习题来源于网络，sql语句是自己写的，部分有参考。欢迎指正。
2019.3.29更新

写完后一年没有看过，没想到这篇文章有这么多人点击。博主工作到一半去考研了，目前已上岸某中部985，也算是比较幸运。非常感谢大家在评论里的留言，留言太多不能一一回复，希望大家见谅。这两天根据评论把文章中的某些错误或者不足的地方更新了下，，希望大家能够继续指出不足之处。
表名和字段
–1.学生表

Student(s_id,s_name,s_birth,s_sex) --学生编号,学生姓名, 出生年月,学生性别

–2.课程表

Course(c_id,c_name,t_id) – --课程编号, 课程名称, 教师编号

–3.教师表

Teacher(t_id,t_name) --教师编号,教师姓名

–4.成绩表

Score(s_id,c_id,s_score) --学生编号,课程编号,分数
测试数据
--建表
--学生表
CREATE TABLE `Student`(
	`s_id` VARCHAR(20),
	`s_name` VARCHAR(20) NOT NULL DEFAULT '',
	`s_birth` VARCHAR(20) NOT NULL DEFAULT '',
	`s_sex` VARCHAR(10) NOT NULL DEFAULT '',
	PRIMARY KEY(`s_id`)
);
--课程表
CREATE TABLE `Course`(
	`c_id`  VARCHAR(20),
	`c_name` VARCHAR(20) NOT NULL DEFAULT '',
	`t_id` VARCHAR(20) NOT NULL,
	PRIMARY KEY(`c_id`)
);
--教师表
CREATE TABLE `Teacher`(
	`t_id` VARCHAR(20),
	`t_name` VARCHAR(20) NOT NULL DEFAULT '',
	PRIMARY KEY(`t_id`)
);
--成绩表
CREATE TABLE `Score`(
	`s_id` VARCHAR(20),
	`c_id`  VARCHAR(20),
	`s_score` INT(3),
	PRIMARY KEY(`s_id`,`c_id`)
);
--插入学生表测试数据
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' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');
--课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');

--教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');

--成绩表测试数据
insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);

练习题和sql语句
-- 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 a.* ,b.s_score as 01_score,c.s_score as 02_score from 
student a 
	join score b on a.s_id=b.s_id and b.c_id='01'
	left join score c on a.s_id=c.s_id and c.c_id='02' or c.c_id = NULL where b.s_score>c.s_score
	
--也可以这样写
	select a.*,b.s_score as 01_score,c.s_score as 02_score from student 		  a,score b,score c 
			where a.s_id=b.s_id 
			and a.s_id=c.s_id 
			and b.c_id='01' 
			and c.c_id='02' 
			and b.s_score>c.s_score
-- 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
	
select a.* ,b.s_score as 01_score,c.s_score as 02_score from 
	student a left join score b on a.s_id=b.s_id and b.c_id='01' or b.c_id=NULL 
	 join score c on a.s_id=c.s_id and c.c_id='02' where b.s_score<c.s_score
			

-- 3、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 b.s_id,b.s_name,ROUND(AVG(a.s_score),2) as avg_score from 
	student b 
	join score a on b.s_id = a.s_id
	GROUP BY b.s_id,b.s_name HAVING avg_score >=60;
	

-- 4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
		-- (包括有成绩的和无成绩的)
		
select b.s_id,b.s_name,ROUND(AVG(a.s_score),2) as avg_score from 
	student b 
	left join score a on b.s_id = a.s_id
	GROUP BY b.s_id,b.s_name HAVING avg_score <60
	union
select a.s_id,a.s_name,0 as avg_score from 
	student a 
	where a.s_id not in (
				select distinct s_id from score);


-- 5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
select a.s_id,a.s_name,count(b.c_id) as sum_course,sum(b.s_score) as sum_score from 
	student a 
	left join score b on a.s_id=b.s_id
	GROUP BY a.s_id,a.s_name;
			
			
-- 6、查询"李"姓老师的数量 
select count(t_id) from teacher where t_name like '李%';
	
-- 7、查询学过"张三"老师授课的同学的信息 
select a.* from 
	student a 
	join score b on a.s_id=b.s_id where b.c_id in(
		select c_id from course where t_id =(
			select t_id from teacher where t_name = '张三'));

