Mysql 基础 [1]之DQL语言

Mysql服务的启动和关闭

方式一：计算机——右击管理——服务
方式二：通过管理员身份运行
net start 服务名（启动服务）如:net start Mysql
net stop 服务名（停止服务）如:net stop Mysql

Mysql的命令行登陆

命令行登陆

win+r命令行登陆

命令行

//mysql -h 主机地址 -P 端口号-u用户名 -p密码
mysql -h localhost -P 3306  -uroot -pok

也可以

mysql -uroot -pok

也可以使用Mysql自带的客户端

安装Mysql的图形化界面此处我使用Navicat 12 @Navicat 12 安装破解

Mysql的常见命令

1.查看当前所有的数据库

show databases;

2.打开指定的库

use 库名

3.查看当前库的所有表

show tables;

4.查看其它库的所有表

show tables from 库名;

5.创建表

create table 表名(
    列名 列类型,
    列名 列类型，

);

6.查看表结构

desc 表名;

查看数据库版本的方式

登陆到 Mysql 的服务端

select version();

未登陆的情况下

mysql --V
mysql --version

查看版本信息

Mysql的规范

1.不区分大小写,但是建议关键字大写.表名,列名小写
2.每条命令最好使用分号结尾
3.每条命令根据需要,可以进行缩进换行,
4.注释
单行注释：#注释文字
单行注释：-- 注释文字
多行注释：/* 注释文字 */

使用图形化界面链接数据库

SQL的语言分类

DQL（Data Query Language）：数据查询语言
select
DML(Data Manipulate Language):数据操作语言
insert 、update、delete
DDL（Data Define Languge）：数据定义语言
create、drop、alter
TCL（Transaction Control Language）：事务控制语言
commit、rollback

建立一个数据库和表

/*
 Navicat Premium Data Transfer

 Source Server         : icanci
 Source Server Type    : MySQL
 Source Server Version : 50540
 Source Host           : localhost:3306
 Source Schema         : myemployees

 Target Server Type    : MySQL
 Target Server Version : 50540
 File Encoding         : 65001

 Date: 13/02/2020 16:44:23
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for departments
-- ----------------------------
DROP TABLE IF EXISTS `departments`;
CREATE TABLE `departments`  (
  `department_id` int(4) NOT NULL AUTO_INCREMENT,
  `department_name` varchar(3) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `manager_id` int(6) NULL DEFAULT NULL,
  `location_id` int(4) NULL DEFAULT NULL,
  PRIMARY KEY (`department_id`) USING BTREE,
  INDEX `loc_id_fk`(`location_id`) USING BTREE,
  CONSTRAINT `loc_id_fk` FOREIGN KEY (`location_id`) REFERENCES `locations` (`location_id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 271 CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of departments
-- ----------------------------
INSERT INTO `departments` VALUES (10, 'Adm', 200, 1700);
INSERT INTO `departments` VALUES (20, 'Mar', 201, 1800);
INSERT INTO `departments` VALUES (30, 'Pur', 114, 1700);
INSERT INTO `departments` VALUES (40, 'Hum', 203, 2400);
INSERT INTO `departments` VALUES (50, 'Shi', 121, 1500);
INSERT INTO `departments` VALUES (60, 'IT', 103, 1400);
INSERT INTO `departments` VALUES (70, 'Pub', 204, 2700);
INSERT INTO `departments` VALUES (80, 'Sal', 145, 2500);
INSERT INTO `departments` VALUES (90, 'Exe', 100, 1700);
INSERT INTO `departments` VALUES (100, 'Fin', 108, 1700);
INSERT INTO `departments` VALUES (110, 'Acc', 205, 1700);
INSERT INTO `departments` VALUES (120, 'Tre', NULL, 1700);
INSERT INTO `departments` VALUES (130, 'Cor', NULL, 1700);
INSERT INTO `departments` VALUES (140, 'Con', NULL, 1700);
INSERT INTO `departments` VALUES (150, 'Sha', NULL, 1700);
INSERT INTO `departments` VALUES (160, 'Ben', NULL, 1700);
INSERT INTO `departments` VALUES (170, 'Man', NULL, 1700);
INSERT INTO `departments` VALUES (180, 'Con', NULL, 1700);
INSERT INTO `departments` VALUES (190, 'Con', NULL, 1700);
INSERT INTO `departments` VALUES (200, 'Ope', NULL, 1700);
INSERT INTO `departments` VALUES (210, 'IT ', NULL, 1700);
INSERT INTO `departments` VALUES (220, 'NOC', NULL, 1700);
INSERT INTO `departments` VALUES (230, 'IT ', NULL, 1700);
INSERT INTO `departments` VALUES (240, 'Gov', NULL, 1700);
INSERT INTO `departments` VALUES (250, 'Ret', NULL, 1700);
INSERT INTO `departments` VALUES (260, 'Rec', NULL, 1700);
INSERT INTO `departments` VALUES (270, 'Pay', NULL, 1700);

