Mysql常见的面试总结

Love The Way You Lie 2023-06-08 03:24 2阅读 0赞

# 第一题 #

CREATE TABLE `table1` (
      `name` VARCHAR(20) DEFAULT NULL,
      `kecheng` VARCHAR(20) DEFAULT NULL,
      `fenshu` INT(10) DEFAULT NULL
    );
    
    
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('张三','语文',81);
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('张三','数学',75);
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('李四','语文',76);
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('李四','数学',76);
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('王五','语文',81);
    INSERT INTO table1(NAME,kecheng,fenshu) VALUES('王五','数学',100);

***\#求出每门课都大于80分的学生姓名***

**第一种方法:**

SELECT NAME FROM table1 GROUP BY NAME 
    HAVING MIN(fenshu)>80

**第二种方法:**

SELECT NAME FROM table1 GROUP BY NAME 
    HAVING COUNT(1)=SUM(CASE WHEN fenshu>80 THEN 1 ELSE 0 END)

**第三种方法:**

SELECT NAME FROM table1 GROUP BY NAME 
    HAVING NAME NOT IN (SELECT NAME FROM table1 WHERE fenshu<=80)

# 第二题 #

CREATE TABLE `table2` (
      `语文` INT(10) DEFAULT NULL,
      `数学` INT(10) DEFAULT NULL,
      `英语` INT(10) DEFAULT NULL
    );
    INSERT INTO table2(语文,数学,英语) VALUES(58,70,80);

**有一张表,3个字段是语文,数学,英语, 有1条记录分别表示语文68,数学70,英语80, 得出结果分数变等级(>=80分是优秀, >=60分是及格, <60是不及格)**

SELECT CASE WHEN 语文>=80 THEN '优秀' WHEN 语文>=60 THEN '及格' WHEN 语文<60 THEN '不及格' END 语文,
    CASE WHEN 数学>=80 THEN '优秀' WHEN 数学>=60 THEN '及格' WHEN 数学<60 THEN '不及格' END 数学,
    CASE WHEN 英语>=80 THEN '优秀' WHEN 英语>=60 THEN '及格' WHEN 英语<60 THEN '不及格' END 英语 
    FROM table2

# 第三题 #

CREATE TABLE `table3` (
      `date` VARCHAR(20) DEFAULT NULL,
      `result` VARCHAR(10) DEFAULT NULL
    )

***\#查询出来的结果：**  
\#date 胜 负  
\#2011-02-01 2 1  
\#2011-02-02 1 1*

SELECT DATE, 
    (SELECT COUNT(1) FROM table3 WHERE DATE=t.date AND result='胜') AS 胜, 
    (SELECT COUNT(1) FROM table3 WHERE DATE=t.date AND result='负') AS 负
    FROM table3 t GROUP BY DATE

# 第四题 #

**\#1 2005001 张三 0001 数学 69  
\#2 2005002 李四 0001 数学 89  
\#3 2005001 张三 0001 数学 69**

CREATE TABLE IF NOT EXISTS `student_test` (
    	id INT UNSIGNED AUTO_INCREMENT,
    	stu_id VARCHAR(10) NULL,
    	NAME VARCHAR(10) NULL,
    	couser_id VARCHAR(10) NULL,
    	couser VARCHAR(10) NULL,
    	score DECIMAL(10,2) NULL,
    	PRIMARY KEY (id) 
    )
    INSERT INTO student_test(id,stu_id,NAME,couser_id,couser,score) VALUES(NULL,'2005001','张三','0001','数学',69);
    INSERT INTO student_test(id,stu_id,NAME,couser_id,couser,score) VALUES(NULL,'2005002','李四','0001','数学',89);
    INSERT INTO student_test(id,stu_id,NAME,couser_id,couser,score) VALUES(NULL,'2005001','张三','0001','数学',69);

**\#删除冗余字段, 保留ID最小的那个**

DELETE FROM student_test WHERE id NOT IN (
     SELECT * FROM (
      SELECT MIN(id) FROM student_test GROUP BY stu_id,NAME,couser_id,couser,score
     ) s
    )