-- 8、查询没学过"张三"老师授课的同学的信息 
select * from 
    student c 
    where c.s_id not in(
        select a.s_id from student a join score b on a.s_id=b.s_id where b.c_id in(
        select a.c_id from course a join teacher b on a.t_id = b.t_id where t_name ='张三'));
-- 9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息

select a.* from 
	student a,score b,score c 
	where a.s_id = b.s_id  and a.s_id = c.s_id and b.c_id='01' and c.c_id='02';
	
-- 10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
			
select a.* from 
	student a 
	where a.s_id in (select s_id from score where c_id='01' ) and a.s_id not in(select s_id from score where c_id='02')
			

-- 11、查询没有学全所有课程的同学的信息 
--@wendiepei的写法
select s.* from student s 
left join Score s1 on s1.s_id=s.s_id
group by s.s_id having count(s1.c_id)<(select count(*) from course)	
--@k1051785839的写法
select *
from student
where s_id not in(
select s_id from score t1  
group by s_id having count(*) =(select count(distinct c_id)  from course)) 
-- 12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息 

select * from student where s_id in(
	select distinct a.s_id from score a where a.c_id in(select a.c_id from score a where a.s_id='01')
	);
			
-- 13、查询和"01"号的同学学习的课程完全相同的其他同学的信息 
--@ouyang_1993的写法
SELECT
 Student.*
FROM
 Student
WHERE
 s_id IN (SELECT s_id FROM Score GROUP BY s_id HAVING COUNT(s_id) = (
    #下面的语句是找到'01'同学学习的课程数
    SELECT COUNT(c_id) FROM Score WHERE s_id = '01'
   )
 )
AND s_id NOT IN (
 #下面的语句是找到学过‘01’同学没学过的课程，有哪些同学。并排除他们
 SELECT s_id FROM Score
 WHERE c_id IN(
   #下面的语句是找到‘01’同学没学过的课程
   SELECT DISTINCT c_id FROM Score
   WHERE c_id NOT IN (
     #下面的语句是找出‘01’同学学习的课程
     SELECT c_id FROM Score WHERE s_id = '01'
    )
  ) GROUP BY s_id
) #下面的条件是排除01同学
AND s_id NOT IN ('01')
--@k1051785839的写法
SELECT
 t3.*
FROM
 (
  SELECT
   s_id,
   group_concat(c_id ORDER BY c_id) group1
  FROM
   score
  WHERE
   s_id &lt;> '01'
  GROUP BY
   s_id
 ) t1
INNER JOIN (
 SELECT
  group_concat(c_id ORDER BY c_id) group2
 FROM
  score
 WHERE
  s_id = '01'
 GROUP BY
  s_id
) t2 ON t1.group1 = t2.group2
INNER JOIN student t3 ON t1.s_id = t3.s_id

-- 14、查询没学过"张三"老师讲授的任一门课程的学生姓名 
select a.s_name from student a where a.s_id not in (
	select s_id from score where c_id = 
				(select c_id from course where t_id =(
					select t_id from teacher where t_name = '张三')));

-- 15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩 
select a.s_id,a.s_name,ROUND(AVG(b.s_score)) from 
	student a 
	left join score b on a.s_id = b.s_id
	where a.s_id in(
			select s_id from score where s_score<60 GROUP BY  s_id having count(1)>=2)
	GROUP BY a.s_id,a.s_name

-- 16、检索"01"课程分数小于60，按分数降序排列的学生信息
select a.*,b.c_id,b.s_score from 
	student a,score b 
	where a.s_id = b.s_id and b.c_id='01' and b.s_score<60 ORDER BY b.s_score DESC;
		