-- ----------------------------
-- Table structure for employees
-- ----------------------------
DROP TABLE IF EXISTS `employees`;
CREATE TABLE `employees`  (
  `employee_id` int(6) NOT NULL AUTO_INCREMENT,
  `first_name` varchar(20) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `last_name` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `email` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `phone_number` varchar(20) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `job_id` varchar(10) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `salary` double(10, 2) NULL DEFAULT NULL,
  `commission_pct` double(4, 2) NULL DEFAULT NULL,
  `manager_id` int(6) NULL DEFAULT NULL,
  `department_id` int(4) NULL DEFAULT NULL,
  `hiredate` datetime NULL DEFAULT NULL,
  PRIMARY KEY (`employee_id`) USING BTREE,
  INDEX `dept_id_fk`(`department_id`) USING BTREE,
  INDEX `job_id_fk`(`job_id`) USING BTREE,
  CONSTRAINT `dept_id_fk` FOREIGN KEY (`department_id`) REFERENCES `departments` (`department_id`) ON DELETE RESTRICT ON UPDATE RESTRICT,
  CONSTRAINT `job_id_fk` FOREIGN KEY (`job_id`) REFERENCES `jobs` (`job_id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 207 CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of employees
-- ----------------------------
INSERT INTO `employees` VALUES (100, 'Steven', 'K_ing', 'SKING', '515.123.4567', 'AD_PRES', 24000.00, NULL, NULL, 90, '1992-04-03 00:00:00');
INSERT INTO `employees` VALUES (101, 'Neena', 'Kochhar', 'NKOCHHAR', '515.123.4568', 'AD_VP', 17000.00, NULL, 100, 90, '1992-04-03 00:00:00');
INSERT INTO `employees` VALUES (102, 'Lex', 'De Haan', 'LDEHAAN', '515.123.4569', 'AD_VP', 17000.00, NULL, 100, 90, '1992-04-03 00:00:00');
INSERT INTO `employees` VALUES (103, 'Alexander', 'Hunold', 'AHUNOLD', '590.423.4567', 'IT_PROG', 9000.00, NULL, 102, 60, '1992-04-03 00:00:00');
INSERT INTO `employees` VALUES (104, 'Bruce', 'Ernst', 'BERNST', '590.423.4568', 'IT_PROG', 6000.00, NULL, 103, 60, '1992-04-03 00:00:00');
INSERT INTO `employees` VALUES (105, 'David', 'Austin', 'DAUSTIN', '590.423.4569', 'IT_PROG', 4800.00, NULL, 103, 60, '1998-03-03 00:00:00');
INSERT INTO `employees` VALUES (106, 'Valli', 'Pataballa', 'VPATABAL', '590.423.4560', 'IT_PROG', 4800.00, NULL, 103, 60, '1998-03-03 00:00:00');
INSERT INTO `employees` VALUES (107, 'Diana', 'Lorentz', 'DLORENTZ', '590.423.5567', 'IT_PROG', 4200.00, NULL, 103, 60, '1998-03-03 00:00:00');
INSERT INTO `employees` VALUES (108, 'Nancy', 'Greenberg', 'NGREENBE', '515.124.4569', 'FI_MGR', 12000.00, NULL, 101, 100, '1998-03-03 00:00:00');
INSERT INTO `employees` VALUES (109, 'Daniel', 'Faviet', 'DFAVIET', '515.124.4169', 'FI_ACCOUNT', 9000.00, NULL, 108, 100, '1998-03-03 00:00:00');
INSERT INTO `employees` VALUES (110, 'John', 'Chen', 'JCHEN', '515.124.4269', 'FI_ACCOUNT', 8200.00, NULL, 108, 100, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (111, 'Ismael', 'Sciarra', 'ISCIARRA', '515.124.4369', 'FI_ACCOUNT', 7700.00, NULL, 108, 100, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (112, 'Jose Manuel', 'Urman', 'JMURMAN', '515.124.4469', 'FI_ACCOUNT', 7800.00, NULL, 108, 100, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (113, 'Luis', 'Popp', 'LPOPP', '515.124.4567', 'FI_ACCOUNT', 6900.00, NULL, 108, 100, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (114, 'Den', 'Raphaely', 'DRAPHEAL', '515.127.4561', 'PU_MAN', 11000.00, NULL, 100, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (115, 'Alexander', 'Khoo', 'AKHOO', '515.127.4562', 'PU_CLERK', 3100.00, NULL, 114, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (116, 'Shelli', 'Baida', 'SBAIDA', '515.127.4563', 'PU_CLERK', 2900.00, NULL, 114, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (117, 'Sigal', 'Tobias', 'STOBIAS', '515.127.4564', 'PU_CLERK', 2800.00, NULL, 114, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (118, 'Guy', 'Himuro', 'GHIMURO', '515.127.4565', 'PU_CLERK', 2600.00, NULL, 114, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (119, 'Karen', 'Colmenares', 'KCOLMENA', '515.127.4566', 'PU_CLERK', 2500.00, NULL, 114, 30, '2000-09-09 00:00:00');
INSERT INTO `employees` VALUES (120, 'Matthew', 'Weiss', 'MWEISS', '650.123.1234', 'ST_MAN', 8000.00, NULL, 100, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (121, 'Adam', 'Fripp', 'AFRIPP', '650.123.2234', 'ST_MAN', 8200.00, NULL, 100, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (122, 'Payam', 'Kaufling', 'PKAUFLIN', '650.123.3234', 'ST_MAN', 7900.00, NULL, 100, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (123, 'Shanta', 'Vollman', 'SVOLLMAN', '650.123.4234', 'ST_MAN', 6500.00, NULL, 100, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (124, 'Kevin', 'Mourgos', 'KMOURGOS', '650.123.5234', 'ST_MAN', 5800.00, NULL, 100, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (125, 'Julia', 'Nayer', 'JNAYER', '650.124.1214', 'ST_CLERK', 3200.00, NULL, 120, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (126, 'Irene', 'Mikkilineni', 'IMIKKILI', '650.124.1224', 'ST_CLERK', 2700.00, NULL, 120, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (127, 'James', 'Landry', 'JLANDRY', '650.124.1334', 'ST_CLERK', 2400.00, NULL, 120, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (128, 'Steven', 'Markle', 'SMARKLE', '650.124.1434', 'ST_CLERK', 2200.00, NULL, 120, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (129, 'Laura', 'Bissot', 'LBISSOT', '650.124.5234', 'ST_CLERK', 3300.00, NULL, 121, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (130, 'Mozhe', 'Atkinson', 'MATKINSO', '650.124.6234', 'ST_CLERK', 2800.00, NULL, 121, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (131, 'James', 'Marlow', 'JAMRLOW', '650.124.7234', 'ST_CLERK', 2500.00, NULL, 121, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (132, 'TJ', 'Olson', 'TJOLSON', '650.124.8234', 'ST_CLERK', 2100.00, NULL, 121, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (133, 'Jason', 'Mallin', 'JMALLIN', '650.127.1934', 'ST_CLERK', 3300.00, NULL, 122, 50, '2004-02-06 00:00:00');
INSERT INTO `employees` VALUES (134, 'Michael', 'Rogers', 'MROGERS', '650.127.1834', 'ST_CLERK', 2900.00, NULL, 122, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (135, 'Ki', 'Gee', 'KGEE', '650.127.1734', 'ST_CLERK', 2400.00, NULL, 122, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (136, 'Hazel', 'Philtanker', 'HPHILTAN', '650.127.1634', 'ST_CLERK', 2200.00, NULL, 122, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (137, 'Renske', 'Ladwig', 'RLADWIG', '650.121.1234', 'ST_CLERK', 3600.00, NULL, 123, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (138, 'Stephen', 'Stiles', 'SSTILES', '650.121.2034', 'ST_CLERK', 3200.00, NULL, 123, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (139, 'John', 'Seo', 'JSEO', '650.121.2019', 'ST_CLERK', 2700.00, NULL, 123, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (140, 'Joshua', 'Patel', 'JPATEL', '650.121.1834', 'ST_CLERK', 2500.00, NULL, 123, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (141, 'Trenna', 'Rajs', 'TRAJS', '650.121.8009', 'ST_CLERK', 3500.00, NULL, 124, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (142, 'Curtis', 'Davies', 'CDAVIES', '650.121.2994', 'ST_CLERK', 3100.00, NULL, 124, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (143, 'Randall', 'Matos', 'RMATOS', '650.121.2874', 'ST_CLERK', 2600.00, NULL, 124, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (144, 'Peter', 'Vargas', 'PVARGAS', '650.121.2004', 'ST_CLERK', 2500.00, NULL, 124, 50, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (145, 'John', 'Russell', 'JRUSSEL', '011.44.1344.429268', 'SA_MAN', 14000.00, 0.40, 100, 80, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (146, 'Karen', 'Partners', 'KPARTNER', '011.44.1344.467268', 'SA_MAN', 13500.00, 0.30, 100, 80, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (147, 'Alberto', 'Errazuriz', 'AERRAZUR', '011.44.1344.429278', 'SA_MAN', 12000.00, 0.30, 100, 80, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (148, 'Gerald', 'Cambrault', 'GCAMBRAU', '011.44.1344.619268', 'SA_MAN', 11000.00, 0.30, 100, 80, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (149, 'Eleni', 'Zlotkey', 'EZLOTKEY', '011.44.1344.429018', 'SA_MAN', 10500.00, 0.20, 100, 80, '2002-12-23 00:00:00');
