MySQL数据库应用DQL之select知识
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一、查看数据
1、查询表的所有数据行
1）命令语法：select <字段1，字段2，...> from <表名> where <表达式>
其中，select，from，where 是不能随便改的，是关键字，支持大小写
2）例：查看表test中所有数据
a.进入指定库后查询
mysql> use oldboy Database changed mysql> select * from test; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | | 3 | inca | | 4 | zuma | | 5 | kaka | +----+---------+ 5 rows in set (0.00 sec) mysql> select id,name from test; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | | 3 | inca | | 4 | zuma | | 5 | kaka | +----+---------+ 5 rows in set (0.00 sec)
2、根据指定条件查询
1）例：查看表test中前2行数据
执行命令：
mysql> select id,name from test limit 2; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | +----+---------+ 2 rows in set (0.00 sec) mysql> select id,name from test limit 0，2; ERROR 1327 (42000): Undeclared variable: 0，2 mysql> select id,name from test limit 0,2; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | +----+---------+ 2 rows in set (0.00 sec)
2）指定固定条件查询数据
执行命令：
mysql> select id,name from test where id=1; +----+--------+ | id | name | +----+--------+ | 1 | oldboy | +----+--------+ 1 row in set (0.00 sec) mysql> select id,name from test where name=‘oldgirl‘; +----+---------+ | id | name | +----+---------+ | 2 | oldgirl | +----+---------+ 1 row in set (0.00 sec)
#提示：字符类型的查询条件的值要带引号。 mysql> select id,name from test where name=‘oldgirl‘ and id=5; Empty set (0.00 sec) mysql> select id,name from test where name=‘oldgirl‘ or id=5; +----+---------+ | id | name | +----+---------+ | 2 | oldgirl | | 5 | kaka | +----+---------+ 2 rows in set (0.11 sec)
3）指定固定条件范围查数据
执行命令：
mysql> select id,name from test where id>2 and id<4; +----+------+ | id | name | +----+------+ | 3 | inca | +----+------+ 1 row in set (0.11 sec) mysql> select id,name from test where id>2 or id<4; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | | 3 | inca | | 4 | zuma | | 5 | kaka | +----+---------+ 5 rows in set (0.11 sec)
4）其他查询功能
a.排序功能
mysql> select id,name from test order by id; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | | 3 | inca | | 4 | zuma | | 5 | kaka | +----+---------+ 5 rows in set (0.00 sec) mysql> select id,name from test order by id asc; +----+---------+ | id | name | +----+---------+ | 1 | oldboy | | 2 | oldgirl | | 3 | inca | | 4 | zuma | | 5 | kaka | +----+---------+ 5 rows in set (0.00 sec) mysql> select id,name from test order by id desc; +----+---------+ | id | name | +----+---------+ | 5 | kaka | | 4 | zuma | | 3 | inca | | 2 | oldgirl | | 1 | oldboy | +----+---------+ 5 rows in set (0.00 sec)
3、多表查询
1）建立几个关联表
要实现多表连表查询，就需要有关联表及数据，表名及字段内容如下：
================================== ——学生表：Student（Sno，Sname，Ssex，Sage，Sdept） ————————（学号-主键，姓名，性别，年龄，所在系） =================================== drop tables student；
Create Table: CREATE TABLE `student` ( `Sno` int(10) NOT NULL COMMENT ‘学号‘, `Sname` varchar(16) NOT NULL COMMENT ‘姓名‘, `Ssex` char(2) NOT NULL COMMENT ‘性别‘, `Sage` tinyint(2) NOT NULL DEFAULT ‘0‘ COMMENT ‘学生年龄‘, `Sdept` varchar(16) DEFAULT NULL COMMENT ‘学生所在系别‘, PRIMARY KEY (`Sno`), KEY `index_Sname` (`Sname`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 ================================== ——课程表：Course（Cno，Cname，Ccredit） ————（课程号-主键，课程名，学分） =================================== Create Table: CREATE TABLE `course` ( `Cno` int(10) NOT NULL COMMENT ‘课程号‘, `Cname` varchar(64) NOT NULL COMMENT ‘课程名‘, `Ccredit` tinyint(2) NOT NULL COMMENT ‘学分‘, PRIMARY KEY (`Cno`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ============================ ——选课表：SC（Sno，Cno，Grade） ————（学号-主键，课程号-主键，成绩） ————其中SC表主键参照Student，Course表 ============================ Create Table: CREATE TABLE `SC` ( `SCid` int(12) NOT NULL AUTO_INCREMENT COMMENT ‘主键‘, `Cno` int(10) NOT NULL COMMENT ‘课程号‘, `Sno` int(10) NOT NULL COMMENT ‘学号‘, `Grade` tinyint(2) NOT NULL COMMENT ‘学生成绩‘, PRIMARY KEY (`SCid`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8