# 第五题 #

CREATE TABLE IF NOT EXISTS `product` (
    	id INT UNSIGNED AUTO_INCREMENT,
    	NAME VARCHAR(10) NULL,
    	STATUS VARCHAR(10) NULL,
    	PRIMARY KEY (id)
    )
    INSERT INTO product(id,NAME,STATUS) VALUES(NULL,'苹果','好吃');
    INSERT INTO product(id,NAME,STATUS) VALUES(NULL,'梨','难吃');
    INSERT INTO product(id,NAME,STATUS) VALUES(NULL,'橘子','好吃');
    INSERT INTO product(id,NAME,STATUS) VALUES(NULL,'葡萄',NULL);

**\#现在需要把status的‘好吃’更新为‘0’，‘难吃’更新为‘1’**

UPDATE product SET STATUS=(CASE WHEN STATUS='好吃' THEN '0' WHEN STATUS='难吃' THEN '1' END)
    1

# 第六题 #

**1) 创建一张学生表，包含以下信息，学号，姓名，年龄，性别，家庭住址，联系电话**

CREATE TABLE IF NOT EXISTS `t_student` (
    	stu_id INT UNSIGNED AUTO_INCREMENT,
    	NAME VARCHAR(10) NULL,
    	age INT NULL,
    	gender VARCHAR(2) NULL,
    	address VARCHAR(50) NULL,
    	phone VARCHAR(11) NULL,
    	PRIMARY KEY (stu_id)
    );

**\#2) 修改学生表的结构，添加一列信息，学历**

ALTER TABLE t_student ADD COLUMN `education` VARCHAR(20) NOT NULL;

**\#3) 修改学生表的结构，删除一列信息，家庭住址**

ALTER TABLE t_student DROP COLUMN address;

**\#4) 向学生表添加如下信息：**

INSERT INTO t_student(stu_id,NAME,age,gender,phone,education) VALUES(NULL,'A',22,'男','123456','小学');
    INSERT INTO t_student(stu_id,NAME,age,gender,phone,education) VALUES(NULL,'B',21,'男','119','中学');
    INSERT INTO t_student(stu_id,NAME,age,gender,phone,education) VALUES(NULL,'C',23,'男','110','高中');
    INSERT INTO t_student(stu_id,NAME,age,gender,phone,education) VALUES(NULL,'D',18,'女','114','大学');

**\#5) 修改学生表的数据，将电话号码以11开头的学员的学历改为“大专”**

UPDATE t_student SET education='大专' WHERE phone LIKE '11%';

**\#6) 删除学生表的数据，姓名以C开头，性别为‘男’的记录删除**

DELETE FROM t_student WHERE NAME LIKE 'C%' AND gender='男';

**\#7) 查询学生表的数据，将所有年龄小于22岁的，学历为“大专”的，学生的姓名和学号示出来**

SELECT stu_id,NAME FROM t_student WHERE age<22 AND education='大专'

**\#8) 查询学生表的数据，查询所有信息，列出前25%的记录**

SELECT * FROM t_student WHERE stu_id<=(SELECT COUNT(*) FROM t_student)*0.25;

**\#9) 查询出所有学生的姓名，性别，年龄降序排列**

SELECT NAME,gender,age FROM t_student ORDER BY age DESC;

**\#10) 按照性别分组查询所有的平均年龄**

SELECT gender,AVG(age) FROM t_student GROUP BY gender;

**第七题**

CREATE TABLE IF NOT EXISTS test1(
    	a DECIMAL(10,2) NOT NULL,
    	b DECIMAL(10,2) NOT NULL,
    	c DECIMAL(10,2) NOT NULL,
    	d VARCHAR(10) NOT NULL
    )
    INSERT INTO test1(a,b,c,d) VALUES(0.5,1.5,2.0,'A1');
    INSERT INTO test1(a,b,c,d) VALUES(1.5,0.5,0.5,'A1');
    INSERT INTO test1(a,b,c,d) VALUES(0.5,0.5,1.5,'A1');
    INSERT INTO test1(a,b,c,d) VALUES(1.5,1.5,1.5,'B1');
    INSERT INTO test1(a,b,c,d) VALUES(0.5,2.0,2.0,'B1');
    INSERT INTO test1(a,b,c,d) VALUES(1.5,2.0,0.5,'B1');

**统计a,b,c三列大于1的个数**

SELECT 
    SUM(CASE WHEN a > 1 THEN 1 ELSE 0 END) a, 
    SUM(CASE WHEN b > 1 THEN 1 ELSE 0 END) b, 
    SUM(CASE WHEN c > 1 THEN 1 ELSE 0 END) c 
    FROM test1;