-- 17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
select a.s_id,(select s_score from score where s_id=a.s_id and c_id='01') as 语文,
				(select s_score from score where s_id=a.s_id and c_id='02') as 数学,
				(select s_score from score where s_id=a.s_id and c_id='03') as 英语,
			round(avg(s_score),2) as 平均分 from score a  GROUP BY a.s_id ORDER BY 平均分 DESC;
--@喝完这杯还有一箱的写法
SELECT a.s_id,MAX(CASE a.c_id WHEN '01' THEN a.s_score END ) 语文, 
MAX(CASE a.c_id WHEN '02' THEN a.s_score END ) 数学, 
MAX(CASE a.c_id WHEN '03' THEN a.s_score END ) 英语, 
avg(a.s_score),b.s_name FROM Score a JOIN Student b ON a.s_id=b.s_id GROUP BY a.s_id ORDER BY 5 DESC		
-- 18.查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率
--及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90
select a.c_id,b.c_name,MAX(s_score),MIN(s_score),ROUND(AVG(s_score),2),
	ROUND(100*(SUM(case when a.s_score>=60 then 1 else 0 end)/SUM(case when a.s_score then 1 else 0 end)),2) as 及格率,
	ROUND(100*(SUM(case when a.s_score>=70 and a.s_score<=80 then 1 else 0 end)/SUM(case when a.s_score then 1 else 0 end)),2) as 中等率,
	ROUND(100*(SUM(case when a.s_score>=80 and a.s_score<=90 then 1 else 0 end)/SUM(case when a.s_score then 1 else 0 end)),2) as 优良率,
	ROUND(100*(SUM(case when a.s_score>=90 then 1 else 0 end)/SUM(case when a.s_score then 1 else 0 end)),2) as 优秀率
	from score a left join course b on a.c_id = b.c_id GROUP BY a.c_id,b.c_name
	
-- 19、按各科成绩进行排序，并显示排名
-- mysql没有rank函数
	select a.s_id,a.c_id,
        @i:=@i +1 as i保留排名,
        @k:=(case when @score=a.s_score then @k else @i end) as rank不保留排名,
        @score:=a.s_score as score
    from (
        select s_id,c_id,s_score from score GROUP BY s_id,c_id,s_score ORDER BY s_score DESC
)a,(select @k:=0,@i:=0,@score:=0)s
--@k1051785839的写法
(select * from (select 
t1.c_id,
t1.s_score,
(select count(distinct t2.s_score) from score t2 where t2.s_score>=t1.s_score and t2.c_id='01') rank
FROM score t1 where t1.c_id='01'
order by t1.s_score desc) t1)
union
(select * from (select 
t1.c_id,
t1.s_score,
(select count(distinct t2.s_score) from score t2 where t2.s_score>=t1.s_score and t2.c_id='02') rank
FROM score t1 where t1.c_id='02'
order by t1.s_score desc) t2)
union
(select * from (select 
t1.c_id,
t1.s_score,
(select count(distinct t2.s_score) from score t2 where t2.s_score>=t1.s_score and t2.c_id='03') rank
FROM score t1 where t1.c_id='03'
order by t1.s_score desc) t3)
-- 20、查询学生的总成绩并进行排名
select a.s_id,
	@i:=@i+1 as i,
	@k:=(case when @score=a.sum_score then @k else @i end) as rank,
	@score:=a.sum_score as score
from (select s_id,SUM(s_score) as sum_score from score GROUP BY s_id ORDER BY sum_score DESC)a,
	(select @k:=0,@i:=0,@score:=0)s
	
-- 21、查询不同老师所教不同课程平均分从高到低显示 
		
	select a.t_id,c.t_name,a.c_id,ROUND(avg(s_score),2) as avg_score from course a
		left join score b on a.c_id=b.c_id 
		left join teacher c on a.t_id=c.t_id
		GROUP BY a.c_id,a.t_id,c.t_name ORDER BY avg_score DESC;
-- 22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
			
			select d.*,c.排名,c.s_score,c.c_id from (
                select a.s_id,a.s_score,a.c_id,@i:=@i+1 as 排名 from score a,(select @i:=0)s where a.c_id='01'  
								ORDER BY a.s_score DESC  
            )c
            left join student d on c.s_id=d.s_id
            where 排名 BETWEEN 2 AND 3
            UNION
            select d.*,c.排名,c.s_score,c.c_id from (
                select a.s_id,a.s_score,a.c_id,@j:=@j+1 as 排名 from score a,(select @j:=0)s where a.c_id='02'  
								ORDER BY a.s_score DESC
            )c
            left join student d on c.s_id=d.s_id
            where 排名 BETWEEN 2 AND 3
            UNION
            select d.*,c.排名,c.s_score,c.c_id from (
                select a.s_id,a.s_score,a.c_id,@k:=@k+1 as 排名 from score a,(select @k:=0)s where a.c_id='03' 
								ORDER BY a.s_score DESC
            )c
            left join student d on c.s_id=d.s_id
            where 排名 BETWEEN 2 AND 3;
			