INSERT INTO `employees` VALUES (150, 'Peter', 'Tucker', 'PTUCKER', '011.44.1344.129268', 'SA_REP', 10000.00, 0.30, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (151, 'David', 'Bernstein', 'DBERNSTE', '011.44.1344.345268', 'SA_REP', 9500.00, 0.25, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (152, 'Peter', 'Hall', 'PHALL', '011.44.1344.478968', 'SA_REP', 9000.00, 0.25, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (153, 'Christopher', 'Olsen', 'COLSEN', '011.44.1344.498718', 'SA_REP', 8000.00, 0.20, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (154, 'Nanette', 'Cambrault', 'NCAMBRAU', '011.44.1344.987668', 'SA_REP', 7500.00, 0.20, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (155, 'Oliver', 'Tuvault', 'OTUVAULT', '011.44.1344.486508', 'SA_REP', 7000.00, 0.15, 145, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (156, 'Janette', 'K_ing', 'JKING', '011.44.1345.429268', 'SA_REP', 10000.00, 0.35, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (157, 'Patrick', 'Sully', 'PSULLY', '011.44.1345.929268', 'SA_REP', 9500.00, 0.35, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (158, 'Allan', 'McEwen', 'AMCEWEN', '011.44.1345.829268', 'SA_REP', 9000.00, 0.35, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (159, 'Lindsey', 'Smith', 'LSMITH', '011.44.1345.729268', 'SA_REP', 8000.00, 0.30, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (160, 'Louise', 'Doran', 'LDORAN', '011.44.1345.629268', 'SA_REP', 7500.00, 0.30, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (161, 'Sarath', 'Sewall', 'SSEWALL', '011.44.1345.529268', 'SA_REP', 7000.00, 0.25, 146, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (162, 'Clara', 'Vishney', 'CVISHNEY', '011.44.1346.129268', 'SA_REP', 10500.00, 0.25, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (163, 'Danielle', 'Greene', 'DGREENE', '011.44.1346.229268', 'SA_REP', 9500.00, 0.15, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (164, 'Mattea', 'Marvins', 'MMARVINS', '011.44.1346.329268', 'SA_REP', 7200.00, 0.10, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (165, 'David', 'Lee', 'DLEE', '011.44.1346.529268', 'SA_REP', 6800.00, 0.10, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (166, 'Sundar', 'Ande', 'SANDE', '011.44.1346.629268', 'SA_REP', 6400.00, 0.10, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (167, 'Amit', 'Banda', 'ABANDA', '011.44.1346.729268', 'SA_REP', 6200.00, 0.10, 147, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (168, 'Lisa', 'Ozer', 'LOZER', '011.44.1343.929268', 'SA_REP', 11500.00, 0.25, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (169, 'Harrison', 'Bloom', 'HBLOOM', '011.44.1343.829268', 'SA_REP', 10000.00, 0.20, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (170, 'Tayler', 'Fox', 'TFOX', '011.44.1343.729268', 'SA_REP', 9600.00, 0.20, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (171, 'William', 'Smith', 'WSMITH', '011.44.1343.629268', 'SA_REP', 7400.00, 0.15, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (172, 'Elizabeth', 'Bates', 'EBATES', '011.44.1343.529268', 'SA_REP', 7300.00, 0.15, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (173, 'Sundita', 'Kumar', 'SKUMAR', '011.44.1343.329268', 'SA_REP', 6100.00, 0.10, 148, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (174, 'Ellen', 'Abel', 'EABEL', '011.44.1644.429267', 'SA_REP', 11000.00, 0.30, 149, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (175, 'Alyssa', 'Hutton', 'AHUTTON', '011.44.1644.429266', 'SA_REP', 8800.00, 0.25, 149, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (176, 'Jonathon', 'Taylor', 'JTAYLOR', '011.44.1644.429265', 'SA_REP', 8600.00, 0.20, 149, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (177, 'Jack', 'Livingston', 'JLIVINGS', '011.44.1644.429264', 'SA_REP', 8400.00, 0.20, 149, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (178, 'Kimberely', 'Grant', 'KGRANT', '011.44.1644.429263', 'SA_REP', 7000.00, 0.15, 149, NULL, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (179, 'Charles', 'Johnson', 'CJOHNSON', '011.44.1644.429262', 'SA_REP', 6200.00, 0.10, 149, 80, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (180, 'Winston', 'Taylor', 'WTAYLOR', '650.507.9876', 'SH_CLERK', 3200.00, NULL, 120, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (181, 'Jean', 'Fleaur', 'JFLEAUR', '650.507.9877', 'SH_CLERK', 3100.00, NULL, 120, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (182, 'Martha', 'Sullivan', 'MSULLIVA', '650.507.9878', 'SH_CLERK', 2500.00, NULL, 120, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (183, 'Girard', 'Geoni', 'GGEONI', '650.507.9879', 'SH_CLERK', 2800.00, NULL, 120, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (184, 'Nandita', 'Sarchand', 'NSARCHAN', '650.509.1876', 'SH_CLERK', 4200.00, NULL, 121, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (185, 'Alexis', 'Bull', 'ABULL', '650.509.2876', 'SH_CLERK', 4100.00, NULL, 121, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (186, 'Julia', 'Dellinger', 'JDELLING', '650.509.3876', 'SH_CLERK', 3400.00, NULL, 121, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (187, 'Anthony', 'Cabrio', 'ACABRIO', '650.509.4876', 'SH_CLERK', 3000.00, NULL, 121, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (188, 'Kelly', 'Chung', 'KCHUNG', '650.505.1876', 'SH_CLERK', 3800.00, NULL, 122, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (189, 'Jennifer', 'Dilly', 'JDILLY', '650.505.2876', 'SH_CLERK', 3600.00, NULL, 122, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (190, 'Timothy', 'Gates', 'TGATES', '650.505.3876', 'SH_CLERK', 2900.00, NULL, 122, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (191, 'Randall', 'Perkins', 'RPERKINS', '650.505.4876', 'SH_CLERK', 2500.00, NULL, 122, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (192, 'Sarah', 'Bell', 'SBELL', '650.501.1876', 'SH_CLERK', 4000.00, NULL, 123, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (193, 'Britney', 'Everett', 'BEVERETT', '650.501.2876', 'SH_CLERK', 3900.00, NULL, 123, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (194, 'Samuel', 'McCain', 'SMCCAIN', '650.501.3876', 'SH_CLERK', 3200.00, NULL, 123, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (195, 'Vance', 'Jones', 'VJONES', '650.501.4876', 'SH_CLERK', 2800.00, NULL, 123, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (196, 'Alana', 'Walsh', 'AWALSH', '650.507.9811', 'SH_CLERK', 3100.00, NULL, 124, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (197, 'Kevin', 'Feeney', 'KFEENEY', '650.507.9822', 'SH_CLERK', 3000.00, NULL, 124, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (198, 'Donald', 'OConnell', 'DOCONNEL', '650.507.9833', 'SH_CLERK', 2600.00, NULL, 124, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (199, 'Douglas', 'Grant', 'DGRANT', '650.507.9844', 'SH_CLERK', 2600.00, NULL, 124, 50, '2014-03-05 00:00:00');
INSERT INTO `employees` VALUES (200, 'Jennifer', 'Whalen', 'JWHALEN', '515.123.4444', 'AD_ASST', 4400.00, NULL, 101, 10, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (201, 'Michael', 'Hartstein', 'MHARTSTE', '515.123.5555', 'MK_MAN', 13000.00, NULL, 100, 20, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (202, 'Pat', 'Fay', 'PFAY', '603.123.6666', 'MK_REP', 6000.00, NULL, 201, 20, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (203, 'Susan', 'Mavris', 'SMAVRIS', '515.123.7777', 'HR_REP', 6500.00, NULL, 101, 40, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (204, 'Hermann', 'Baer', 'HBAER', '515.123.8888', 'PR_REP', 10000.00, NULL, 101, 70, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (205, 'Shelley', 'Higgins', 'SHIGGINS', '515.123.8080', 'AC_MGR', 12000.00, NULL, 101, 110, '2016-03-03 00:00:00');
INSERT INTO `employees` VALUES (206, 'William', 'Gietz', 'WGIETZ', '515.123.8181', 'AC_ACCOUNT', 8300.00, NULL, 205, 110, '2016-03-03 00:00:00');