2）往关联表中填充数据
a.学生表插入数据：
insert into student values(0001,‘宏志‘,‘男‘,‘30‘,‘计算机网络‘); insert into student values(0002,‘王硕‘,‘男‘,‘30‘,‘computer application‘); insert into student values(0003,‘oldboy‘,‘男‘,‘28‘,‘物流管理‘); insert into student values(0004,‘脉动‘,‘男‘,‘29‘,‘computer application‘); insert into student values(0005,‘oldgirl‘,‘女‘,‘26‘,‘计算机科学与技术‘); insert into student values(0006,‘莹莹‘,‘女‘,‘22‘,‘护士‘);
b.课程表插入数据：
insert into course values(1001,‘Linux中高级运维‘,3); insert into course values(1002,‘Linux高级架构师‘,5); insert into course values(1003,‘MySQL高级DBA‘,4); insert into course values(1004,‘Python运维开发‘,4); insert into course values(1005,‘Java web 开发‘,3);
c.选课表插入数据
insert into SC(Sno,Cno,Grade) values(0001,1001,4); insert into SC(Sno,Cno,Grade) values(0001,1002,3); insert into SC(Sno,Cno,Grade) values(0001,1003,1); insert into SC(Sno,Cno,Grade) values(0001,1004,6); insert into SC(Sno,Cno,Grade) values(0002,1001,3); insert into SC(Sno,Cno,Grade) values(0002,1002,2); insert into SC(Sno,Cno,Grade) values(0002,1003,2); insert into SC(Sno,Cno,Grade) values(0002,1004,8); insert into SC(Sno,Cno,Grade) values(0003,1001,4); insert into SC(Sno,Cno,Grade) values(0003,1002,4); insert into SC(Sno,Cno,Grade) values(0003,1003,2); insert into SC(Sno,Cno,Grade) values(0003,1004,8); insert into SC(Sno,Cno,Grade) values(0004,1001,1); insert into SC(Sno,Cno,Grade) values(0004,1002,1); insert into SC(Sno,Cno,Grade) values(0004,1003,2); insert into SC(Sno,Cno,Grade) values(0004,1004,3); insert into SC(Sno,Cno,Grade) values(0005,1001,5); insert into SC(Sno,Cno,Grade) values(0005,1002,3); insert into SC(Sno,Cno,Grade) values(0005,1003,2); insert into SC(Sno,Cno,Grade) values(0005,1004,9);
查看三张表的结果：
mysql> select * from student; +-----+---------+------+------+--------------------------+ | Sno | Sname | Ssex | Sage | Sdept | +-----+---------+------+------+--------------------------+ | 1 | 宏志 | 男 | 30 | 计算机网络 | | 2 | 王硕 | 男 | 30 | computer applica | | 3 | oldboy | 男 | 28 | 物流管理 | | 4 | 脉动 | 男 | 29 | computer applica | | 5 | oldgirl | 女 | 26 | 计算机科学与技术 | | 6 | 莹莹 | 女 | 22 | 护士 | +-----+---------+------+------+--------------------------+ 6 rows in set (0.00 sec) mysql> select * from course; +------+----------------------+---------+ | Cno | Cname | Ccredit | +------+----------------------+---------+ | 1001 | Linux中高级运维 | 3 | | 1002 | Linux高级架构师 | 5 | | 1003 | MySQL高级DBA | 4 | | 1004 | Python运维开发 | 4 | | 1005 | Java web 开发 | 3 | +------+----------------------+---------+ 5 rows in set (0.00 sec) mysql> select * from SC; +------+------+-----+-------+ | SCid | Cno | Sno | Grade | +------+------+-----+-------+ | 1 | 1001 | 1 | 4 | | 2 | 1002 | 1 | 3 | | 3 | 1003 | 1 | 1 | | 4 | 1004 | 1 | 6 | | 5 | 1001 | 2 | 3 | | 6 | 1002 | 2 | 2 | | 7 | 1003 | 2 | 2 | | 8 | 1004 | 2 | 8 | | 9 | 1001 | 3 | 4 | | 10 | 1002 | 3 | 4 | | 11 | 1003 | 3 | 2 | | 12 | 1004 | 3 | 8 | | 13 | 1001 | 4 | 1 | | 14 | 1002 | 4 | 1 | | 15 | 1003 | 4 | 2 | | 16 | 1004 | 4 | 3 | | 17 | 1001 | 5 | 5 | | 18 | 1002 | 5 | 3 | | 19 | 1003 | 5 | 2 | | 20 | 1004 | 5 | 9 | +------+------+-----+-------+ 20 rows in set (0.00 sec)