-- 23、统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[0-60]及所占百分比


		select distinct f.c_name,a.c_id,b.`85-100`,b.百分比,c.`70-85`,c.百分比,d.`60-70`,d.百分比,e.`0-60`,e.百分比 from score a
				left join (select c_id,SUM(case when s_score >85 and s_score <=100 then 1 else 0 end) as `85-100`,
											ROUND(100*(SUM(case when s_score >85 and s_score <=100 then 1 else 0 end)/count(*)),2) as 百分比
								from score GROUP BY c_id)b on a.c_id=b.c_id
				left join (select c_id,SUM(case when s_score >70 and s_score <=85 then 1 else 0 end) as `70-85`,
											ROUND(100*(SUM(case when s_score >70 and s_score <=85 then 1 else 0 end)/count(*)),2) as 百分比
								from score GROUP BY c_id)c on a.c_id=c.c_id
				left join (select c_id,SUM(case when s_score >60 and s_score <=70 then 1 else 0 end) as `60-70`,
											ROUND(100*(SUM(case when s_score >60 and s_score <=70 then 1 else 0 end)/count(*)),2) as 百分比
								from score GROUP BY c_id)d on a.c_id=d.c_id
				left join (select c_id,SUM(case when s_score >=0 and s_score <=60 then 1 else 0 end) as `0-60`,
											ROUND(100*(SUM(case when s_score >=0 and s_score <=60 then 1 else 0 end)/count(*)),2) as 百分比
								from score GROUP BY c_id)e on a.c_id=e.c_id
				left join course f on a.c_id = f.c_id
				 
-- 24、查询学生平均成绩及其名次 

		select a.s_id,
				@i:=@i+1 as '不保留空缺排名',
				@k:=(case when @avg_score=a.avg_s then @k else @i end) as '保留空缺排名',
				@avg_score:=avg_s as '平均分'
		from (select s_id,ROUND(AVG(s_score),2) as avg_s from score GROUP BY s_id ORDER BY avg_s DESC)a,(select @avg_score:=0,@i:=0,@k:=0)b;
-- 25、查询各科成绩前三名的记录
			-- 1.选出b表比a表成绩大的所有组
			-- 2.选出比当前id成绩大的 小于三个的
		select a.s_id,a.c_id,a.s_score from score a 
			left join score b on a.c_id = b.c_id and a.s_score<b.s_score
			group by a.s_id,a.c_id,a.s_score HAVING COUNT(b.s_id)<3
			ORDER BY a.c_id,a.s_score DESC

-- 26、查询每门课程被选修的学生数 

		select c_id,count(s_id) from score a GROUP BY c_id

-- 27、查询出只有两门课程的全部学生的学号和姓名 
		select s_id,s_name from student where s_id in(
				select s_id from score GROUP BY s_id HAVING COUNT(c_id)=2);

-- 28、查询男生、女生人数 
		select s_sex,COUNT(s_sex) as 人数  from student GROUP BY s_sex

-- 29、查询名字中含有"风"字的学生信息

		select * from student where s_name like '%风%';

-- 30、查询同名同性学生名单，并统计同名人数 
		
		select a.s_name,a.s_sex,count(*) from student a  JOIN 
					student b on a.s_id !=b.s_id and a.s_name = b.s_name and a.s_sex = b.s_sex
		GROUP BY a.s_name,a.s_sex



-- 31、查询1990年出生的学生名单
		
		select s_name from student where s_birth like '1990%'

-- 32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列 

	select c_id,ROUND(AVG(s_score),2) as avg_score from score GROUP BY c_id ORDER BY avg_score DESC,c_id ASC

-- 33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩 

	select a.s_id,b.s_name,ROUND(avg(a.s_score),2) as avg_score from score a
		left join student b on a.s_id=b.s_id GROUP BY s_id HAVING avg_score>=85
	
