MYSQL query operation

--Logical Operators

• and
• select * from classes where age >= 18 and age <= 28;
• or
• select * from classes where age>18 or height >=100;
• not
• select * form classes where not (age > 18 and gender=2);

-- Fuzzy query (where name like the data to be queried)

• like
• % replace any
• _ replace 1
• Search for names starting with "small"
• select * from classes where name like "小%"

-- Range query

• in (1,3,6) means in a non-contiguous range
• Search for names with ages 18 and 34
• select * from classes where age in (18,34);
• not in non-contiguous range
• select * from classes where age between 18 and 34;
• not between...and.. means not in a continuous range
• Empty judgment is null and non-null judgment is not null
• select * from classes where height is null

--sort

• order by
• asc sort from small to large, ascending order
• desc sort from largest to smallest, descending
• select * from classes where (age between 18 and 34) and gender = 1 order by age asc,name desc;

--aggregate function

• total
• -count
• select count(*) from classes where gender = 1;
• maximum value
• max
• minimum
• min
• sum
• sum
• average value
• avg
• round(123.23 , 1) round to 1 decimal place (rounding)

--grouping (can be used with aggregate functions)

• group by
• Group by gender, query all genders
• select gender from classes group by gender;
• group_concat(...)
• 查询同种性别中的姓名
• select group_concat(name),gender from classes group by gender;
• having(having 类似where和group by组合使用)
• 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
• select gender,group_concat(name) from students group by gender having avg(age) > 30;

--分页

• limit start,count
• select * from classes limit 0,2;

--连接查询

• inner join ... on
• select ... from 表A inner join 表B;
• select * from students inner join classes on students.cls_id = classes.id;
• left join(左连接)
• right join(右连接)
• 以左边或者右边的表作为基准进行查询，没有的信息将以null显示

-- 子查询

• 标量子查询:子查询返回的结果是一个数据(一行一列)
• 列子查询:返回的结果是一列(一列多行)
• 行子查询:返回的结果是一行(一行多列)
• 1 查出平均身高
• select avg(height) from students ; -- 172
• 2 查出高于平均身高的信息
• select * from students where height >(select avg(height) from students) ;