Linux mysql 联表查询

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

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3

查询密码是fedora,并且住址是Santa Clara的人

SELECT
    res.firstname
FROM
    ( SELECT * FROM u_name, u_passwd WHERE u_name.userid = u_passwd.uid AND u_passwd.PASSWORD = 'fadora' ) res,
    u_loc
WHERE
    res.userid = u_loc.uid
    AND u_loc.LOCATION = 'Santa Clara';

解析：

　　res为新表，res.firstname 为新表中的字段

　　首先查询所有密码是fadora的人，然后将查询结果作为新表，命名为res，然后继续联表查询

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