**按a分组, 用b排序取top1**  
**其实用hive或者Oracle很简单了, 直接用row\_number即可**  
**方法一**

SELECT test1.* 
    FROM test1 
    JOIN (SELECT a,MAX(b) maxb FROM test1 GROUP BY a) t 
    ON test1.b=t.maxb 
    GROUP BY a

**方法二**

SELECT a, SUBSTRING_INDEX(GROUP_CONCAT(b ORDER BY b DESC),',',1) b, 
    SUBSTRING_INDEX(GROUP_CONCAT(c ORDER BY b DESC),',',1) c, 
    SUBSTRING_INDEX(GROUP_CONCAT(d ORDER BY b DESC),',',1) d 
    FROM test1 GROUP BY a;

**用到了下面的两个函数:**

> group\_concat(v)

**先看看它的语法:**

> group\_concat(\[DISTINCT\] 要连接的字段 \[ORDER BY ASC/DESC 排序字段\] \[SEPARATOR  
> ‘分隔符’\])

**举例:**

select GROUP_CONCAT(c ORDER BY b DESC) from test1 group a
    #解析: 按a进行分组, 把c字段的值打印在一起, 逗号分割(默认), 并且c字段按b排序

> substring\_index(v1, v2, v3) – 将v1按照v2进行分割取下标为v3的值

**从表中随机取出3条数据**  
**下面这种方法虽然可以取出随机记录, 但是不推荐, 数据量大的话会很慢很慢**

SELECT * FROM test1 ORDER BY RAND() LIMIT 3

可以使用下面这种方法

SELECT * 
    FROM (SELECT test1.*, RAND() r FROM test1) t 
    ORDER BY t.r LIMIT 3

**第八题**  
**Table表有三个字段ID,Name,Location, 请以两种方式写出SQL语句删除表中ID以及Name都重复的记录.**

CREATE TABLE table4(
      ID INT NOT NULL,
      NAME VARCHAR(10) NOT NULL,
      Location VARCHAR(12) NOT NULL
    );
    
    INSERT INTO table4(ID,NAME,Location) VALUES(1,'zhangsan','bj');
    INSERT INTO table4(ID,NAME,Location) VALUES(1,'zhangsan','bj');
    INSERT INTO table4(ID,NAME,Location) VALUES(2,'lisi','bj');
    INSERT INTO table4(ID,NAME,Location) VALUES(3,'wangwu','sh');

**第一种方法**

DELETE FROM table4 WHERE (id,NAME) IN (
    SELECT * FROM (SELECT id,NAME FROM table4 GROUP BY ID,NAME HAVING COUNT(1)>1) t)

**第二种方法**

DELETE FROM table4 WHERE (id,NAME) IN (
    SELECT * FROM(
    SELECT id,NAME FROM table4 t1 WHERE (
      (SELECT COUNT(1) FROM table4 t2 WHERE t1.ID=t2.ID AND t1.Name=t2.Name)>1
    )
    ) tmp)

# 第九题 #

**现有BOOKS表, 查询所有图书中价格小于60元的作者有多少本图书在售**

CREATE TABLE books(
      bno INT NOT NULL,
      bname VARCHAR(10) NOT NULL,
      author VARCHAR(10) NOT NULL,
      price DECIMAL(10,2) NOT NULL
    )
    INSERT INTO books VALUES(1,'倚天屠龙记','金庸',12.99);
    INSERT INTO books VALUES(2,'雪山飞狐','金庸',64.99);
    INSERT INTO books VALUES(3,'东方不败','金庸',12.99);
    INSERT INTO books VALUES(4,'仙逆','耳根',72.99);
    INSERT INTO books VALUES(5,'求魔','耳根',12.99);

**答案:**  
**使用子查询查出图书价格小于60的作者有哪些, 然后按照作者分组, 查询这些作者的图书数量.**

SELECT author,COUNT(1) FROM books GROUP BY author HAVING author IN 
    (SELECT DISTINCT(author) FROM books WHERE price<60)

# 第十题 #

**表中有A,B,C三列, 用SQL语句实现, 依次取A,B,C中第一个不为空的值**

CREATE TABLE table_null(
      A VARCHAR(10),
      B VARCHAR(10),
      C VARCHAR(10)
    )
    
