Mysql database foundation: sub query

【MySQL Database foundation ：8. Subquery 】 

author ：zhenyi

special column ：mysql database 1

brief introduction ：MySQL Basic column , fit MySQL Learning and reading in the basic stage .

Documents used in the article , Contact the author for ！ If it helps you , Pay attention , Ongoing update , Thank you for your support . If there is a problem or a mistake , Welcome to point out your personal letter and contact me , thank you .

Catalog

One 、where or having Back

1. Scalar subquery

example 1： Whose pay ratio is Abel high ?

example 2： return job_id And 141 Same as employee No ,salary Than 143 Employees with more employees full name ,job_id And wages

example 3： Return to the lowest paid employees of the company last_name,job_id and salary

  example 4： The minimum wage is greater than 50 The Department of minimum wage id And its minimum wage

2. Column query （ Multi line sub query ）

example 1： return location_id yes 1400 or 1700 The names of all employees in the Department

example 2： Go back to other jobs job_id by ‘IT_PROG’ The employee number of any employee with a low wage 、 full name 、job_id as well as salary

example 3： Return to other departments job_id by ‘IT_PROG’ Employee number of all employees in the Department with low salary 、 full name 、job_id as well as salary

3、 Line sub query （ The result set has one row and multiple columns or multiple rows and multiple columns ）

example 1： Query the employee information with the lowest employee number and the highest salary

Two 、select Back

example 1： Query the number of employees in each department

3、 ... and 、from Back

example 1： Query the average wage level of each department

Four 、exists Back （ Correlation subquery ）

example 1： Check the Department name of the employee

---

Subquery meaning ：
In other statements select sentence , Called subquery or inner query
External query statements , Called main query or external query

classification ：

Click the location where the subquery appears ：
    select Back ：
         Only scalar subqueries are supported
    from Back ：
         Support table subquery
    where or having Back ：
         Scalar subquery （ A single ） 
         Column query  （ Multiple lines ） 
         Line sub query
    exists Back （ Correlation subquery ）
         Table sub query

Depending on the number of rows and columns in the result set ：
     Scalar subquery （ The result set has only one row and one column ）
     Column query （ The result set has only one column and multiple rows ）
     Line sub query （ The result set has one row and more columns ）
     Table sub query （ The result set is usually multi row and multi column ）

One 、where or having Back

1、 Scalar subquery （ Single line sub query ）
2、 Column query （ Multi line sub query ）

3、 Line sub query （ More columns, more lines ）

characteristic ：
① Subqueries in parentheses
② Subqueries are usually placed on the right side of the condition
③ Scalar subquery , Usually used with a single line operator
> < >= <= = <>

Column query , Usually used with multiline operators
in、any/some、all

④ The execution of the subquery takes precedence over the execution of the main query , The conditions of the main query use the results of the sub query

1. Scalar subquery

example 1： Whose pay ratio is Abel high ?

SELECT *
FROM employees
WHERE salary>(

    SELECT salary
    FROM employees
    WHERE last_name = 'Abel'

);

SELECT *
FROM employees
WHERE salary >(
	SELECT salary
	FROM employees
	WHERE last_name='Abel'
);

The query results are as follows ：

example 2： return job_id And 141 Same as employee No ,salary Than 143 Employees with more employees full name ,job_id And wages

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

);

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
);

The query results are as follows ：

example 3： Return to the lowest paid employees of the company last_name,job_id and salary

SELECT last_name,job_id,salary
FROM employees
WHERE salary=(
    SELECT MIN(salary)
    FROM employees
);

SELECT last_name,job_id,salary
FROM employees
WHERE salary =(
			SELECT MIN(salary)
			FROM employees
);

The query results are as follows ：

  example 4： The minimum wage is greater than 50 The Department of minimum wage id And its minimum wage

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT  MIN(salary)
    FROM employees
    WHERE department_id = 50
);

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
	SELECT MIN(salary)
	FROM employees
	WHERE department_id=50
);

The query results are as follows ：

 

Illegal use of scalar subqueries

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT  salary
    FROM employees
    WHERE department_id = 250
);

2. Column query （ Multi line sub query ）

example 1： return location_id yes 1400 or 1700 The names of all employees in the Department

SELECT last_name
FROM employees
WHERE department_id  <>ALL(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);

SELECT last_name
FROM employees
WHERE department_id <>ALL(
		SELECT DISTINCT department_id
		FROM departments
		WHERE location_id IN(1400,1700)
);

The query results are as follows ：

 

example 2： Go back to other jobs job_id by ‘IT_PROG’ The employee number of any employee with a low wage 、 full name 、job_id as well as salary

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

or
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MAX(salary)
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
		SELECT DISTINCT salary
		FROM employees
		WHERE job_id ='IT_PROG'
)AND job_id<>'IT_PROG';

The query results are as follows ：

 

example 3： Return to other departments job_id by ‘IT_PROG’ Employee number of all employees in the Department with low salary 、 full name 、job_id as well as salary

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

or

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MIN( salary)
    FROM employees
    WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
			SELECT MIN(salary)
			FROM employees
			WHERE job_id='IT_PROG'
) AND job_id <>'IT_PROG';

The query results are as follows ：

 

3、 Line sub query （ The result set has one row and multiple columns or multiple rows and multiple columns ）

example 1： Query the employee information with the lowest employee number and the highest salary

SELECT *
FROM employees
WHERE employee_id=(
    SELECT MIN(employee_id)
    FROM employees
)AND salary=(
    SELECT MAX(salary)
    FROM employees

);

SELECT *
FROM employees
WHERE employee_id=(
			SELECT MIN(employee_id)
			FROM employees
) AND salary=(
			SELECT MAX(salary)
			FROM employees
);

The query results are as follows ：

 

Two 、select Back

Only scalar subqueries are supported

example 1： Query the number of employees in each department

SELECT d.*,(SELECT COUNT(*) FROM employees AS e WHERE e.department_id=d.department_id) AS Number
FROM departments AS d;

SELECT d.*,(SELECT COUNT(*) FROM employees AS e WHERE e.department_id=d.department_id) AS  Number 
FROM departments AS d;

The query results are as follows ：

 

3、 ... and 、from Back

Use the subquery results as a table , The alias is required

example 1： Query the average wage level of each department

① Query the average salary of each department
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id

SELECT * FROM job_grades;

#② Connect ① The result set and job_grades surface , Screening criteria average wage between lowest_sal and highest_sal

SELECT  ag_dep.*,g.`grade_level`
FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

Four 、exists Back （ Correlation subquery ）

grammar ：
exists( Complete query statement )
result ：
1 or 0

SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);

example 1： Check the Department name of the employee

SELECT department_name
FROM departments d
WHERE EXISTS(
    SELECT *
    FROM employees e
    WHERE d.`department_id`=e.`department_id`
);

SELECT department_name
FROM departments AS d
WHERE EXISTS(
			SELECT *
			FROM employees AS e
			WHERE d.department_id=e.department_id
);

The query results are as follows ：

 

If it helps you , Pay attention , Ongoing update , Thank you for your support . If there is a problem or a mistake , Welcome to contact me , thank you .