John','wang'); /*!40000 ALTER TABLE `u_name` ENABLE KEYS */; UNLOCK TABLES; -- -- Table structure for table `u_passwd` -- DROP TABLE IF EXISTS `u_passwd`; /*!40101 SET @saved_cs_client = @@character_set_client */; /*!40101 SET character_set_client = utf8 */; CREATE TABLE `u_passwd` ( `uid` int(11) NOT NULL AUTO_INCREMENT, `password` varchar(50) NOT NULL, PRIMARY KEY (`uid`) ) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=latin1; /*!40101 SET character_set_client = @saved_cs_client */; -- -- Dumping data for table `u_passwd` -- LOCK TABLES `u_passwd` WRITE; /*!40000 ALTER TABLE `u_passwd` DISABLE KEYS */; INSERT INTO `u_passwd` VALUES (1,'redhat'),(2,'fedora'),(3,'centos'),(4,'centes'),(5,'redhrt'),(6,'ridhat'),(7,'redfat'),(8,'fadora'),(9,'cantos'),(10,'redhap'),(11,'contos'),(12,'fcdora'),(13,'cendora'),(14,'tangene'),(15,'tangrine'),(16,'tangerone'),(17,'tangeine'),(18,'taangerine'),(19,'tangerine'),(20,'tanggerine'),(21,'anggerine'),(22,'aggerine'),(23,'taggerine'),(24,'tanerine'),(25,'tannerine'); /*!40000 ALTER TABLE `u_passwd` ENABLE KEYS */; UNLOCK TABLES; -- -- Table structure for table `u_loc` -- DROP TABLE IF EXISTS `u_loc`; /*!40101 SET @saved_cs_client = @@character_set_client */; /*!40101 SET character_set_client = utf8 */; CREATE TABLE `u_loc` ( `uid` int(11) NOT NULL AUTO_INCREMENT, `location` varchar(50) NOT NULL, PRIMARY KEY (`uid`) ) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=latin1; /*!40101 SET character_set_client = @saved_cs_client */; -- -- Dumping data for table `u_loc` -- LOCK TABLES `u_loc` WRITE; /*!40000 ALTER TABLE `u_loc` DISABLE KEYS */; INSERT INTO `u_loc` VALUES (1,'Shenzhen'),(2,'Guangzhou'),(3,'Santa Clara'),(4,'Santa Clara'),(5,'San Francisco'),(6,'Santa Calara'),(7,'Santa Clare'),(8,'Santa Clara'),(9,'Florida'),(10,'Santa Claraa'),(11,'Florida'),(12,'State of Texas'),(13,'State of Texas'),(14,'Santa Claraa'),(15,'Santa Clara'),(16,'Colorado?CO'),(17,'Hawaii?HI'),(18,'Santa Calara'),(19,'Santa Clra'),(20,'Santa Clara'),(21,'Santa Clara'),(22,'Minnesota'),(23,'Vermont'),(24,'Santa Clara'),(25,'Minnesota'); /*!40000 ALTER TABLE `u_loc` ENABLE KEYS */; UNLOCK TABLES; /*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */; /*!40101 SET SQL_MODE=@OLD_SQL_MODE */; /*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */; /*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */; /*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */; /*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */; /*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */; /*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */; -- Dump completed on 2016-07-24 18:17:53