DQL query:
2. Query constants, expressions, functions
4. The role of the plus sign and the string splicing function: concat();
Group by expression or function, and summarize:
Seven*, internal and external links of SQL99 grammar
2. Column sub-query (multi-line sub-query)
3. Subquery placed after select
4. Subquery placed after exists
1. Basic query
1. Query field:
SHOW DATABASES;
#基础查询
/*
语法：
select 查询列表 from 表名；
特点：
查询列表可以是表中的字段，常量值、表达式、函数
查询结果是一个虚拟的表格
*/
#先启用指定的库：
USE myemployees;
#1.查询表中单个字段
SELECT last_name FROM employees;
#2.查询表中的多个字段
#为了防止字段和关键字重名，可以加上着重号：``
SELECT `last_name`,`salary`,`email` FROM employees;
#3、查询表中的所有字段
# 如果用*，那么查询顺序和表中一样。
SELECT * FROM employees;
#运行：先选中，然后F9即可Query result: (the text is omitted)
2. Query constants, expressions, functions
#4.查询常量值
SELECT 100;
#sql中不区分字符和字符串，都用单引号即可。
SELECT 'john';
#5.查询表达式
SELECT 100%98;
#6.查询函数
SELECT VERSION(); 
3. Get an alias
#7.起别名
#方式一：便于理解，如果要查询的字段有重名的情况，使用别名可以区分开。
SELECT 100%98 AS 结果;
SELECT last_name AS 姓 FROM employees;
#方式二：使用空格
SELECT last_name 姓 FROM employees;
#案例：查询salary，显示结果为 out put 这里Out put中间有特殊符号空格，所以建议加上双引号
SELECT salary AS "out put" FROM employees; 
3. De-duplication
Remove duplicate fields:
#8.去重
#案例：查询员工表中涉及到的所有部门编号
SELECT DISTINCT department_id FROM employees;
4. The role of the plus sign and the string splicing function: concat();
#+号的作用
-- java中的+号
-- 1、运算符，两个操作数都为数值型
-- 2、连接符，只要有一个操作数为字符串
--
-- MySQL中，只有一个功能：运算符
-- select 100+90;这就是加法。
-- 两个操作数为数值型，做加法运算；
-- select '123' +90，有其中一方为字符型，试图将字符型数值转换成数值型
-- 如果转换成功，则继续做加法运算；
-- 如果转换失败，如select 'john' +90
-- 则将字符型数值转换成0，然后继续加。
-- 这里答案是90.
--
-- 如果其中一方为Null，不管另一方是什么，则结果肯定为Null.
如果要字符串拼接，使用concat函数。如下：
CONCAT('a','b','c') AS 结果；
#案例：查询员工名和姓，连接成一个字段，并显示为 姓名
SELECT CONCAT(last_name,first_name) AS 姓名 FROM employees; 
2. Conditional query
overall framework:
Advanced 2: Conditional query
Syntax:
SELECT query list FROM table name WHERE filter condition;
Classification:
1. Filter by conditional expression
Conditional operator:> <= <> >= <=
2. Filter by logical expression
Logical operator: && ||!