    INSERT INTO table_null VALUES(NULL,NULL,NULL);
    INSERT INTO table_null VALUES('abc',NULL,NULL);
    INSERT INTO table_null VALUES(NULL,'ccc',NULL);
    INSERT INTO table_null VALUES(NULL,NULL,'ddd');

**答案:**  
**分别按照每一个字段分组, 所有为NUll的分为一组, 然后取不为null的第一条即可.**

SELECT * FROM 
    (SELECT A FROM table_null GROUP BY A LIMIT 1,1) t1 JOIN
    (SELECT B FROM table_null GROUP BY B LIMIT 1,1) t2 JOIN
    (SELECT C FROM table_null GROUP BY C LIMIT 1,1) t3

# 第十一题 #

**有一张表stu\_score, 所有课程都>=90则特优, 所有课程都>=80则优秀, 都>=60则及格, 否则挂科.**

CREATE TABLE stu_score(
      kecheng VARCHAR(10),
      stu_id INT,
      score INT
    )
    INSERT INTO stu_score VALUES('001',1,90);
    INSERT INTO stu_score VALUES('002',1,92);
    INSERT INTO stu_score VALUES('001',2,80);
    INSERT INTO stu_score VALUES('002',2,92);
    INSERT INTO stu_score VALUES('001',3,76);
    INSERT INTO stu_score VALUES('002',3,92);
    INSERT INTO stu_score VALUES('001',4,50);
    INSERT INTO stu_score VALUES('002',4,92);

**答案:**  
**按照学号分组, 使用 CASE WHEN 判断最小分数 MIN(score) 是否满足条件.**

SELECT 
       stu_id, (CASE WHEN MIN(score)>=90 THEN '特优' WHEN MIN(score)>=80 THEN '优秀' WHEN MIN(score)>=60 THEN '及格' ELSE '挂科' END) stat
    FROM stu_score GROUP BY stu_id

# 第十二题 #

**运用SQL实现数据转换  
转换前:**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNDEyMjg5_size_16_color_FFFFFF_t_70]  
**转换后:**  
![在这里插入图片描述][20191016092057525.png]

CREATE TABLE test33(
      grade_id INT,
      subject_name VARCHAR(10),
      max_score INT
    )
    INSERT INTO test33 VALUES(1,'语文',98);
    INSERT INTO test33 VALUES(2,'数学',95);
    INSERT INTO test33 VALUES(2,'政治',87);
    INSERT INTO test33 VALUES(5,'语文',97);
    INSERT INTO test33 VALUES(5,'数学',100);
    INSERT INTO test33 VALUES(5,'政治',92);

**答案:**

SELECT 年级,
    SUM(CASE WHEN  语文 IS NULL THEN 0 ELSE 语文 END) 语文,
    SUM(CASE WHEN  数学 IS NULL THEN 0 ELSE 数学 END) 数学,
    SUM(CASE WHEN  政治 IS NULL THEN 0 ELSE 政治 END) 政治
    FROM
    (
    (SELECT 
    (CASE grade_id WHEN 1 THEN '一年级' WHEN 2 THEN '二年级' WHEN 5 THEN '五年级' END) 年级,
    (CASE subject_name WHEN '语文' THEN max_score ELSE NULL END) 语文,
    (CASE subject_name WHEN '数学' THEN max_score ELSE NULL END) 数学,
    (CASE subject_name WHEN '政治' THEN max_score ELSE NULL END) 政治
    FROM test33
    )  UNION (
    SELECT 
    (CASE grade_id WHEN 1 THEN '一年级' WHEN 2 THEN '二年级' WHEN 5 THEN '五年级' END) 年级,
    (CASE subject_name WHEN '语文' THEN max_score ELSE NULL END) 语文,
    (CASE subject_name WHEN '数学' THEN max_score ELSE NULL END) 数学,
    (CASE subject_name WHEN '政治' THEN max_score ELSE NULL END) 政治
    FROM test33
    )  UNION (
    SELECT 
    (CASE grade_id WHEN 1 THEN '一年级' WHEN 2 THEN '二年级' WHEN 5 THEN '五年级' END) 年级,
    (CASE subject_name WHEN '语文' THEN max_score ELSE NULL END) 语文,
    (CASE subject_name WHEN '数学' THEN max_score ELSE NULL END) 数学,
    (CASE subject_name WHEN '政治' THEN max_score ELSE NULL END) 政治
    FROM test33
    )
    ) t1 GROUP BY 年级

