MySQL经典面试题

建表语句

CREATE TABLE `t_dept` (
 `id` INT(11) NOT NULL AUTO_INCREMENT,
 `deptName` VARCHAR(30) DEFAULT NULL,
 `address` VARCHAR(40) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
CREATE TABLE `t_emp` (
 `id` INT(11) NOT NULL AUTO_INCREMENT,
 `name` VARCHAR(20) DEFAULT NULL,
  `age` INT(3) DEFAULT NULL,
 `deptId` INT(11) DEFAULT NULL,
empno int  not null,
 PRIMARY KEY (`id`),
 KEY `idx_dept_id` (`deptId`)
 #CONSTRAINT `fk_dept_id` FOREIGN KEY (`deptId`) REFERENCES `t_dept` (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

预置数据

INSERT INTO t_dept(deptName,address) VALUES('华山','华山');
INSERT INTO t_dept(deptName,address) VALUES('丐帮','洛阳');
INSERT INTO t_dept(deptName,address) VALUES('峨眉','峨眉山');
INSERT INTO t_dept(deptName,address) VALUES('武当','武当山');
INSERT INTO t_dept(deptName,address) VALUES('明教','光明顶');
INSERT INTO t_dept(deptName,address) VALUES('少林','少林寺');
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('风清扬',90,1,100001);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('岳不群',50,1,100002);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('令狐冲',24,1,100003);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('洪七公',70,2,100004);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('乔峰',35,2,100005);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('灭绝师太',70,3,100006);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('周芷若',20,3,100007);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('张三丰',100,4,100008);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('张无忌',25,5,100009);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('韦小宝',18,null,100010);
 
ALTER TABLE `t_dept` add  CEO  INT(11)  ;
 
update t_dept set CEO=2 where id=1;
update t_dept set CEO=4 where id=2;
update t_dept set CEO=6 where id=3;
update t_dept set CEO=8 where id=4;
update t_dept set CEO=9 where id=5;

练习题 

1、列出自己的掌门比自己年龄小的人员

SELECT a.`name`,a.`age`,c.`name` ceoname,c.`age` ceoage 
FROM t_emp a
LEFT JOIN t_dept b ON a.`deptId`= b.`id`
LEFT JOIN t_emp c ON b.`CEO`= c.`id`
WHERE c.`age`     
#优化：CREATE INDEX idx_age ON emp(age)；

2、列出所有年龄低于自己门派平均年龄的人员

SELECT c.`name`,c.`age`,aa.avr_age FROM t_emp c INNER JOIN
(
    SELECT a.`deptId`,AVG(a.`age`)avr_age FROM t_emp a
    WHERE a.`deptId` IS NOT NULL
    GROUP BY a.`deptId`
 )aa ON c.`deptId`=aa.deptid 
 WHERE c.`age`< aa.avr_age

#优化 ： CREATE INDEX idx_deptid ON emp(deptid)； CREATE INDEX idx_deptid_age ON emp(deptid,age)

3、列出至少有2个年龄大于40岁的成员的门派

 SELECT b.`deptName`,COUNT(*) FROM t_emp a 
 INNER JOIN t_dept b ON b.`id` = a.`deptId`
 WHERE a.age >40
 GROUP BY b.`deptName`,b.`id` 
 HAVING COUNT(*)>=2
 
 #优化 ： CREATE INDEX  idx_deptid_age ON emp(deptid,age)； CREATE INDEX  idx_deptname ON dept(deptname)；

STRAIGHT_JOIN 强制确定驱动表和被驱动表 1、概念非常明确 2、对数据量的比例非常明确

4、至少有2位非掌门人成员的门派

SELECT * FROM t_emp a WHERE a.id NOT IN
(
 SELECT b.`ceo` FROM t_dept b WHERE b.`ceo`IS NOT NULL
)

NOT IN —>LEFT JOIN xxx ON xx WHERE xx IS NULL

SELECT c.deptname,  c.id,COUNT(*) FROM t_emp a 
INNER JOIN t_dept c ON a.`deptId` =c.`id`
LEFT JOIN t_dept b ON a.`id`=b.`ceo`
WHERE b.`id` IS NULL
GROUP BY c.`id` ,c.deptname
HAVING COUNT(*)>=2

 

#优化 

CREATE INDEX idx_ceo_deptnam ON dept(ceo,deptname)
CREATE INDEX idx_deptnam ON dept(deptname)
CREATE INDEX idx_deptid ON emp(deptid)

SELECT b.`id`,b.`deptName` ,COUNT(*) FROM t_emp a INNER JOIN  t_dept b ON a.`deptId`= b.`id`
GROUP BY b.`deptName`,b.`id`

