MySQL——基础50题

MySql数据库50题

准备工作

参考答案

use PraticeSql;
create table SC(SId varchar(10),CId varchar(10),score decimal(18,1));
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98);

create table Course(CId varchar(10),Cname nvarchar(10),TId varchar(10));
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');

create table Teacher(TId varchar(10),Tname varchar(10));
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');

create table Student(SId varchar(10),Sname varchar(10),Sage datetime,Ssex varchar(10));
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-12-20' , '男');
insert into Student values('04' , '李云' , '1990-12-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-01-01' , '女');
insert into Student values('07' , '郑竹' , '1989-01-01' , '女');
insert into Student values('09' , '张三' , '2017-12-20' , '女');
insert into Student values('10' , '李四' , '2017-12-25' , '女');
insert into Student values('11' , '李四' , '2012-06-06' , '女');
insert into Student values('12' , '赵六' , '2013-06-13' , '女');
insert into Student values('13' , '孙七' , '2014-06-01' , '女');

course表：

sc表：

student表：

teacher表：

关系：

题目

1. 查询" 01 “课程比” 02 "课程成绩高的学生的信息及课程分数

SELECT * FROM
  (SELECT `SId` AS sno1, `CId`AS cno1, score FROM sc WHERE `CId`=01) a
LEFT JOIN
  (SELECT `SId` AS sno2, `CId`AS cno2, score FROM sc WHERE `CId`=02) b
ON a.sno1 = b.sno2
WHERE a.score > b.score

left join：代表选择的是前面一个表的全部。

左连接是以左表为标准，只查询在左边表中存在的数据，当然需要两个表中的键值一致。语法如下：

SELECT 列名1 FROM 表1 LEFT OUTER JOIN 表2 ON 表1.外键=表2.主键 WhERE 条件语句;

1.1 查询同时存在" 01 “课程和” 02 "课程的情况

SELECT * FROM
   (SELECT `SId` AS sno1, `CId`AS cno1, score FROM sc WHERE `CId`=01) a
LEFT JOIN
  (SELECT `SId` AS sno2, `CId`AS cno2, score FROM sc WHERE `CId`=02) b
ON a.sno1 = b.sno2
WHERE sno2 IS NOT NULL;

使用IS NULL运算符

查询包含空值的记录

SELECT last_name, manager_id

FROM employees

WHERE manager_id IS NULL;

1.2 查询存在" 01 “课程但可能不存在” 02 "课程的情况(不存在时显示为 null )

SELECT * FROM
  (SELECT `SId` AS sno1, `CId`AS cno1, score FROM sc WHERE `CId`=01) a
LEFT JOIN
  (SELECT `SId` AS sno2, `CId`AS cno2, score FROM sc WHERE `CId`=02) b
ON a.sno1 = b.sno2;

1.3 查询不存在" 01 “课程但存在” 02 "课程的情况

SELECT *
FROM sc
WHERE `CId`= '02' AND `SId` NOT IN(
    SELECT `SId`
    FROM sc
    WHERE 'CId' = '01'
    )

使用NOT运算符

NOT是取反的意思

SELECT last_name, job_id

FROM employees

WHERE job_id NOT IN (‘CLERK’,‘MANAGER’,‘ANALYST’);

2.查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩

SELECT a.SId,a.avg_score,b.Sname
FROM(
    SELECT `SId`,AVG(score) AS avg_score