-- ----------------------------
-- Table structure for jobs
-- ----------------------------
DROP TABLE IF EXISTS `jobs`;
CREATE TABLE `jobs`  (
  `job_id` varchar(10) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NOT NULL,
  `job_title` varchar(35) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `min_salary` int(6) NULL DEFAULT NULL,
  `max_salary` int(6) NULL DEFAULT NULL,
  PRIMARY KEY (`job_id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of jobs
-- ----------------------------
INSERT INTO `jobs` VALUES ('AC_ACCOUNT', 'Public Accountant', 4200, 9000);
INSERT INTO `jobs` VALUES ('AC_MGR', 'Accounting Manager', 8200, 16000);
INSERT INTO `jobs` VALUES ('AD_ASST', 'Administration Assistant', 3000, 6000);
INSERT INTO `jobs` VALUES ('AD_PRES', 'President', 20000, 40000);
INSERT INTO `jobs` VALUES ('AD_VP', 'Administration Vice President', 15000, 30000);
INSERT INTO `jobs` VALUES ('FI_ACCOUNT', 'Accountant', 4200, 9000);
INSERT INTO `jobs` VALUES ('FI_MGR', 'Finance Manager', 8200, 16000);
INSERT INTO `jobs` VALUES ('HR_REP', 'Human Resources Representative', 4000, 9000);
INSERT INTO `jobs` VALUES ('IT_PROG', 'Programmer', 4000, 10000);
INSERT INTO `jobs` VALUES ('MK_MAN', 'Marketing Manager', 9000, 15000);
INSERT INTO `jobs` VALUES ('MK_REP', 'Marketing Representative', 4000, 9000);
INSERT INTO `jobs` VALUES ('PR_REP', 'Public Relations Representative', 4500, 10500);
INSERT INTO `jobs` VALUES ('PU_CLERK', 'Purchasing Clerk', 2500, 5500);
INSERT INTO `jobs` VALUES ('PU_MAN', 'Purchasing Manager', 8000, 15000);
INSERT INTO `jobs` VALUES ('SA_MAN', 'Sales Manager', 10000, 20000);
INSERT INTO `jobs` VALUES ('SA_REP', 'Sales Representative', 6000, 12000);
INSERT INTO `jobs` VALUES ('SH_CLERK', 'Shipping Clerk', 2500, 5500);
INSERT INTO `jobs` VALUES ('ST_CLERK', 'Stock Clerk', 2000, 5000);
INSERT INTO `jobs` VALUES ('ST_MAN', 'Stock Manager', 5500, 8500);

