1.删除除了学号字段以外，其它字段都相同的冗余记录，只保留一条！（也就是要删除王五和赵六中一条重复数据只留一条）
要求的预期效果:
原始数据创建表结构：
CREATE TABLE tb_student (
id int(16) NOT NULL,
name varchar(10) DEFAULT NULL,
sex varchar(10) DEFAULT NULL,
age char(2) DEFAULT NULL,
PRIMARY KEY (id)
)插入数据：
insert into tb_student (id, name, sex, age) values('2','李四','男','21');
insert into tb_student (id, name, sex, age) values('3','张三','女','17');
insert into tb_student (id, name, sex, age) values('4','李四','男','12');
insert into tb_student (id, name, sex, age) values('6','王五','女','20');
insert into tb_student (id, name, sex, age) values('5','王五','女','20');
insert into tb_student (id, name, sex, age) values('7','赵六','男','18');
insert into tb_student (id, name, sex, age) values('1','赵六','男','18');
insert into tb_student (id, name, sex, age) values('8','张三','男','17');sql如下：
DELETE FROM tb_student WHERE id NOT IN
( SELECT id FROM
(( SELECT min(id) id FROM tb_student GROUP BY name, sex,age ))
t)2.查询各科成绩都及格的学员（要求查询出参加考试的各科成绩都高于60分，不管参加了多少科考试）
要求结果：
CREATE TABLE tb_score (
id int(10) NOT NULL,
username varchar(20) DEFAULT NULL,
course varchar(20) DEFAULT NULL,
score int(10) DEFAULT NULL,
PRIMARY KEY (id)
)insert into tb_score (id, username, course, score) values('1','张三','语文','50');
insert into tb_score (id, username, course, score) values('2','张三','数学','80');
insert into tb_score (id, username, course, score) values('3','张三','英语','90');
insert into tb_score (id, username, course, score) values('4','李四','语文','70');
insert into tb_score (id, username, course, score) values('5','李四','数学','80');
insert into tb_score (id, username, course, score) values('6','李四','英语','80');
insert into tb_score (id, username, course, score) values('7','王五','语文','50');
insert into tb_score (id, username, course, score) values('8','王五','英语','70');
insert into tb_score (id, username, course, score) values('9','赵六','数学','90');sql如下：
select username,score from tb_score where id
not in (select id from tb_score where score < 60)3.mysql完成行转列操作
原始数据如下：
行转列后的数据：
完成上面的后就可以看到只需要将结果以名称分组并按列sum求和或用max取最大值就可以获取到满足的结构了（因为不对应学科的记录，其分数必然是0），完善结果如下 :
总结：if主要是用来创建新列，并将非对应学科的分数写为0，用sum或max配合group by保证取出的值是学科对应的值，这样就可以完成行转列了。