AND OR NOT
3. Fuzzy query
LIKE
BETWEEN AND
IN
IS NULL
-- 一 按条件表达式筛选
-- 查询工资大于12000的员工信息
SELECT * FROM employees WHERE salary>12000;
-- 案例2：查询部门编号不等于90号的员工名和部门编号
SELECT
last_name,department_id
FROM
employees
WHERE
department_id <> 90;
按逻辑表达式筛选
查询工资在10000到20000之间的员工名、工资以及奖金
SELECT
last_name,salary,commission_pct
FROM
employees
WHERE
salary>=10000 AND salary<=20000;
案例2 查询部门编号不是在90到110之间，或者工资高于15000的员工信息
SELECT
*
FROM
employees
WHERE
NOT(department_id>=90 AND department_id<=110) OR salary>15000;Fuzzy query:
三、模糊查询
LIKE
BETWEEN AND
IN
IS NULL
IS NOT NULL
1.like
案例 查询员工名中包含字符a的员工信息
SELECT
*
FROM
employees
WHERE
last_name LIKE '%a%';
百分号代表任意位置，a的前面可能有字符，也可能后面有字符
案例2 查询员工中第三个字符为e，第五个字符为a的员工名和工资
前面单个字符用下划线代替
SELECT
last_name,
salary
FROM
employees
WHERE
last_name LIKE '__e_a%';
案例3 查询员工名中第二个字符为_的员工名
SELECT
last_name
FROM
employees
WHERE
last_name LIKE '_\_%';
在需要的字符前面加上\即可转义。
2、 BETWEEN AND
查询员工编号在100到120之间的员工信息
SELECT
*
FROM
employees
WHERE
employee_id BETWEEN 100 AND 120;
可以提高语句简洁度
两个临界值需要升序。
3、in关键字
查询员工的工种编号是 IT_PROG、AD_VP AD_PRES中的一个员工名和工种编号
SELECT
last_name,
job_id
FROM
employees
WHERE
job_id = 'IT_PROT' OR job_id = 'AD_VP' OR job_id = 'AD_PRES';
SELECT
last_name,
job_id
FROM
employees
WHERE
job_id IN ('IT_PROT','AD_VP','AD_PRES');
特点：1.提高语句简洁度
2.in列表的值必须一致或兼容
3.不支持模糊搜索，不能用%或者_
4.is NULL
案例：查询没有奖金的员工名和奖金率
SELECT
last_name,
commission_pct
FROM
employees
WHERE
commission_pct IS NULL;
有的话，就是 IS NOT NULL;
=或<>不能用于判断null值
Three, sort query
排序查询
语法：
SELECT 查询列表
FROM 表
【where 筛选条件】
ORDER BY 排序列表 ASC|DESC
案例 查询员工信息，要求工资从高到低排序
SELECT
*
FROM
employees
ORDER BY
salary DESC;
默认升序
案例2 查询部门编号大于等于90的员工信息，按入职时间的先后进行排序
SELECT
*
FROM
employees
WHERE
department_id >=90
ORDER BY
hiredate ASC;
案例3 按表达式排序 按年薪的高低显示员工信息和年薪
SELECT
*,salary*12*(1+IFNULL(commission_pct,0)) AS 年薪
FROM
employees
ORDER BY
salary*12*(1+IFNULL(commission_pct,0)) DESC;
案例4 按年薪的高低显示员工的信息和年薪 【按别名排序】
SELECT *,salary*12*(1+IFNULL(commission_pct,0)) AS 年薪
FROM
employees
ORDER BY
年薪;
案例5：按姓名的长度显示员工的姓名和工资【按函数排序】
SELECT LENGTH(last_name) AS 字节长度,last_name,salary
FROM employees
ORDER BY 字节长度 DESC;
案例6：查询员工信息，要求先按工资排序，再按员工编号排序【按多个字段排序】
SELECT
*
FROM
employees
ORDER BY
salary ASC,employee_id ASC;
ORDER BY 字句可以支持单个字段，多个字段，表达式，函数，别名
ORDER BY 字句一般放在查询语句的最后面，limit字句除外
案例：查询员工的姓名和部门号和年薪，按年薪降序，按姓名升序
SELECT
last_name,department_id,salary*12*(1+IFNULL(commission_pct,0)) AS 年薪
FROM
employees
ORDER BY
年薪 DESC,last_name ASC;Four, common functions
One, one-line function
1. Character function
concat stitching
substr intercept substring
convert upper to uppercase
lower case
trim to remove the specified spaces and characters before and after
ltrim go to the left space
rtrim goes to the right space
replace
lpad left padding
rpad right padding
instr returns the index of the first occurrence of the substring
length Get the number of bytes
2. Mathematical functions
round round
rand random number
floor rounded down
ceil improvement arrangement
mod take over
truncate
3. Date function
now current system date+time
curdate current system date
curtime current system time
str_to_date converts characters to dates
date_format converts dates into characters
4. Process control function