多表查询练习：
mysql> select student.Sno,student.Sname,course.Cname,SC.Grade from student,course,SC where student.Sno=SC.Sno and course.Cno=SC.Cno; +-----+---------+----------------------+-------+ | Sno | Sname | Cname | Grade | +-----+---------+----------------------+-------+ | 1 | 宏志 | Linux中高级运维 | 4 | | 2 | 王硕 | Linux中高级运维 | 3 | | 3 | oldboy | Linux中高级运维 | 4 | | 4 | 脉动 | Linux中高级运维 | 1 | | 5 | oldgirl | Linux中高级运维 | 5 | | 1 | 宏志 | Linux高级架构师 | 3 | | 2 | 王硕 | Linux高级架构师 | 2 | | 3 | oldboy | Linux高级架构师 | 4 | | 4 | 脉动 | Linux高级架构师 | 1 | | 5 | oldgirl | Linux高级架构师 | 3 | | 1 | 宏志 | MySQL高级DBA | 1 | | 2 | 王硕 | MySQL高级DBA | 2 | | 3 | oldboy | MySQL高级DBA | 2 | | 4 | 脉动 | MySQL高级DBA | 2 | | 5 | oldgirl | MySQL高级DBA | 2 | | 1 | 宏志 | Python运维开发 | 6 | | 2 | 王硕 | Python运维开发 | 8 | | 3 | oldboy | Python运维开发 | 8 | | 4 | 脉动 | Python运维开发 | 3 | | 5 | oldgirl | Python运维开发 | 9 | +-----+---------+----------------------+-------+ 20 rows in set (0.00 sec) 排序： mysql> select student.Sno,student.Sname,course.Cname,SC.Grade from student,course,SC where student.Sno=SC.Sno and course.Cno=SC.Cno order by Sno; +-----+---------+----------------------+-------+ | Sno | Sname | Cname | Grade | +-----+---------+----------------------+-------+ | 1 | 宏志 | Linux中高级运维 | 4 | | 1 | 宏志 | Python运维开发 | 6 | | 1 | 宏志 | MySQL高级DBA | 1 | | 1 | 宏志 | Linux高级架构师 | 3 | | 2 | 王硕 | Linux中高级运维 | 3 | | 2 | 王硕 | Python运维开发 | 8 | | 2 | 王硕 | MySQL高级DBA | 2 | | 2 | 王硕 | Linux高级架构师 | 2 | | 3 | oldboy | Linux中高级运维 | 4 | | 3 | oldboy | Python运维开发 | 8 | | 3 | oldboy | MySQL高级DBA | 2 | | 3 | oldboy | Linux高级架构师 | 4 | | 4 | 脉动 | Python运维开发 | 3 | | 4 | 脉动 | MySQL高级DBA | 2 | | 4 | 脉动 | Linux高级架构师 | 1 | | 4 | 脉动 | Linux中高级运维 | 1 | | 5 | oldgirl | Python运维开发 | 9 | | 5 | oldgirl | MySQL高级DBA | 2 | | 5 | oldgirl | Linux高级架构师 | 3 | | 5 | oldgirl | Linux中高级运维 | 5 | +-----+---------+----------------------+-------+ 20 rows in set (0.00 sec)
4、使用explain查询select查询语句执行计划
即使用索引情况
mysql> show tables; +------------------+ | Tables_in_oldboy | +------------------+ | SC | | course | | student | | test | +------------------+ 4 rows in set (0.00 sec) mysql> desc test; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(4) | NO | PRI | NULL | auto_increment | | name | char(20) | NO | | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.00 sec) mysql> select * from test where name=‘oldboy‘; +----+--------+ | id | name | +----+--------+ | 1 | oldboy | +----+--------+ 1 row in set (0.00 sec) mysql> explain select * from test where name=‘oldboy‘G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: test type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 5 Extra: Using where 1 row in set (0.00 sec) mysql> create index index_name on test(name); Query OK, 0 rows affected (0.63 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc test; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(4) | NO | PRI | NULL | auto_increment | | name | char(20) | NO | MUL | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.00 sec) mysql> explain select * from test where name=‘oldboy‘G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: test type: ref possible_keys: index_name key: index_name key_len: 20 ref: const rows: 1 Extra: Using where; Using index 1 row in set (0.00 sec)
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