-- 34、查询课程名称为"数学"，且分数低于60的学生姓名和分数 
	
		select a.s_name,b.s_score from score b join student a on a.s_id=b.s_id where b.c_id=(
					select c_id from course where c_name ='数学') and b.s_score<60

-- 35、查询所有学生的课程及分数情况； 
	
		
		select a.s_id,a.s_name,
					SUM(case c.c_name when '语文' then b.s_score else 0 end) as '语文',
					SUM(case c.c_name when '数学' then b.s_score else 0 end) as '数学',
					SUM(case c.c_name when '英语' then b.s_score else 0 end) as '英语',
					SUM(b.s_score) as  '总分'
		from student a left join score b on a.s_id = b.s_id 
		left join course c on b.c_id = c.c_id 
		GROUP BY a.s_id,a.s_name


 -- 36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数； 
			select a.s_name,b.c_name,c.s_score from course b left join score c on b.c_id = c.c_id
				left join student a on a.s_id=c.s_id where c.s_score>=70

		

-- 37、查询不及格的课程
		select a.s_id,a.c_id,b.c_name,a.s_score from score a left join course b on a.c_id = b.c_id
			where a.s_score<60 
		
--38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名； 
		select a.s_id,b.s_name from score a LEFT JOIN student b on a.s_id = b.s_id
			where a.c_id = '01'	and a.s_score>80

-- 39、求每门课程的学生人数 
		select count(*) from score GROUP BY c_id;

-- 40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩

		
		-- 查询老师id	
		select c_id from course c,teacher d where c.t_id=d.t_id and d.t_name='张三'
		-- 查询最高分（可能有相同分数）
		select MAX(s_score) from score where c_id='02'
		-- 查询信息
		select a.*,b.s_score,b.c_id,c.c_name from student a
			LEFT JOIN score b on a.s_id = b.s_id
			LEFT JOIN course c on b.c_id=c.c_id
			where b.c_id =(select c_id from course c,teacher d where c.t_id=d.t_id and d.t_name='张三')
			and b.s_score in (select MAX(s_score) from score where c_id='02')


-- 41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩 
	select DISTINCT b.s_id,b.c_id,b.s_score from score a,score b where a.c_id != b.c_id and a.s_score = b.s_score
	

-- 42、查询每门功成绩最好的前两名 
		-- 牛逼的写法
	select a.s_id,a.c_id,a.s_score from score a
		where (select COUNT(1) from score b where b.c_id=a.c_id and b.s_score>=a.s_score)<=2 ORDER BY a.c_id


-- 43、统计每门课程的学生选修人数（超过5人的课程才统计）。要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列  
		select c_id,count(*) as total from score GROUP BY c_id HAVING total>5 ORDER BY total,c_id ASC
		
-- 44、检索至少选修两门课程的学生学号 
		select s_id,count(*) as sel from score GROUP BY s_id HAVING sel>=2

-- 45、查询选修了全部课程的学生信息 
		select * from student where s_id in(		
			select s_id from score GROUP BY s_id HAVING count(*)=(select count(*) from course))


--46、查询各学生的年龄
	-- 按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一

	select s_birth,(DATE_FORMAT(NOW(),'%Y')-DATE_FORMAT(s_birth,'%Y') - 
				(case when DATE_FORMAT(NOW(),'%m%d')>DATE_FORMAT(s_birth,'%m%d') then 0 else 1 end)) as age
		from student;


-- 47、查询本周过生日的学生
	select * from student where WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))=WEEK(s_birth)
	select * from student where YEARWEEK(s_birth)=YEARWEEK(DATE_FORMAT(NOW(),'%Y%m%d'))
	
	select WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))

-- 48、查询下周过生日的学生
	select * from student where WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))+1 =WEEK(s_birth)

-- 49、查询本月过生日的学生

	select * from student where MONTH(DATE_FORMAT(NOW(),'%Y%m%d')) =MONTH(s_birth)
	
-- 50、查询下月过生日的学生
	select * from student where MONTH(DATE_FORMAT(NOW(),'%Y%m%d'))+1 =MONTH(s_birth)

