2020年MySQL数据库常见的面试题【乐搏TestPRO】

• 数据库常见问答题

1. 什么情况下用group by?

数据分组，需求里常见关键字：各，不同…

1. Where和having有什么区别？

1. Where在group by之前，having在group by之后
2. Where针对select语句(原数据表)的结果进行筛选的。Having是针对分组后的数据(虚拟表)进行筛选。

1. 内连接和外连接有什么区别？左外连接和右外连接有什么区别

内连接：是多表的交集部分

外连接：通过外连接关键字指定的部分

左外/右外连接：（1）左外使用left outer join.. on  右外使用right outer join…on

（2）左外：以关键字左表为主，左表的字段全部显示，右外相反

 

1. 模糊查询用什么关键字？通配符%和_有什么区别？

Like %多个任意数据，_一个任意数据

1. 运算符in和between…and有什么区别？

1. In指定的数据都需要写在括号里。between…and把数据范围边界作为参数。
2. In指定数据没有数据类型限制。between…and只能接Int, float, date类型数据。

1. 删除操作drop和delete有什么区别？

Drop 一般用于删除数据库和数据表 ，是删除表结构 ，delete用于删除表数据内容

1. 表的主键有什么作用？有什么特征？

主键用于指示数据的唯一性。特征：唯一、非空

 

1. 分页查询如何操作，比如查询表的2~7行数据？

Limit ，limit 1, 6

1. count(*) 和count(字段)有什么区别？

Count（*） 针对非空或空数据都做计数。Count（字段）针对该字段非空的数据。

1. 怎么做数据排序操作？

Order by关键字+排序的字段，asc升序(默认)， desc降序

 

• 数据库常见实操题

---------------------------------day1---------------------------------

#1.统计项目表中项目名称以“管理”开头的记录一共多少条

SELECT COUNT(*) FROM project p WHERE p.pname LIKE '管理%';

#2.查询出项目表中code内容长度大于11的所有记录

SELECT * FROM project p WHERE lengh(p.`porg_code`)>11;

#3.统计项目人员信息表中一共有多少条记录

SELECT COUNT(*) FROM project_user;

#4.统计项目人员信息表中一共有多少员工

SELECT COUNT(uname) FROM project_user;

#5.查询出哪个项目的参与人员数量最多

SELECT MAX(c.sers)，p,name FROM

(SELECT p.name AS proname,COUNT(u.uer_id) AS users FROM project p INNER JOIN project_user u ON u.project_id=p.id AND u.type=3 GROUP BY u.uer_id) c;

 

---------------------------------day2---------------------------------

#学生表

CREATE TABLE Students(

  Sid INT PRIMARY KEY, #学生学号

  Sname VARCHAR(10) NOT NULL, #学生姓名

  Sage INT,  #学生年龄

  Ssex NVARCHAR(5) #学生性别

) CHARSET utf8;

 

#教师表

CREATE TABLE Teacher(

  Tid INT PRIMARY KEY,  #教师号

  Tname VARCHAR(10) NOT NULL  #教师名字

) CHARSET utf8;

 

#课程表

CREATE TABLE Course(

  Cid INT PRIMARY KEY, #课程ID

  Cname VARCHAR(15), #课程名称

  Cteacherid INT,  #教师ID

  CONSTRAINT Course_fk FOREIGN KEY (Cteacherid) REFERENCES Teacher(Tid)

) CHARSET utf8;

 

#成绩表

CREATE TABLE Record(

  Rstuid INT NOT NULL, #学生学号

  Rcouid INT NOT NULL, #课程ID

  Rmark INT,  #分数

  CONSTRAINT RecordStu_fk FOREIGN KEY (Rstuid) REFERENCES Student(Sid),

  CONSTRAINT RecordCou_fk FOREIGN KEY (Rcouid) REFERENCES Course(Cid)

) CHARSET utf8;

 

#4个表数据插入

INSERT INTO Students VALUES(001,"张三",20,"男");

INSERT INTO Students VALUES(002,"李四",24,"男");

INSERT INTO Students VALUES(003,"王五",24,"女");

INSERT INTO Students VALUES(004,"赵六",22,"男");

INSERT INTO Students VALUES(005,"孙七",21,"女");

INSERT INTO Students VALUES(006,"钱八",26,"男");

INSERT INTO Students VALUES(007,"吴用",27,"女");

SELECT * FROM Students;

 

INSERT INTO Teacher VALUES(1,"乐老师");

INSERT INTO Teacher VALUES(2,"郭老师");

INSERT INTO Teacher VALUES(3,"小美老师");

INSERT INTO Teacher VALUES(4,"贺老师");

SELECT * FROM Teacher;

 

INSERT INTO Course VALUES(1,"Python",1);

INSERT INTO Course VALUES(2,"MySQL",2);

INSERT INTO Course VALUES(3,"Java",3);