-- ----------------------------
-- Table structure for locations
-- ----------------------------
DROP TABLE IF EXISTS `locations`;
CREATE TABLE `locations`  (
  `location_id` int(11) NOT NULL AUTO_INCREMENT,
  `street_address` varchar(40) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `postal_code` varchar(12) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `city` varchar(30) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `state_province` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  `country_id` varchar(2) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,
  PRIMARY KEY (`location_id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 3201 CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of locations
-- ----------------------------
INSERT INTO `locations` VALUES (1000, '1297 Via Cola di Rie', '00989', 'Roma', NULL, 'IT');
INSERT INTO `locations` VALUES (1100, '93091 Calle della Testa', '10934', 'Venice', NULL, 'IT');
INSERT INTO `locations` VALUES (1200, '2017 Shinjuku-ku', '1689', 'Tokyo', 'Tokyo Prefecture', 'JP');
INSERT INTO `locations` VALUES (1300, '9450 Kamiya-cho', '6823', 'Hiroshima', NULL, 'JP');
INSERT INTO `locations` VALUES (1400, '2014 Jabberwocky Rd', '26192', 'Southlake', 'Texas', 'US');
INSERT INTO `locations` VALUES (1500, '2011 Interiors Blvd', '99236', 'South San Francisco', 'California', 'US');
INSERT INTO `locations` VALUES (1600, '2007 Zagora St', '50090', 'South Brunswick', 'New Jersey', 'US');
INSERT INTO `locations` VALUES (1700, '2004 Charade Rd', '98199', 'Seattle', 'Washington', 'US');
INSERT INTO `locations` VALUES (1800, '147 Spadina Ave', 'M5V 2L7', 'Toronto', 'Ontario', 'CA');
INSERT INTO `locations` VALUES (1900, '6092 Boxwood St', 'YSW 9T2', 'Whitehorse', 'Yukon', 'CA');
INSERT INTO `locations` VALUES (2000, '40-5-12 Laogianggen', '190518', 'Beijing', NULL, 'CN');
INSERT INTO `locations` VALUES (2100, '1298 Vileparle (E)', '490231', 'Bombay', 'Maharashtra', 'IN');
INSERT INTO `locations` VALUES (2200, '12-98 Victoria Street', '2901', 'Sydney', 'New South Wales', 'AU');
INSERT INTO `locations` VALUES (2300, '198 Clementi North', '540198', 'Singapore', NULL, 'SG');
INSERT INTO `locations` VALUES (2400, '8204 Arthur St', NULL, 'London', NULL, 'UK');
INSERT INTO `locations` VALUES (2500, 'Magdalen Centre, The Oxford Science Park', 'OX9 9ZB', 'Oxford', 'Oxford', 'UK');
INSERT INTO `locations` VALUES (2600, '9702 Chester Road', '09629850293', 'Stretford', 'Manchester', 'UK');
INSERT INTO `locations` VALUES (2700, 'Schwanthalerstr. 7031', '80925', 'Munich', 'Bavaria', 'DE');
INSERT INTO `locations` VALUES (2800, 'Rua Frei Caneca 1360 ', '01307-002', 'Sao Paulo', 'Sao Paulo', 'BR');
INSERT INTO `locations` VALUES (2900, '20 Rue des Corps-Saints', '1730', 'Geneva', 'Geneve', 'CH');
INSERT INTO `locations` VALUES (3000, 'Murtenstrasse 921', '3095', 'Bern', 'BE', 'CH');
INSERT INTO `locations` VALUES (3100, 'Pieter Breughelstraat 837', '3029SK', 'Utrecht', 'Utrecht', 'NL');
INSERT INTO `locations` VALUES (3200, 'Mariano Escobedo 9991', '11932', 'Mexico City', 'Distrito Federal,', 'MX');

