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。