问题背景
通常我们开发的时候，需要联合控制台和Navicat/PLSQL等工具进行语句的拼接检查，如果只是输出了一堆？？？，那么将极大降低我们的效率。因此我们需要输出完整的SQL语句以便调试。

Update

2020-Oct ：  新增LOG DEBUG LEVEL模式
2020-July ： 新增官方p6spy打印分析sql语句方案


解决方案（StdOutImpl）
请注意: 部分朋友反馈不生效，估计跟引入的包有一定关系，druid+mybatis-plus-boot-starter 就亲测有用。请检查是否有log4j相关实现类。
如果是application.yml
#by zhengkai.blog.csdn.net
#mybatis-plus配置控制台打印完整带参数SQL语句
mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

如果是application.properties，添加：
#mybatis-plus配置控制台打印完整带参数SQL语句
mybatis-plus.configuration.log-impl=org.apache.ibatis.logging.stdout.StdOutImpl

Mybatis内置的日志工厂提供日志功能，具体的日志实现有以下几种方式：

SLF4J
Apache Commons Logging
Log4j 2
Log4j
JDK logging
no logging

具体选择哪个日志实现由MyBatis的LogFactory内置日志工厂确定。它会使用最先找到的（按上文列举的顺序查找）。 如果一个都未找到，日志功能就会被禁用。
    static {
        tryImplementation(LogFactory::useSlf4jLogging);
        tryImplementation(LogFactory::useCommonsLogging);
        tryImplementation(LogFactory::useLog4J2Logging);
        tryImplementation(LogFactory::useLog4JLogging);
        tryImplementation(LogFactory::useJdkLogging);
        tryImplementation(LogFactory::useNoLogging);
    }

不少应用服务器的classpath中已经包含Commons Logging，如Tomcat和WebShpere， 所以MyBatis会把它作为具体的日志实现。
记住这点非常重要。这意味着，在诸如 WebSphere的环境中——WebSphere提供了Commons Logging的私有实现，你的Log4J配置将被忽略。
这种做法不免让人悲摧，MyBatis怎么能忽略你的配置呢？事实上，因Commons Logging已经存 在，按优先级Log4J自然就被忽略了！
控制台输出
--- [ XNIO-1 task-12] c.s.cms.controller.IndexController       : username-admin-password-123456-****
Creating a new SqlSession
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@708e9ffd] was not registered for synchronization because synchronization is not active
--- [ XNIO-1 task-12] com.alibaba.druid.pool.DruidDataSource   : {dataSource-1} inited
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@62b13210] will not be managed by Spring
==>  Preparing: select * from user t where t.user_code='admin' and t.password='123456' 
==> Parameters: 
<==    Columns: user_id, user_code, create_date, modify_date, user_name, password, status, role_id, department_id, major_id, classes_id, year
<==        Row: 1, admin, 2020-02-15 22:14:32, 2020-02-18 23:38:51, Moshow K ZHENG, 123456, 1, 9, 1, 13, 113, 2020
<==      Total: 1
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@708e9ffd]

解决方案（手写MybatisPlusOutImpl）
配置文件
mybatis-plus:
  configuration:
#    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
#    改为自己写的
    log-impl: com.softdev.system.config.MybatisPlusOutImpl

java类 MybatisPlusOutImpl
package com.softdev.system.config;

import org.apache.ibatis.logging.Log;
/**
 * @Description MybatisPlusOutImpl，直接使用控制台输出日志
 * @Author zhengkai.blog.csdn.net
 **/
public class MybatisPlusOutImpl implements Log {
    public MybatisPlusOutImpl(String clazz) {
        System.out.println("MybatisPlusOutImpl::"+clazz);
    }

    public boolean isDebugEnabled() {
        return true;
    }

    public boolean isTraceEnabled() {
        return true;
    }

    public void error(String s, Throwable e) {
        System.err.println(s);
        e.printStackTrace(System.err);
    }

    public void error(String s) {
        System.err.println("MybatisPlusOutImpl::"+s);
    }

    public void debug(String s) {
        System.out.println("MybatisPlusOutImpl::"+s);
    }

    public void trace(String s) {
        System.out.println("MybatisPlusOutImpl::"+s);
    }

    public void warn(String s) {
        System.out.println("MybatisPlusOutImpl::"+s);
    }
}