SET FOREIGN_KEY_CHECKS = 1;

进阶1：基础查询

语法：
SELECT 要查询的东西
[FROM 表名];
select name from user

类似于Java中 :System.out.println(要打印的东西);
特点：
①通过select查询完的结果，是一个虚拟的表格，不是真实存在
② 要查询的东西可以是常量值、可以是表达式、可以是字段、可以是函数

进阶2：条件查询

    条件查询：根据条件过滤原始表的数据，查询到想要的数据
    语法：
    select 
        要查询的字段|表达式|常量值|函数
    from 
        表
    where 
        条件 ;
查询常量值:SELECT 100; SELECT 
查询表达式:100%98;
查询函数:SELECT VERSION();
起别名
1.便于理解
2.如果要查询的字段有重名的情况,使用别名可取区分开来
SELECT VERSION() as version; //方式1
SELECT VERSION() version;  //方式2
注意 有空格需要使用单引号或者双引号抱起来
SELECT VERSION() "mysql version"; //不然会报错
去重复 DISTINCT 关键字
SELECT DISTINCT xxx FROM xxx;
+ 的作用:
只有一个功能,运算符  select 100+60; //都为数值类型
select '100'+20; select '100.05'+20;//如果一个为数字类型,会试图进行类型转换 如果转换成功,就会进行加法运算 如果转换失败,就会把字符型转换为0
select null;//只要有一方为null 结构就是null

拼接名字
select concat('a','b') as 性别;
IFNULL 函数 SELECT IFNULL(id,0) 前面是可能出现null的字段,后面是出现null需要替换的值

通配符查询 
select * from user ;会查询所有的user 数据
分类：
一、条件表达式
    示例：salary>10000
    条件运算符：
    > < >= <= = != <>[就是不等于的意思]

二、逻辑表达式
示例：salary>10000 && salary<20000

逻辑运算符：

    and（&&）:两个条件如果同时成立，结果为true，否则为false
    or(||)：两个条件只要有一个成立，结果为true，否则为false
    not(!)：如果条件成立，则not后为false，否则为true

三、模糊查询 
like 
between and //包含临界值 顺序不可以颠倒 
in
is null  //seelct * from user where user is null ;// null值不可使用 = 判断
<=> 安全等于 既可以判断 null 也可以判断普通的数据
% 表示任意多个字符
_ 表示单个字符
需要查询第二个字符为 _ 的员工  此时需要转义
select * from user where username  like '_\_%' //方法1
select * from user where username  like '_$_%' ESCAPE '$';// 指定转义的符号
示例：last_name like 'a%'

进阶3:排序查询

语法

select
    要查询的东西
from
    表
where 
    条件
order by 排序的字段|表达式|函数|别名 [asc|desc] asc升序[默认] desc[降序]
order by 后面可以放多个条件,上面的都可以 order by 一般放在查询语句的最后 limit 除外

进阶4：常见函数

调用:select 函数名(实参列表) from 表

一、单行函数

1、字符函数
concat拼接
substr截取子串

 SELECT SUBSTR('哈希哈哈哈',1,2) out_put; 
#结果哈希  下标从1开始

upper转换成大写
lower转换成小写
trim去前后指定的空格和字符默认就是去除空格去除字符的写法

SELECT TRIM('a' FROM   'aaaaa哈希aaa') as 哈希 ;

ltrim去左边空格
rtrim去右边空格
replace替换

SELECT REPLACE('333333demo','demo','1') ;
//3333331

lpad左填充

SELECT LPAD('demo',10,'3') as out_put;
//333333demo 长度是填充之后的长度

rpad右填充
instr返回子串第一次出现的索引找不到就返回0
length 获取字节个数
2、数学函数
round 四舍五入

SELECT ROUND(-1.63);
// -2
SELECT ROUND(-1.636,2);
//-1.64

rand 随机数
floor向下取整返回<=该参数的最大整数

SELECT FLOOR(1.003);
//1

ceil向上取整返回>=该参数的最小整数

SELECT CEIL(1.003);
//2

mod取余 : MOD(a,b):a-a/b*b

SELECT MOD(10,3);
//1

truncate截断

SELECT TRUNCATE(1.4545645,1);
//1.4

3、日期函数
now当前系统日期+时间

SELECT NOW()
//2020-02-14 10:25:51

curdate当前系统日期

SELECT CURDATE();
//2020-02-14

curtime当前系统时间

SELECT CURTIME();
//10:27:04

SELECT YEAR(NOW());
//2020
SELECT MONTH(NOW());
//2
SELECT DAY(NOW());
//14
//还有 时分秒

str_to_date 将字符转换成日期

SELECT STR_TO_DATE('1998-3-2','%Y-%c-%d')
//1998-3-2

date_format将日期转换成字符

SELECT DATE_FORMAT('2020/1/2','%Y年%m月%d日');
//2020年01月02日

格式

4、流程控制函数
if 处理双分支

SELECT IF(10>5,'大','小');
// 大

case语句处理多分支
情况1：处理等值判断
情况2：处理条件判断

case 要判断的字段或者表达式
when 常量1 then 要显示的值1或语句1
when 常量2 then 要实现的值2或语句2
when 常量3 then 要实现的值3或语句3
...
else 要显示的值或者语句n;
end

