SQL_DQL_ query statement

DQL_ query records in the table

select * from 表名;

• 1. Syntax :

		select
					字段列表
		from
					表名列表
		where
					条件列表
		group by
					分组字段
		having
					分组之后的条件
		order by
					排序
		limit
					分页限定

• 2. Basic query

2.1 Query of multiple fields

		select 字段名1，字段名2... from 表名；
* 注意：
			* 如果查询所有字段，则可以使用*来替代字段列表。

2.2 Remove duplication:

		* distinct

2.3. Calculation columns
* Generally, four arithmetic operations can be used to calculate the values ​​of some columns. (Generally only numerical calculations are performed)

		* ifnull(表达式1,表达式2)：null参与的运算，计算结果都为null
			* 表达式1：哪个字段需要判断是否为null
			* 如果该字段为null后的替换值。

2.4. Alias:

		* as：as也可以省略

Example:
Create a student table

Query name and age:

Query addresses, and remove duplicate addresses.

Calculate the sum of math and English scores. Create


aliases

• 3. Condition query

3.1 where clause followed by conditions

3.2 Operators
*>, <, <=, >=, =, <>
* BETWEEN...AND
* IN (collection)
* LIKE: fuzzy query
* placeholder:
* _: single arbitrary character
* %: multiple arbitrary characters
* IS NULL
* and or &&
* or or ||
* not or!

Example:

				-- 查询年龄大于20岁

				SELECT * FROM student WHERE age > 20;
				
				SELECT * FROM student WHERE age >= 20;
				
				-- 查询年龄等于20岁
				SELECT * FROM student WHERE age = 20;
				
				-- 查询年龄不等于20岁
				SELECT * FROM student WHERE age != 20;
				SELECT * FROM student WHERE age <> 20;
				
				-- 查询年龄大于等于20 小于等于30
				
				SELECT * FROM student WHERE age >= 20 &&  age <=30;
				SELECT * FROM student WHERE age >= 20 AND  age <=30;
				SELECT * FROM student WHERE age BETWEEN 20 AND 30;
				
				-- 查询年龄22岁，18岁，25岁的信息
				SELECT * FROM student WHERE age = 22 OR age = 18 OR age = 25
				SELECT * FROM student WHERE age IN (22,18,25);
				
				-- 查询英语成绩为null
				SELECT * FROM student WHERE english = NULL; -- 不对的。null值不能使用 = （!=） 判断
				
				SELECT * FROM student WHERE english IS NULL;
				
				-- 查询英语成绩不为null
				SELECT * FROM student WHERE english  IS NOT NULL;
	


				-- 查询姓马的有哪些？ like
				SELECT * FROM student WHERE NAME LIKE '马%';
				-- 查询姓名第二个字是化的人
				
				SELECT * FROM student WHERE NAME LIKE "_化%";
				
				-- 查询姓名是3个字的人
				SELECT * FROM student WHERE NAME LIKE '___';
				
				
				-- 查询姓名中包含德的人
				SELECT * FROM student WHERE NAME LIKE '%德%';

Supplement: DQL: query statement

• 1. Sort query

  • Syntax: order by clause
    * order by sort field 1 sort method 1, sort field 2 sort method 2...

  • Sorting method:
    * ASC: Ascending order, the default.
    * DESC: descending order.

  • Note:
    * If there are multiple sorting conditions, the second condition will be judged only when the current condition value is the same.

• 2. Aggregate functions:
• Take a column of data as a whole and perform vertical calculations.
  1. Count: calculate the number.
  Generally select non-empty columns: primary key
  count(*)
  2. max: calculate the maximum value
  3. min: calculate the minimum value
  4. sum: calculate and
  5. avg: calculate the average
  

* 注意：聚合函数的计算，排除null值。
		解决方案：
			
			1. 选择不包含非空的列进行计算
			2. IFNULL函数

• 3. Group query:
  1. Syntax: group by group field;
  2. Note:

  	1. 分组之后查询的字段：分组字段、聚合函数
  	2. where 和 having 的区别？
  		1. where 在分组之前进行限定，如果不满足条件，则不参与分组。
  		 having在分组之后进行限定，如果不满足结果，则不会被查询出来
  		2. where 后不可以跟聚合函数，having可以进行聚合函数的判断。
  

-- 按照男女性别分组，分别查询男、女同学的平均分

SELECT sex,AVG(math) FROM student GROUP BY sex;

-- 按照性别分组。分别查询男女同学的平均分，人数

SELECT sex,AVG(math),COUNT(id) FROM student GROUP BY sex;

-- 按照性别分组。分别查询男女同学的平均分，人数。要求：分数低于80分的人。不参与分组

SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 80 GROUP BY sex;

-- 按照性别分组。分别查询男女同学的平均分，人数。要求：分数低于80分的人。不参与分组,人数大于3

SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 80 GROUP BY sex HAVING COUNT(id) > 3;

• 4. Paging query

1. Syntax: index starting with limit, the number of queries per page;
2. Formula: starting index = (current page number-1) * number of items displayed on each page

			-- 每页显示3条记录 

			SELECT * FROM student LIMIT 0,3; -- 第1页
			
			SELECT * FROM student LIMIT 3,3; -- 第2页
			
			SELECT * FROM student LIMIT 6,3; -- 第3页

3. limit is a MySQL "dialect"