if handles double branches
case statement handles multiple branches
Case 1: Processing equivalence judgment
SELECT salary AS 原始工资,department_id,
CASE department_id
WHEN 30 THEN salary*1.1
WHEN 40 THEN salary*1.2
WHEN 50 THEN salary*1.3
ELSE salary
END AS 新工资
FROM employees; 
Case 2: Judgment of processing conditions
5. Other functions
version version
database current library
user currently connected user
Five, grouping function
sum
max
min minimum
avg average
count count
Features:
1. The above five grouping functions ignore null values, except for count(*)
2. Sum and avg are generally used to process numerical values max, min, count, and can process any data type
3. Both can be used with distinct to count the results after deduplication
4. The parameter of count can support:
Field, *, constant value, generally put 1
It is recommended to use count(*)
Exercise:
1.查询公司员工工资的最大值，最小值，平均值，总和
SELECT
MAX(salary) mx_sal,
MIN(salary) xi_sal,
ROUND(AVG(salary),2) ag_sal,
SUM(salary) sm_sal
FROM
employees;
round保留2位小数
2.查询员工表中的最大入职时间和最小入职时间的相差天数
SELECT DATEDIFF(MAX(hiredate),MIN(hiredate));
3.查询部门编号为90的员工个数
SELECT COUNT(*) 个数
FROM employees
WHERE department_id = 90;Six, group query
1.查询每个工种的最高工资
SELECT
MAX(salary),job_id
FROM
employees
GROUP BY
job_id;
2.查询每个位置上的部门个数
SELECT COUNT(*),location_id
FROM
departments
GROUP BY
location_id;
添加筛选条件
1.查询邮箱中包含a字符的，每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;
案例2：查询有奖金的每个领导手下员工的最高工资
SELECT
MAX(salary),manager_id
FROM
employees
WHERE
commission_pct IS NOT NULL
GROUP BY
manager_id;
添加复杂筛选条件
1.查询哪个部门的员工个数>2
1 查询每个部门的员工个数
2 筛选出大于2的部门
having用于分组后的筛选
SELECT
department_id,COUNT(*)
FROM
employees
GROUP BY
department_id
HAVING COUNT(*)>2;
案例2：查询每个工种有奖金的员工的最高工资>12000的工种编号和最高工资
1 查询每个工种有奖金的员工的最高工资
SELECT
MAX(salary),job_id
FROM
employees
WHERE
commission_pct IS NOT NULL
GROUP BY
job_id;
2 根据1的结果继续筛选，最高工资>12000
SELECT
MAX(salary),job_id
FROM
employees
WHERE
commission_pct IS NOT NULL
GROUP BY
job_id
HAVING
MAX(salary)>12000;
案例3：查询领导编号>102的每个领导手下的最低工资>5000的领导编号
以及其最低工资
SELECT
MIN(salary),manager_id
FROM
employees
WHERE
manager_id >102
GROUP BY
manager_id
HAVING
MIN(salary)>5000;
分组查询的筛选条件分为两类
数据源不一样
分组前筛选：原始表 使用where 在 GROUP BY 前面
分组后筛选：分组后的结果集 使用having 在 GROUP BY 后面
分组函数做条件肯定是放在having子句中
能用分组前筛选的，就优先考虑分组前。Group by expression or function, and summarize:
按表达式或函数分组
案例：按员工姓名长度分组
查询每一组的员工个数，筛选员工个数大于5的有哪些
SELECT
COUNT(*),LENGTH(last_name) len_name
FROM
employees
GROUP BY
LENGTH(last_name)
HAVING
COUNT(*) >5;
按多个字段分组
案例：查询每个部门每个工种的员工的平均工资
SELECT
AVG(salary),department_id,job_id
FROM
employees
GROUP BY
department_id,job_id;
#添加排序
案例 查询每个部门每个工种的员工的平均工资，
并按照平均工资的高低显示
SELECT AVG(salary),department_id,job_id
FROM
employees
GROUP BY
job_id,department_id
ORDER BY AVG(salary) DESC;
GROUP BY 子句支持单个字段分组，多个字段分组，
多个字段用逗号隔开没有顺序要求，表达式或函数（用的较少）
也可以添加排序（放在最后）
练习：
1.查询各job_id的员工工资的最大值，最小值，平均值，总和，并按照job_id升序
SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY job_id ASC;
2.查询员工最高工资和最低工资的差距
SELECT MAX(salary)-MIN(salary) difference
FROM employees;
3.查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，
没有管理者的员工不计算在内
SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary)>=6000;