5、其他函数
version版本
database当前库
user当前连接用户

SELECT VERSION();
//5.5.40
SELECT USER();
//root@localhost
SELECT DATABASE();
//myemployees

二、分组函数

sum 求和

SELECT SUM(salary) FROM employees;
//691400.00

max 最大值

SELECT MAX(salary) FROM employees;

min 最小值

SELECT MIN(salary) FROM employees;

avg 平均值

SELECT AVG(salary) FROM employees;

count 计数

SELECT COUNT(*) FROM employees;

SELECT SUM(DISTINCT salary) FROM employees;

特点：
1、以上五个分组函数都忽略null值，除了count()
2、sum和avg一般用于处理数值型 max、min、count可以处理任何数据类型
3、都可以搭配distinct使用，用于统计去重后的结果
4、count的参数可以支持：字段、、常量值，一般放1
建议使用 count(*)

求日期的时间间隔

SELECT DATEDIFF('2020-1-7',NOW());
//-38
SELECT DATEDIFF('2020-10-7',NOW());
//236
此函数是前面的时间减去后面的时间的 天数

进阶5：分组查询

语法：

select 查询的字段，分组函数
from 表
group by 分组的字段
特点：
1、可以按单个字段分组
2、和分组函数一同查询的字段最好是分组后的字段
3、分组筛选
        针对的表    位置          关键字
分组前筛选：  原始表     group by的前面     where
分组后筛选：  分组后的结果集 group by的后面     having
4、可以按多个字段分组，字段之间用逗号隔开
5、可以支持排序 放在分组的后面
6、having后可以支持别名

SELECT AVG(salary),department_id,email
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id


SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2

1.分组函数做条件肯定是放在HAVING子句中
2.能用分组前筛选的,就放在分组之前

进阶6：多表连接查询

/*
 Navicat Premium Data Transfer

 Source Server         : icanci
 Source Server Type    : MySQL
 Source Server Version : 50540
 Source Host           : localhost:3306
 Source Schema         : girls

 Target Server Type    : MySQL
 Target Server Version : 50540
 File Encoding         : 65001

 Date: 14/02/2020 13:58:35
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for admin
-- ----------------------------
DROP TABLE IF EXISTS `admin`;
CREATE TABLE `admin`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `password` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 3 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of admin
-- ----------------------------
INSERT INTO `admin` VALUES (1, 'john', '8888');
INSERT INTO `admin` VALUES (2, 'lyt', '6666');

-- ----------------------------
-- Table structure for beauty
-- ----------------------------
DROP TABLE IF EXISTS `beauty`;
CREATE TABLE `beauty`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `sex` char(1) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT '女',
  `borndate` datetime NULL DEFAULT '1987-01-01 00:00:00',
  `phone` varchar(11) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `photo` blob NULL,
  `boyfriend_id` int(11) NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 13 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of beauty
-- ----------------------------
INSERT INTO `beauty` VALUES (1, '柳岩', '女', '1988-02-03 00:00:00', '18209876577', NULL, 8);
INSERT INTO `beauty` VALUES (2, '苍老师', '女', '1987-12-30 00:00:00', '18219876577', NULL, 9);
INSERT INTO `beauty` VALUES (3, 'Angelababy', '女', '1989-02-03 00:00:00', '18209876567', NULL, 3);
INSERT INTO `beauty` VALUES (4, '热巴', '女', '1993-02-03 00:00:00', '18209876579', NULL, 2);
INSERT INTO `beauty` VALUES (5, '周冬雨', '女', '1992-02-03 00:00:00', '18209179577', NULL, 9);
INSERT INTO `beauty` VALUES (6, '周芷若', '女', '1988-02-03 00:00:00', '18209876577', NULL, 1);
INSERT INTO `beauty` VALUES (7, '岳灵珊', '女', '1987-12-30 00:00:00', '18219876577', NULL, 9);
INSERT INTO `beauty` VALUES (8, '小昭', '女', '1989-02-03 00:00:00', '18209876567', NULL, 1);
INSERT INTO `beauty` VALUES (9, '双儿', '女', '1993-02-03 00:00:00', '18209876579', NULL, 9);
INSERT INTO `beauty` VALUES (10, '王语嫣', '女', '1992-02-03 00:00:00', '18209179577', NULL, 4);
INSERT INTO `beauty` VALUES (11, '夏雪', '女', '1993-02-03 00:00:00', '18209876579', NULL, 9);
INSERT INTO `beauty` VALUES (12, '赵敏', '女', '1992-02-03 00:00:00', '18209179577', NULL, 1);

-- ----------------------------
-- Table structure for boys
-- ----------------------------
DROP TABLE IF EXISTS `boys`;
CREATE TABLE `boys`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `boyName` varchar(20) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `userCP` int(11) NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of boys
-- ----------------------------
INSERT INTO `boys` VALUES (1, '张无忌', 100);
INSERT INTO `boys` VALUES (2, '鹿晗', 800);
INSERT INTO `boys` VALUES (3, '黄晓明', 50);
INSERT INTO `boys` VALUES (4, '段誉', 300);