# 第十三题 #

**A表与B表结构相同, 现在想用B表的数据更新A表的用户姓名, 存在就更新, 不存在就添加.  
表A**

CREATE TABLE A(
      user_id VARCHAR(10),
      user_name VARCHAR(20)
    )
    CREATE TABLE B(
      user_id VARCHAR(10),
      user_name VARCHAR(20)
    )

**答案:**  
**mysql有这样的语法: ON DUPLICATE KEY UPDATE 对唯一索引或主键索引可以实现更新.**

**\#给B表添加唯一索引**

ALTER TABLE B ADD UNIQUE KEY user_name(user_name)

**\#存在相同的user\_name就执行UPDATE**

INSERT INTO B (user_id,user_name) SELECT user_id,user_name FROM A 
    ON DUPLICATE KEY UPDATE user_id=VALUES(user_id),user_name=VALUES(user_name)

**下面列几个常见的Hive必考HQL  
\#1. 级联求和问题 \#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#**

select a.username, a.month, max(a.salSum), sum(b.salSum) as salSum
    from 
      (select username, month, sum(salary) as salSum from t_salary_detail group by username,month) a
      join
      (select username, month, sum(salary) as salSum from t_salary_detail group by username,month) b
      on a.username=b.username
    where a.month>=b.month
    group by a.username,a.month;

**\#2. 连续3个月都有销售额的商家 \#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#**

select shopid,count(flag) as cnt
    from 
      (select shopid,dt,sale,rn,date_sub(to_date(dt),rn) as flag 
       from 
         (select shopid,dt,sale,
            row_number() over(partition by shopid order by dt) as rn 
          from t_jd
         ) tmp
      ) tmp2
    group by shopid,flag having cnt>=3

**\#3. 求TOPN问题 \#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#**

--统计每日最热门页面的top10
    select month,day,request,request_count, od 
    from 
     (select 
     	month,day,request,request_count,
        row_number() over(distribute by concat(month,day) sort by request_count desc) as od
      from 
        (select month,day,request,count(1) as request_count 
         from ods_weblog_detail
         where datestr='20130918' 
         group by request,month,day having request is not null
        ) b
     ) c where od<=10

**\#4. 行转列, 列转行问题 \#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#**

**–当前有用户人生阶段表lifeStage, 有用户唯一ID字段uid,用户人生阶段字段stage,其中stage字段内容为各个人生阶段标签按照英文逗号分割的拼接内容,  
–如:计划买车,已买房, 并且每个用户的内容不同, 请使用hive sql统计每个人生阶段的用户量.  
–考察点: lateral view使用, explode函数**

select stage_someone,count(distinct uid) as uids
    from lifeStage
      lateral view explode(split(stage,',')) lifeStage_tmp as stage_someone
    group by stage_someone;

**–和上一题相同的数据场景, 但是lifeStage中每行数据存储一个用户的人生阶段数据  
–如: 一行数据uid是43, stage内容为计划买车, 另一行数据uid字段为43,stage字段为已买房, 请输出类似于uid为43,stage字段为计划买车,已买房这样的新整合数据.  
–考察点: collect\_set函数, concat\_ws函数**

select uid,concat_ws(',',collect_set(stage_someone)) as stage
    from lifestage_multline
    group by uid

**\#5. 多表关联的基础上, 行转列与列转行问题 \#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#\#**

**–权限组表(g)中记录了组id和组名称，用户表(u)记录了用户id和用户名称，权限表(gu)是权限组表和用户表的关系，记录了每一个权限组与用户的对应关系  
–题目是根据gt和ut表将gu表中的所有id转换为名称**

create table gu_trans as
    select t.gname as gname,concat_ws(',',collect_set(t.uname)) as uname
    from
     (select g.gname,u.uname
      from
       (select gid,s_uid
       from gu lateral view explode(split(uid,',')) b as s_uid) tmp 
       join g on tmp.gid=g.gid
       join u on tmp.s_uid=u.uid
     ) t
    group by t.gname

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[20191016092057525.png]: /images/20230531/1a8801e6a9604831ae5c465eeff4f887.png