INSERT INTO Course VALUES(4,"Linux",4);

SELECT * FROM Course;

 

INSERT INTO Record VALUES(1,1,80);

INSERT INTO Record VALUES(1,2,90);

INSERT INTO Record VALUES(1,3,70);

INSERT INTO Record VALUES(1,4,78);

 

INSERT INTO Record VALUES(2,1,70);

INSERT INTO Record VALUES(2,2,95);

INSERT INTO Record VALUES(2,3,60);

INSERT INTO Record VALUES(2,4,75);

 

INSERT INTO Record VALUES(3,1,90);

INSERT INTO Record VALUES(3,2,85);

INSERT INTO Record VALUES(3,3,70);

INSERT INTO Record VALUES(3,4,85);

 

INSERT INTO Record VALUES(4,1,70);

INSERT INTO Record VALUES(4,2,40);

INSERT INTO Record VALUES(4,3,50);

INSERT INTO Record VALUES(4,4,55);

 

INSERT INTO Record VALUES(5,1,80);

SELECT * FROM Record;

 

SELECT * FROM Student;

SELECT * FROM Teacher;

SELECT * FROM Course;  #课程表

SELECT * FROM Record; #成绩表

#查询平均成绩大于60分的同学的学号和平均成绩

SELECT r.`Rstuid` '学号',AVG(r.`Rmark`) '平均成绩'

FROM Record r GROUP BY r.`Rstuid` HAVING AVG(r.`Rmark`)>60;

 

#按各科平均成绩从低到高和及格率的百分数从高到低顺序查询

SELECT r.`Rcouid`,AVG(r.Rmark) avgrmark,

SUM(r.Rmark>=60)/COUNT(r.Rstuid) AS percent

FROM Record r

GROUP BY r.`Rcouid` ORDER BY avgrmark ASC, percent DESC;

 

#查询语文课程成绩在70分以上的学生姓名、课程名称和分数

SELECT s.`sname`,c.`Cname`,r.`Rmark`

FROM Student s INNER JOIN Course c INNER JOIN Record r

ON s.`id`=r.`Rstuid` AND c.`Cid`=r.`Rcouid`

AND c.`Cname`='语文' AND r.`Rmark`>70;

 

#删除002同学的001课程的成绩

DELETE FROM Record WHERE Rstuid='002' AND Rcouid='001';

 

#增加一条数据：学生为张三报考教师王老师数学学课程的考试成绩

#方法一：

SET @sid = (SELECT sid FROM Students WHERE sname='张三');

SET @tname = (SELECT t.tid FROM Teacher t,Course c

WHERE t.tname='贺老师' AND c.cname='MySQL'

AND c.Cteacherid=t.Tid);

INSERT INTO Record VALUES (@sid,@tname,99);

 

#方法二：

INSERT INTO Record VALUES

(SELECT sid FROM Students WHERE sname='张三',

SELECT t.tid FROM Teacher t,Course c

WHERE t.tname='贺老师' AND c.cname='MySQL'

AND c.Cteacherid=t.Tid,99);

 

---------------------------------day3---------------------------------

CREATE DATABASE information;

CREATE TABLE inf

(

iname VARCHAR(4),-- 姓名

applyno VARCHAR(4) PRIMARY KEY,-- 申请号

tel VARCHAR(11),-- 手机号

inf_id INT-- 证件号

)CHARSET utf8;

 

INSERT INTO inf VALUES

('张三',1001,'A001',1370755),

('李四',1004,'A002',1370756);

INSERT INTO inf VALUES

('陈六',1006,'A004',1320761),

('何七',1007,'A005',1340752),

('李四',1003,'A006',1370757),

('王五',1005,'A007',1370758);

 

SELECT * FROM inf;

 

CREATE TABLE apply

(

applyno VARCHAR(4),-- 申请号

custno VARCHAR(4),-- 客户号

aprocess VARCHAR(10),-- 申请过程

accto VARCHAR(5),-- 账户号

CONSTRAINT apply_fk FOREIGN KEY (applyno) REFERENCES inf(applyno)-- 外键必须是另一个表的主键或者唯一键;外键的字段类型必须和主键的字段类型一致

)CHARSET utf8;

INSERT INTO apply VALUES

('1001','C003','pass','TT003'),

('A002','C004','pass','TT004'),

('A003','C003','reject','TT003'),

('A004','C006','pass','TT006'),

('A005','C007','reject','TT007'),

('A006','C004','running','TT004'),

('A007','C005','running','TT005');

SELECT * FROM apply;

 

CREATE TABLE trans

(

custno VARCHAR(4),-- 客户号

amt INT,-- 交易金额

t_status VARCHAR(10),-- 状态

t_time DATETIME-- 时间

)CHARSET utf8;

 

INSERT INTO trans VALUES

('C003',500,'S00','2016-3-1 10:50');

