计算所有行的平均值
计算ID=1的学生的的平均分
select AVG(grade) as '平均分' from score where student_id = 1;
1.3 MAX、MIN
计算所有行的最大值和最小值
select MAX(grade) 最高分,MIN(grade) 最低分 from score where student_id = 1;
1.4 AVG
计算所有行的平均值
select AVG(grade) as '平均分' 最低分 from score where student_id = 1;
1.5 COUNT
计算值不为NULL的行
select COUNT(*) from student;
select COUNT(1) from student;
select COUNT(name) from student;
select COUNT(NULL) from student;
2. 分组
分组查询就是按某列的值进行分组，相同的值分成一组，然后可以对此组内进行求平均、求和等计算
2.1 语法
SELECT 列名,查询表达式
FROM <表名>
WHERE <条件>
GROUP BY <分组字段>
HAVING 分组后的过滤条件
ORDER BY 列名 [ASC,DESC]
LIMIT 偏移量,条数
SELECT列表中只能包含：
被分组的列
为每个分组返回一个值的表达式，如聚合函数
2.2 练习
统计每位同学的平均成绩-单列分组
select student_id,avg(grade) from score group by student_id;
统计每门课程的最高分，并按分数从高到低排列
select course_id,max(grade) 平均分 from score group by course_id order by max(grade) desc
统计各省的男女同学人数-多列分组
select province,gender,COUNT(*) from student group by province,gender
2.3 分组筛选
2.3.1 语法
SELECT FROM <表名>
WHERE
GROUP BY {col_name|expr|position}
HAVING {col_name|expr|position}
ORDER BY {col_name|expr|position} [ASC|DESC]
LIMIT offset,row_count
WHERE用于过滤掉不符合条件的记录
HAVING 用于过滤分组后的记录
GROUP BY用于对筛选后的结果进行分组
2.3.2 练习
统计学生人数超过1人的省份
select province,COUNT(*) from student group by province having COUNT(*)>1
不及格次数大于1次的学生
select student_id,COUNT(*) 不及格次数 from score where grade <60 group by student_id having COUNT(*)>1
3. 子查询
子查询就是指出现在其它SQL语句中的SELECT语句,必须始终出现在圆括号中
子查询可以包含多个关键字或条件
子查询的外层查询可以是: SELECT、INSERT、UPDATE、SET等
子查询可以返回常量、一行数据、一列数据或其它子查询
3.1 比较运算符的子查询
>= 大于等于
<= 小于等于
!= 不等于
<=> 安全不等于
3.2 查询年龄大于平均年龄的学生
SELECT ROUND(AVG(age),2) FROM student;
SELECT * from student WHERE age > (SELECT ROUND(AVG(age),2) FROM student)
3.2 ANY SOME ALL
年龄大于陕西省任何一位同学
SELECT * from student WHERE age > ANY (SELECT age FROM student WHERE province = '陕西省');
年龄大于陕西省某些同学
SELECT * from student WHERE age > SOME (SELECT age FROM student WHERE province = '陕西省');
年龄大于陕西省所有同学
SELECT * from student WHERE age > ALL (SELECT age FROM student WHERE province = '陕西省');
3.3 查询一下有考试成绩的学生信息
[NOT IN]
SELECT * FROM student where id in (SELECT distinct student_id from score);
[EXISTS]
[NOTEXISTS]
SELECT * FROM student where EXISTS (SELECT distinct student_id from score where student.id = score.student_id )
4. 表连接
4.1 连接类型
INNER JOIN 内连接
LEFT JOIN 左外连接
RIGHT JOIN 右外连接
ON 连接条件
4.2 连接条件
使用ON关键字来设定连接条件，也可以使用WHERE来代替
ON来设定连接条件
也可以使用WHERE来对结果进行过滤
4.3 内连接
显示左表和右表中符合条件的
SELECT * FROM student INNER JOIN score ON student.id = score.student_id;
4.4 左外连接
显示左表的全部和右表符合条件的
SELECT * FROM student LEFT JOIN score ON student.id = score.student_id;
4.5 右外连接
显示右表的全部和左表符合条件的
SELECT * FROM student RIGHT JOIN score ON student.id = score.student_id;
4.6 多表连接
SELECT student.name,course.name,score.grade FROM score
INNER JOIN student ON student.id = score.student_id
INNER JOIN course ON course.id = score.course_id;
4.7 无限分类[自身连接]
4.7.1 建表
CREATE table category(
id int(11) PRIMARY KEY AUTO_INCREMENT NOT NULL,
name varchar(50),
parent_id int(11)
4.7.2 插入语句
INSERT INTO category(id,name,parent_id)
VALUES (1,'数码产品',0),(2,'服装',0),(3,'食品',0),
(4,'iPad',1),(5,'李宁',2),(6,'康师傅',3);
4.7.3 查询所有的顶级分类下面分类的数量
SELECT c1.id,c1.name,COUNT(1)
FROM category c1 INNER JOIN category c2 ON c1.id = c2.parent_id
WHERE c1.parent_id = 0
GROUP BY c1.id;
4.7.4 父类变成名称
SELECT c1.id,c1.name,p.name
FROM category c1 LEFT JOIN category p ON c1.parent_id = p.id
4.8 删除重复记录[多表删除]
INSERT INTO category(id,name,parent_id)
VALUES
(7,'iPad',1),
(8,'李宁',2),
(9,'康师傅',3);
4.8.1 子查询找要删除的ID
SELECT * FROM category c1 LEFT JOIN
(SELECT id,name from category GROUP BY name HAVING COUNT(1)>1) c2
ON c1.name = c2.name WHERE c1.id != c2.id
4.8.2 通过IN找要删除的ID
SELECT * FROM category c1
WHERE c1.name IN
(SELECT name from category GROUP BY name HAVING COUNT(1)>1)
AND c1.id NOT IN
(SELECT MIN(id) from category GROUP BY name HAVING COUNT(1)>1)
4.8.3 删除重复记录
DELETE FROM category
WHERE name IN
(SELECT NAME FROM ( SELECT name from category GROUP BY name HAVING COUNT(1)>1) AS T1 )
AND id NOT IN
(SELECT id FROM (SELECT MIN(id) id from category GROUP BY name HAVING COUNT(1)>1) AS T2)
4.9 多表更新
4.9.1 (插入省份)INSERT SELECT
CREATE TABLE province(id int PRIMARY KEY AUTO_INCREMENT,name varchar(50))
INSERT INTO province(name) SELECT DISTINCT province FROM student;
4.9.2 更新省份
UPDATE student INNER JOIN province ON student.province=province.name
SET student.province=province.id
4.9.3 修改字段
ALTER TABLE student
CHANGE COLUMN `province` `province_id` int(11);
王立志_LEO
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