4.查询所有部门的编号，员工数量和工资平均值，并按照平均工资降序
SELECT department_id,COUNT(*),AVG(salary)
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) DESC;
5.选择具有各个job_id的员工人数
SELECT COUNT(*),job_id
FROM employees
WHERE job_id IS NOT NULL
GROUP BY job_Id;Seven, connection query
Meaning: also known as multi-table query
Cartesian product phenomenon
Table 1 has m rows, Table 2 has n rows, and the
result has m*n rows
Cause: No valid connection conditions
How to avoid: Add valid connection conditions
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.boyfriend_id = boys.id;
Remember to add scope!
Classification: Classification
by age: SQL 92 only supports inner connection
SQL 99 supports inner connection + outer connection (left and right) + cross connectionClassified by function:
Internal connection:
Equivalent connection
Non-equivalent connection
Self-connectionExternal connection:
Left external connection
Right external connection
Full external connectionCross connect
1. Equivalent connection
1.等值连接
案例1 查询女神名和对应男神名
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.boyfriend_id = boys.id;
USE myemployees;
#案例2 查询员工名和对应的部门名
SELECT last_name,department_name
FROM employees,departments
WHERE employees.`department_id` = departments.`department_id`;
2.查询员工号，工种号，工种名
SELECT last_name,employees.job_id,job_title
FROM employees,jobs
WHERE employees.`job_id` = jobs.`job_id`;
3.查询有奖金的员工名，部门名
使用AND加筛选
SELECT last_name,department_name,commission_pct
FROM employees e,departments d
WHERE e.department_id = d.department_id
AND e.commission_pct IS NOT NULL;
案例2 查询城市名中第二个字符为o的部门名和城市名
SELECT department_name,city
FROM departments d,locations l
WHERE d.location_id = l.location_id
AND l.city LIKE '_o%';
是否能加分组？
案例1：查询每个城市的部门个数
SELECT COUNT(*),city
FROM departments d,locations l
GROUP BY city;
案例2 查询有奖金的每个部门的部门名和部门的领导编号，和该部门的最低工资
SELECT department_name,d.manager_id,MIN(salary)
FROM departments d,employees e
WHERE d.department_id = e.department_id
AND commission_pct IS NOT NULL
GROUP BY department_name,d.manager_id;
三表连接：
案例 查询员工名，部门名和所在城市
SELECT
e.last_name,d.department_name,l.city
FROM
employees e,departments d,locations l
WHERE
e.department_id = d.department_id
AND
d.location_id = l.location_id;
2. Non-equivalent connection
Use the same table as two tables
Query employee's salary and salary level
SELECT salary,grade_level
FROM employees e,job_grades g
WHERE salary BETWEEN g.lowest_Sal AND g.highest_sal;3. Self-connection
Case: Query the name of the employee and the name of the superior
Seven*, internal and external links of SQL99 grammar
Syntax:
SELECT query list
FROM table 1 alias [connection type]
JOIN table 1 alias [connection type]
ON connection condition
[where filter condition]
[group BY grouping]
[having filter condition]
[order BY sort list]Category:
inner connection: inner
left outer connection: left OUTER
right outer connection: right OUTER
full outer connection: full OUTER
cross connection: cross
1. Equivalent connection
1 等值连接
案例一 查询员工名，部门名
SELECT last_name,department_name
FROM employees e
INNER JOIN departments d
ON e.department_id = d.department_id;
案例二 查询名字中包含e的员工名和工种名（筛选）
SELECT last_name,job_title