INSERT INTO trans VALUES

('C004',400,'S00','2016-4-8 14:32'),

('C003',700,'F01','2016-4-9 12:41'),

('C006',214,'E23','2016-4-9 13:20'),

('C007',339,'F01','2015-12-21 19:52'),

('C004',780,'E99','2016-4-6 16:22'),

('C005',1100,'S00','2016-4-8 16:24');

SELECT * FROM trans;

 

-- 1.查询A表手机号不为空的所有信息

SELECT* FROM inf WHERE tel<>'' OR tel IS NOT NULL;

 

-- 2.查询张三这个客户发起的交易总额

#写法一: 内连接

SELECT i.`iname`,SUM(t.`amt`) '交易总额' FROM trans t, inf i,apply a

WHERE a.`applyno`=i.`applyno` AND t.`custno`=a.`custno` AND i.`iname`='张三';

 

#写法二：子查询

SELECT i.`iname`,SUM(amt) '交易总额' FROM trans WHERE custno=

(SELECT DISTINCT custno FROM inf,apply WHERE inf.`applyno`=apply.`applyno` AND iname='张三');

 

-- 3.查询手机号是137开头的人的姓名、申请号和客户号

SELECT i.iname '姓名',i.`applyno` '申请号',a.custno '客户号'

FROM inf i,apply a

WHERE i.`applyno`=a.`applyno` AND tel LIKE '137%';

 

-- 4.查询交易金额小于1000的数据，并按时间降序排列

SELECT * FROM trans WHERE amt<1000 ORDER BY t_time DESC;

 

-- 5.把C表中状态为E99和E23的改为S00

UPDATE trans SET t_status='S00'

WHERE t_status IN('E99','E23');

 

------------------------day4-----------------------

CREATE TABLE Students

(

  id INT,

  sno VARCHAR(10) PRIMARY KEY,

  sname VARCHAR(20),

  sage INT,

  ssex VARCHAR(3)

) CHARSET=utf8;

CREATE TABLE Mark

(

  id INT PRIMARY KEY,

  Sno_M VARCHAR(10),

  Cname VARCHAR(10),

  Scroe INT,

  CONSTRAINT mark_fk FOREIGN KEY (Sno_M) REFERENCES Studentss (sno)

) CHARSET=utf8;

 

INSERT INTO Students VALUES(1,"SA001","张三",16,"男");

INSERT INTO Students VALUES(2,"SA002","李四",15,"女");

INSERT INTO Students VALUES(3,"SA003","王二",14,"男");

 

INSERT INTO Mark VALUES(1,"SA001","语文",96);

INSERT INTO Mark VALUES(2,"SA002","数学",89);

INSERT INTO Mark VALUES(3,"SA003","英语",90);

INSERT INTO Mark VALUES(4,"SA002","语文",88);

INSERT INTO Mark VALUES(5,"SA001","数学",89);

INSERT INTO Mark VALUES(6,"SA003","语文",78);

 

#查询平均成绩大于60分的同学的学号和平均成绩；

SELECT m.Sno_M,AVG(m.scroe) FROM mark m GROUP BY m.Sno_M HAVING AVG(m.scroe)>60

#查询平均成绩大于60分的同学的姓名和平均成绩；

SELECT s.Sname,AVG(m.scroe) FROM student s,mark m  WHERE s.sno=m.Sno_M

GROUP BY m.Sno_M HAVING AVG(m.scroe)>60

 

#查询”语文”比”数学”高的所有学生的学号

SELECT scroe1.Sno_M FROM

(SELECT * FROM mark WHERE Cname="语文") scroe1,

(SELECT * FROM mark WHERE Cname="数学") scroe2

WHERE scroe1.Scroe>scroe2.Scroe AND scroe1.Sno_M=scroe2.Sno_M

 

#查询”语文”比”数学”高的所有学生的姓名

SELECT s.sname,scroe1.Sno_M FROM

student s,

(SELECT * FROM mark WHERE Cname="语文") scroe1,

(SELECT * FROM mark WHERE Cname="数学") scroe2

WHERE scroe1.Scroe>scroe2.Scroe AND scroe1.Sno_M=scroe2.Sno_M

AND s.sno=scroe1.Sno_M

 

-----------------------day5------------------------

SELECT * FROM `students`;

SELECT * FROM `teacher`;

SELECT * FROM `course`;

 

#查询学生表每个年纪有多少人，并按年级人数从多到少排序，查询出的列名为年级人数

SELECT grade,COUNT(*) 'nums' FROM students GROUP BY grade ORDER BY nums DESC;

#查询每门课程对应的年级和对应的教师，对于没有老师教的课程，也需要展示出来

SELECT c.`Cname`,t.`Tname`,c.`grade` FROM course c LEFT JOIN teacher t

ON c.`Cteacherid`=t.`Tid`;

 

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