SELECT b.`id`,b.`deptName`, COUNT(*) FROM emp a INNER JOIN  dept b ON a.`deptId`= b.`id`
GROUP BY b.`deptName`,b.`id`

UPDATE t_dept SET deptname='明教' WHERE id=5

5、列出全部人员，并增加一列备注“是否为掌门”，如果是掌门人显示是，不是掌门人显示否

SELECT  a.`name`, CASE WHEN b.`id` IS NULL THEN '否' ELSE '是' END '是否为掌门'
FROM  t_emp a 
LEFT JOIN t_dept b ON a.`id`=b.`ceo`

6、列出全部门派，并增加一列备注“老鸟or菜鸟”，若门派的平均值年龄>50显示“老鸟”，否则显示“菜鸟”

SELECT b.`deptName`,
IF (AVG(a.age)>50,'老鸟','菜鸟')'老鸟or菜鸟'
FROM t_emp a
INNER JOIN t_dept b ON a.`deptId`= b.`id`
GROUP BY b.`id` ,b.`deptName`

 

7、显示每个门派年龄最大的人

SELECT NAME,age FROM t_emp a
INNER JOIN
(
SELECT deptid,MAX(age) maxage
FROM t_emp
WHERE deptid IS NOT NULL
GROUP BY deptid
) aa ON a.`age`= aa.maxage AND a.`deptId`=aa.deptid

#优化 
EXPLAIN SELECT SQL_NO_CACHE NAME,age FROM emp a
INNER JOIN
(
SELECT deptid,MAX(age) maxage
FROM emp
WHERE deptid IS NOT NULL
GROUP BY deptid
) aa ON a.`age`= aa.maxage AND a.`deptId`=aa.deptid
CREATE INDEX idx_deptid_age ON emp(deptid,age)

#错例
SELECT b.`deptName`,a.`name`,MAX(a.`age`)FROM t_dept b
   LEFT JOIN t_emp a ON b.`id`=a.`deptId`
   WHERE a.name IS NOT NULL
   GROUP BY b.`deptName`

UPDATE t_emp SET age=100 WHERE id =2

8、求所有人物对应的掌门名称答案 

-- No1
SELECT a.name,c.name ceoname FROM t_emp a
INNER JOIN t_dept b ON a.deptId = b.id
INNER JOIN t_emp c ON b.CEO = c.id;

-- No2
SELECT a.name,(SELECT c.name FROM t_emp c WHERE c.id=b.CEO) ceoname 
FROM t_emp a
INNER JOIN t_dept b ON a.`deptId` = b.`id`;

-- No3
SELECT ab.name,c.name ceoname FROM
(SELECT a.name,b.CEO FROM t_emp a
INNER JOIN t_dept b ON a.`deptId` = b.`id`) ab
INNER JOIN t_emp c ON ab.ceo = c.id;

-- No4
SELECT c.name ,ab.ceoname FROM t_emp c 
INNER JOIN
(SELECT a.name ceoname,a.deptId FROM t_emp a 
INNER JOIN t_dept b ON b.CEO = a.id) ab
ON c.deptId = ab.deptId;

 

9、显示每个门派年龄第二大的人

SET @rank=0;
SET @last_deptid=0;
SELECT a.deptid,a.name,a.age
FROM(    
    SELECT t.*,
     IF(@last_deptid=deptid,@rank:=@rank+1,@rank:=1) AS rk,
     @last_deptid:=deptid AS last_deptid
    FROM t_emp t
    ORDER BY deptid,age DESC
    
 )a WHERE a.rk=2;

拓展：分组排序，选取TOPN

方式一：使用变量（旧版本推荐）

SET @rank=0;
SET @last_deptid=0;
SELECT * FROM
(
 SELECT t.*,
     IF(@last_deptid=deptid,@rank:=@rank+1,@rank:=1) AS rk,
     @last_deptid:=deptid AS last_deptid
    FROM t_emp t
    ORDER BY deptid,age DESC
) a WHERE a.rk <=5

方式二：使用自身左链接

SELECT * from (
SELECT a.* 
from t_emp a left  JOIN t_emp b 
on a.deptid= b.deptid and a.age 

方式三：使用子查询

SELECT * from t_emp a
where 5 > (SELECT count(*) from t_emp where a.deptid=deptid and  a.age 

注意，使用子查询数据量一大就玩完。

方式四：使用窗口函数（新版推荐）

SELECT * from (
SELECT *,rank() over (PARTITION by deptid ORDER BY age DESC) as ranking
from t_emp ) b
where b.ranking<=5;

如果表有重复，只能用dense_rank() 和rank（）窗口函数在mysql必知必会中没有涉及，因为mysql8.0才开始支持。（执行效率非常高）