FROM employees e
INNER JOIN jobs j
ON e.job_id = j.job_id
WHERE last_name LIKE '%e%';
案例三 查询部门个数>3的城市名和部门个数（添加分组和筛选）
SELECT COUNT(*),city,department_id
FROM departments d
INNER JOIN locations l
ON d.location_id = l.location_id
GROUP BY city
HAVING COUNT(*) > 3;
案例4 查询哪个部门的员工个数>3的部门名和员工个数，并按个数降序
SELECT e.department_id,department_name,COUNT(*)
FROM departments d
INNER JOIN employees e
ON d.department_id = e.department_id
GROUP BY department_name
HAVING COUNT(*) > 3
ORDER BY COUNT(*) DESC;
案例5 查询员工名、部门名、工种名，并按部门名降序
SELECT last_name,department_name,job_title
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
INNER JOIN jobs j ON e.job_id = j.job_id
ORDER BY d.department_name DESC;2. Non-equivalent connection
查询员工工资级别
SELECT salary,grade_level
FROM employees e
INNER JOIN job_grades g
ON e.salary BETWEEN g.lowest_sal AND g.highest_sal;
查询工资级别>2的个数，并且按工资级别降序
SELECT COUNT(*),salary,grade_level
FROM employees e
INNER JOIN job_grades g
ON e.salary BETWEEN g.lowest_sal AND g.highest_sal;
GROUP BY grade_level
HAVING COUNT(*) >20
ORDER BY grade_level DESC;3. Self-connection
自连接
查询员工名字，上级的名字
SELECT e.last_name,m.last_name
FROM employees e
INNER JOIN employees m
ON e.manager_id = m.employee_id;Outer join
The outer join is used to query the situation where there is in one table but not in one table.
There are master table and slave table
Outer join query records all records in the main table
If there is a match from the table, match
If there is no match, it becomes NULL
Outer join result = inner join result + records in the master table but not in the slave table.
Left outer join: the main table left join from the table
Right outer join: right join the main table from the table
The same effect can be achieved by swapping the order of the left outer and right outer.
外连接
引入 查询男朋友不在男神表的女神名
USE girls;
外连接用于查询一个表中有，一个表中没有的情况
引入 查询男朋友不在男神表的女神名
SELECT b.name,boys.*
FROM beauty b
LEFT OUTER JOIN boys
ON b.boyfriend_id = boys.id;
USE myemployees;
案例1 查询哪个部门没有员工
左外
SELECT d.department_name,employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.department_id = e.department_id
WHERE e.employee_id IS NULL;
右外
SELECT d.department_name,employee_id
FROM employees e
RIGHT OUTER JOIN departments d
ON d.department_id = e.department_id
WHERE e.employee_id IS NULL;4. Full external connection
5. Cross-connect
That is, the Cartesian product.
Eight, subqueries
The select statement that appears in other statements is called a subquery or inner query. The
external query statement is called the main query
Example:
Classification:
According to the position where the subquery appears:
behind select
Only supports scalar subquery
from behind
Table subquery
where or having [important]
scalar subquery,
column subquery,
row subquery
After exists (related subquery)
table subquery
According to the number of rows and columns of the result set
Scalar subquery: The result set has only one row and one column.
Row subquery: The result set has one row and multiple columns.
Column subquery: The result set has only one column and multiple rows.
Table subquery: The result set is generally multiple rows and multiple columns.
