1. Select multi-clause single-table advanced practice

1.1 select multi-clause advanced syntax

The difference between where and having is that the latter is ranked after grouping
select syntax:select <字段1, 字段2, ...> from <表名> [where 条件]
other statements
1. group by 分组条件: group by the specified column
2. having 条件: Conditional judgment or filtering after grouping
3. order {col_name | expr | position} [ASC | DESC]: Sort ASC ascending order, DESC descending order
4. LIMIT {[offset,] row_count | row_count OFFSET offset}: limit the number of result sets

1.2 Aggregate functions

What is an aggregate function: an aggregate function is a prerequisite for group by

Common aggregate functions

serial number	aggregate function	usefulness
1	count()	Returns the number of data in the specified group, with column names in parentheses
2	sum()	Returns the sum of the data in the specified group, only for numeric columns
3	avg()	Returns the average value in the specified group
4	max()	Returns the maximum value of the specified data
5	min()	Returns the minimum value of the specified data
6	group_concat()	Return the specified data, separated by commas into one line

1.3 group by practice

View table structure

MariaDB [world]> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+

Count the totals for each country

mistake

select countrycode,sum(population) 
from city;

correct

select countrycode,sum(population) 
from city 
group by countrycode;

Count the number of cities in each province of China

SELECT District,COUNT(ID)
	FROM city 
	WHERE CountryCode='chn'
	GROUP BY District;

Statistics of the population of each province in China

SELECT District,SUM(Population)
	FROM city 
	WHERE CountryCode='chn'
	GROUP BY District;

1.4 having screening

Purpose: to filter after group by

example

Count the number of cities and the list of cities in each province of China, and output more than ten cities

SELECT district,COUNT(*),GROUP_CONCAT(NAME) 
  FROM city 
  WHERE countrycode='CHN' GROUP BY district 
  having count(*)>10;

1.5 order by sorting

Query the urban population of China and sort the output by population

SELECT NAME,Population
	FROM city 
	WHERE CountryCode= 'CHN'
	ORDER BY Population DESC;

Count the total population of each province in China, filter and output the total population of more than 1000w, and sort the output from large to small

SELECT District,SUM(Population)
	FROM city 
	WHERE CountryCode= 'CHN'
	GROUP BY District
	HAVING SUM(Population)>10000000
	ORDER BY SUM(Population) DESC;

1.6 limit

Function and syntax: used to display the specified number of data rows, generally used after order by sorting, for example, select top3 or the last 3
example
- Display the first two lines:limit 2
- Display lines 3-6:LIMIT 2,5

2. Multi-table join

What is a multi-table join: join multiple tables into one table with the same field
Classification of connections
Syntax: select 字段1,字段2 from 表1 join 表2 on(Note that the field representation method at this time should be: table.field)
For practice, please refer to: https://www.cnblogs.com/oldboy666/p/15637461.html

example

Query the names of the students taught by teacher oldboy

SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc 
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldboy'
GROUP BY teacher.tno;

Query the average score of the courses taught by oldboy

SELECT teacher.tname,AVG(sc.score) 
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc 
ON course.cno=sc.cno
WHERE teacher.tname='oldboy'
GROUP BY teacher.tno;