解决方案（LOG-DEBUG模式）
# 在application.yml 中增加配置，指定 mapper 文件所在的包，进入DEBUG模式
logging:
  level:
    com.baomidou.example.mapper: debug

官方解决方案p6spy（不建议）
查看p6spy最新版本 ，请注意，该方案为侵入式的JDBC级方案。
pom.xml引入
<!-- https://mvnrepository.com/artifact/p6spy/p6spy -->
<dependency>
    <groupId>p6spy</groupId>
    <artifactId>p6spy</artifactId>
    <version>3.9.1</version>
</dependency>

这是yaml版本,还没试过,待我实验一下.
spring:
  datasource:
    driver-class-name: com.p6spy.engine.spy.P6SpyDriver
    url: jdbc:p6spy:h2:mem:test
    ...

这是官方提供的properties版本
#3.2.1以上使用
modulelist=com.baomidou.mybatisplus.extension.p6spy.MybatisPlusLogFactory,com.p6spy.engine.outage.P6OutageFactory
#3.2.1以下使用或者不配置
#modulelist=com.p6spy.engine.logging.P6LogFactory,com.p6spy.engine.outage.P6OutageFactory
# 自定义日志打印
logMessageFormat=com.baomidou.mybatisplus.extension.p6spy.P6SpyLogger
#日志输出到控制台
appender=com.baomidou.mybatisplus.extension.p6spy.StdoutLogger
# 使用日志系统记录 sql
#appender=com.p6spy.engine.spy.appender.Slf4JLogger
# 设置 p6spy driver 代理
deregisterdrivers=true
# 取消JDBC URL前缀
useprefix=true
# 配置记录 Log 例外,可去掉的结果集有error,info,batch,debug,statement,commit,rollback,result,resultset.
excludecategories=info,debug,result,commit,resultset
# 日期格式
dateformat=yyyy-MM-dd HH:mm:ss
# 实际驱动可多个
#driverlist=org.h2.Driver
# 是否开启慢SQL记录
outagedetection=true
# 慢SQL记录标准 2 秒
outagedetectioninterval=2

 

在控制台上打印sql语句

mybatis:
  # spring boot集成mybatis的方式打印sql
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

 

在log日志中打印

logging:
  level:
    com.eth.wallet.mapper: debug  # com.eth.wallet.mapper 根据你的项目设定


 

springboot+mybatis整合过程中，开启控制台sql语句打印的多种方式：

附：mybatis官方文档

方法一：

1.在mybatis的配置文件中添加：

<settings>
    <!-- 打印sql日志 -->
    <setting name="logImpl" value="STDOUT_LOGGING" />
</settings>


mybatis的配置文件----mybatis-config.xml如下：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <!-- 打印sql日志 -->
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
</configuration>

2.在springboot的配置文件----appcation.yml中添加：

mybatis:
    configuration:
      log-impl: org.apache.ibatis.logging.stdout.StdOutImpl


ps：

          IDEA中，springboot默认配置文件是application.properties文件，但是yml文件在语法上更加简洁，更有层次感，所以此处是用yml语法，properties中好像是这么写的：mybatis.configuration.log-impl= org.apache.ibatis.logging.stdout.StdOutImpl

控制台可以打印了。。。。。



方法二：

在springboot+mybatis整合中，可以将springboot的配置文件添加如下一段也可：

logging:
  level:
    com.lucifer.springboot.cache.mapper: debug



ps:  com.lucifer.springboot.cache.mapper是包名

方法三：

如果你使用的是springboot+mybatis-plus的话：

  <dependency>
       <groupId>com.baomidou</groupId>
       <artifactId>mybatis-plus-boot-starter</artifactId>
       <version>3.3.1</version>
  </dependency>

application.yml: 

mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

控制台打印： 

JDBC Connection [HikariProxyConnection@1006460161 wrapping com.mysql.cj.jdbc.ConnectionImpl@37cccae8] will not be managed by Spring
==>  Preparing: SELECT id,user_name,age FROM user WHERE (user_name = ?) 
==> Parameters: 张三(String)
<==    Columns: id, user_name, age
<==        Row: 1, 张三, 18
<==      Total: 1