SET FOREIGN_KEY_CHECKS = 1;

笛卡尔乘积：如果连接条件省略或无效则会出现
解决办法：添加上连接条件
一、传统模式下的连接：等值连接——非等值连接
1.等值连接的结果 = 多个表的交集
2.n表连接，至少需要n-1个连接条件
3.多个表不分主次，没有顺序要求
4.一般为表起别名，提高阅读性和性能

SELECT * FROM beauty,boys 
WHERE beauty.boyfriend_id = boys.idSELECT * FROM beauty,boys 
WHERE beauty.boyfriend_id = boys.id

二、sql99语法：通过join关键字实现连接
含义：1999年推出的sql语法
支持：
等值连接、非等值连接自链接（内连接,查询自己）
外连接
左外链接右外链接全外链接
交叉连接

语法：

好处：语句上，连接条件和筛选条件实现了分离，简洁明了！

等值链接

SELECT last_name,department_name
FROM employees e
INNER JOIN departments d
ON e.department_id=d.department_id

等值链接 添加筛选

SELECT last_name,job_title 
FROM employees e
INNER JOIN jobs j
ON e.job_id=j.job_id
WHERE e.last_name LIKE '%e%'

等值链接 添加分组 + 筛选

SELECT city,COUNT(*) 部门个数
FROM departments d
INNER JOIN locations l
ON d.location_id=l.location_id
GROUP BY city
HAVING COUNT(*) >3

添加排序

SELECT COUNT(*),department_name
FROM employees e
INNER JOIN departments d
ON e.department_id=d.department_id
GROUP BY department_name
HAVING COUNT(*) >3
ORDER BY COUNT(*) DESC;

三表链接


SELECT last_name,department_name,job_title
FROM employees e
INNER JOIN departments d ON e.department_id=d.department_id
INNER JOIN jobs j ON e.job_id=j.job_id
ORDER BY department_name DESC;

特点:
1.添加排序,分组,筛选
2.inner 可以省略
3.筛选条件放在where后面链接条件放在 on 后面
4.inner join 的和sql92的等值链接效果是一样的

外连接应用场景:用于查询一个表中有,另外一个表没有的记录
特点
1.外连接的查询结果为主表的所有记录
2.如果从表中有和他匹配的,就显示匹配的值
4.如果从表中没有和他匹配的,就显示为null
5.外连接查询结果等于内连接结果+主表中有而从表中没有的记录
左外链接:left join左边的是主表
右外链接;right jpin 右边的是主表
左外和右外交换两个表的顺序,可以实现同样的效果
全外链接 = 内连接的结果 + 表1中有但是表2没有+表2中有但是表1没有

SELECT d.*,e.employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.department_id = e.department_id
WHERE e.employee_id IS NULL;

链接总结

三、自连接
案例：查询员工名和直接上级的名称
sql99

SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m ON e.`manager_id`=m.`employee_id`;

sql92

SELECT e.last_name,m.last_name
FROM employees e,employees m 
WHERE e.`manager_id`=m.`employee_id`;

进阶7：子查询

含义：
一条查询语句中又嵌套了另一条完整的select语句，其中被嵌套的select语句，称为子查询或内查询
在外面的查询语句，称为主查询或外查询
特点：

1、子查询都放在小括号内
2、子查询可以放在from后面、select后面、where后面、having后面，但一般放在条件的右侧
3、子查询优先于主查询执行，主查询使用了子查询的执行结果
4、子查询根据查询结果的行数不同分为以下两类：
① 单行子查询
结果集只有一行
一般搭配单行操作符使用：> < = <> >= <=
非法使用子查询的情况：
a、子查询的结果为一组值
b、子查询的结果为空
② 多行子查询3
结果集有多行
一般搭配多行操作符使用：any、all、in、not in
in：属于子查询结果中的任意一个就行
any和all往往可以用其他查询代替

SELECT
    * 
FROM
    employees 
WHERE
    salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );

#行子查询
SELECT * 
FROM employees
WHERE(employee_id,salary) =(
    SELECT MIN(employee_id),MAX(salary)
    FROM employees
);

#行子查询
SELECT * 
FROM employees
WHERE(employee_id,salary) =(
    SELECT MIN(employee_id),MAX(salary)
    FROM employees
);

进阶8：分页查询

应用场景：
实际的web项目中需要根据用户的需求提交对应的分页查询的sql语句

select 字段|表达式,...
from 表
[where 条件]
[group by 分组字段]
[having 条件]
[order by 排序的字段]
limit [起始的条目索引，]条目数;

特点：

1.起始条目索引从0开始

2.limit子句放在查询语句的最后

3.公式：select * from  表 limit （page-1）*sizePerPage,sizePerPage
假如:
每页显示条目数sizePerPage
要显示的页数 page

进阶9：联合查询

引入：
union 联合、合
语法：

elect 字段|常量|表达式|函数 【from 表】 【where 条件】 union 【all】
select 字段|常量|表达式|函数 【from 表】 【where 条件】 union 【all】
select 字段|常量|表达式|函数 【from 表】 【where 条件】 union  【all】
.....
select 字段|常量|表达式|函数 【from 表】 【where 条件】

特点：

1、多条查询语句的查询的列数必须是一致的
2、多条查询语句的查询的列的类型几乎相同
3、union代表去重，union all代表不去重

SELECT * FROM employees WHERE email LIKE '%a
%' OR department_id>90

SELECT * FROM employees WHERE email LIKE '%a'
UNION
SELECT * FROM employees WHERE department_id>90;`