1. Scalar quantum query
一.where或者having后
1.标量子查询
特点：
1.子查询放在小括号内
2.子查询一般放在条件的右侧
3.标量子查询，一般搭配着单行操作符使用
4.列子查询，一般搭配着多行操作符使用
标量子查询
案例一：谁的工资比阿贝尔高？
1.查询阿贝尔的工资
USE myemployees;
SELECT salary
FROM employees
WHERE last_name = 'Abel';
SELECT last_name
FROM employees
WHERE salary>(SELECT salary
FROM employees
WHERE last_name = 'Abel');
案例2 返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id和工资
SELECT job_id
FROM employees
WHERE employee_id = 141;
SELECT salary
FROM employees
WHERE employee_id = 143;
SELECT last_name,job_id,salary
FROM employees
WHERE
job_id = (SELECT job_id
FROM employees
WHERE employee_id = 141)
AND
salary>(SELECT salary
FROM employees
WHERE employee_id = 143);
案例3：返回公司工资最少的员工的信息
SELECT last_name,job_id,salary
FROM employees
WHERE salary =(SELECT MIN(salary)
FROM employees) ;2. Column sub-query (multi-line sub-query)
Multiple rows in one column
案例1：返回location_id是1400或1700的部门中所有员工姓名
1.先查location_id是1400或1700的部门编号
SELECT department_id
FROM departments
WHERE location_id = 1400 OR location_id = 1700;
2.再查询姓名，要求部门号是1中某一个
SELECT last_name
FROM employees
WHERE department_id IN
(SELECT department_id
FROM departments
WHERE location_id = 1400 OR location_id = 1700);
3.返回其他部门中比job_id为 IT_PROG 部门任一工资低的员工的员工号，姓名，job_id以及salary
SELECT salary
FROM employees
WHERE job_id ='IT_PROG';
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ANY
(SELECT DISTINCT salary
FROM employees
WHERE job_id ='IT_PROG')AND job_id<>'IT_PORG';
4.返回其他部门中比job_id为 IT_PROG 部门所有工资低的员工的员工号，姓名，job_id以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ALL
(SELECT DISTINCT salary
FROM employees
WHERE job_id ='IT_PROG')AND job_id<>'IT_PORG';3. Subquery placed after select
4. Subquery placed after exists
See if there is any value after the subquery
(In way)
(The way of exists)
Nine, pagination query
Application scenarios:
In the actual web project, the corresponding paging query SQL statement needs to be submitted according to the user's needs
grammar:
select field|expression,...
from 表
[Where condition]
[Group by group field]
[Having conditions]
[Order by sorted field]
limit [starting item index,] the number of items;
Features:
1. The index of the starting entry starts from 0
2. The limit clause is placed at the end of the query statement
3.公式：select * from 表 limit （page-1）*sizePerPage,sizePerPage
if:
Number of items displayed per page sizePerPage
The number of pages to display page
案例1 查询前五条员工信息
SELECT *
FROM employees
LIMIT 0,5;
案例2 查询第11条~第25条
SELECT *
FROM employees
LIMIT 10,15;
案例3 有奖金的员工信息，并且工资较高的前10名显示出来
SELECT *
FROM employees
WHERE commission_pct IS NOT NULL
ORDER BY salary DESC
LIMIT 0,10;
特点：
1.limit语句放在查询语句最后
2.公式：
要显示的页数是page
每页条目数是size
SELECT 查询列表 FROM 表
LIMIT (page-1)*size,size;
10. Union joint query
Introduce:
union union, merge
grammar:
select field|constant|expression|function [from table] [where condition] union [all]
select field|constant|expression|function [from table] [where condition] union [all]
select field|constant|expression|function [from table] [where condition] union [all]
.....
select field|constant|expression|function [from table] [where condition]
Features:
1. The number of columns in multiple query statements must be consistent
2. The query column types of multiple query statements are almost the same
3. Union means de-duplication, union all means not de-duplication
联合查询
UNION 联合，合并 将多条查询语句的结果合并成一个结果
引入的案例：查询部门编号大于90或者邮箱中包含a的员工信息
SELECT *
FROM employees
WHERE email LIKE '%a%'
UNION
SELECT *
FROM employees
WHERE department_id>90;
应用场景
查询中国用户中男性的信息以及外国用户中男性的信息
SELECT id,cname,csex
FROM t_ca
WHERE csex = '男'
UNION
SELECT t_id,tName,tGender
FROM t_ua
WHERE tGender = 'male';