Mysql-16-subquery

Subquery refers to a query that is nested within another query statement by one query statement , This feature comes from MySQL 4.1 Start introducing .

SQL The use of neutron queries has greatly enhanced SELECT The ability to query , Because many times queries need to get data from the result set , Or you need to calculate a data result from the same table first , Then with this data result （ It could be some scalar , It could also be a collection ） Compare .

1. Demand analysis and problem solving

1.1 Practical problems

Existing solutions ：

# Mode one ：
SELECT salary
FROM employees
WHERE last_name = 'Abel';
SELECT last_name,salary
FROM employees
WHERE salary > 11000;

# Mode two ： Self join 
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name = 'Abel'
AND e1.`salary` < e2.`salary`

# Mode three ： Subquery 
SELECT last_name,salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);

1.2 The basic use of subqueries

• The basic syntax structure of subquery ：
  
• Subquery （ Internal query ） Execute once before the main query .
• The results of the sub query are by the main query （ External query ） Use .
• matters needing attention
  • Subqueries should be enclosed in parentheses
  • Place the subquery to the right of the comparison criteria
  • Single line operators correspond to single line subqueries , Multiline operators correspond to multiline subqueries

1.3 Classification of subqueries

Classification 1：
We return one or more records according to the results of internal query , Sub queries are divided into Single line sub query 、 Multi line sub query .

• Single line sub query
  

• Multi line sub query
  
  Classification 2：

We check whether the query is executed more than once , Divide the subquery into relevant ( Or associated ) Subquery and Unrelated ( Or unrelated ) Subquery .

The subquery queries the data results from the data table , If this data result is executed only once , The data result is then executed as a condition of the main query , Then such a subquery is called an unrelated subquery .

Again , If the subquery needs to be executed multiple times , That is, in the form of circulation , Start with an external query , Each time, the sub query is passed in for query
Inquiry , The results are then fed back to the outside world , This nested execution is called related subquery .

2. Single line sub query

2.1 Single line comparison operator

The operator	meaning
=	equal to
>	greater than
>=	greater than or equal to
<	less than
<=	less than or equal to
<>	not equal to

2.2 Code example

subject ： Query salary greater than 149 Information of the employee whose salary is No


subject ： return job_id And 141 Same as employee No ,salary Than 143 The name of the employee with a large number of employees ,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);

subject ： 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);

subject ： Query and 141 Number or 174 Of employee No manager_id and department_id Of the same other employees employee_id,manager_id,department_id

 Realization way 1： Unpaired comparison 
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id IN
           (SELECT manager_id
                  FROM employees
                  WHERE employee_id IN (174,141))
AND department_id IN
           (SELECT department_id
                  FROM employees
                  WHERE employee_id IN (174,141))
AND employee_id NOT IN(174,141);

 Realization way 2： Compare in pairs 
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id, department_id) IN
               (SELECT manager_id, department_id
                FROM employees
                WHERE employee_id IN (141,174))
AND employee_id NOT IN (141,174);

2.3 HAVING Subqueries in

• First, execute the subquery .
• To the... In the main query HAVING Clause returns the result .
  subject ： The minimum wage is greater than 50 The Department of minimum wage id And its minimum wage

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

2.4 CASE Subqueries in

stay CASE Single column subqueries are used in expressions ：

subject ： Explicit employee employee_id,last_name and location. among , If employee department_id And location_id by 1800 Of department_id identical , be location by ’Canada’, The rest are ’USA’.

SELECT employee_id, last_name,
             (CASE department_id
              WHEN
                          (SELECT department_id FROM departments
              WHERE location_id = 1800)
             THEN 'Canada' ELSE 'USA' END) location
FROM employees;

2.5 Null value problem in subquery

SELECT last_name, job_id
FROM employees
WHERE job_id =
               (SELECT job_id
                FROM employees
               WHERE last_name = 'Haas');

The subquery does not return any rows

2.5 Illegal use of subquery

SELECT employee_id, last_name
FROM employees
WHERE salary =
              (SELECT MIN(salary)
               FROM employees
               GROUP BY department_id);

Multi line subqueries use single line comparators

3. Multi line sub query

• Also known as set comparison subquery
• Inner query returns multiple rows
• Use the multiline comparison operator

3.1 Multiline comparison operator

The operator	meaning
IN	Equal to any one of the list
ANY	It needs to be used with the single line comparison operator , Compare with a value returned by a subquery
ALL	It needs to be used with the single line comparison operator , Compare with all the values returned by the subquery
SOME	It's actually ANY Another name for , The same effect , Commonly used ANY

experience ANY and ALL The difference between

3.2 Code example

subject ： Return to other job_id Middle ratio job_id by ‘IT_PROG’ Employee number of any low paid employee in the Department 、 full name 、job_id as well as salary


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


subject ： Query the Department with the lowest average wage id

# The way 1：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
            SELECT MIN(avg_sal)
            FROM (
                  SELECT AVG(salary) avg_sal
                  FROM employees
                  GROUP BY department_id
                 ) dept_avg_sal
                      )

# The way 2：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (
                            SELECT AVG(salary) avg_sal
                            FROM employees
                            GROUP BY department_id
                           )

3.3 The null problem

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
             SELECT manager_id
             FROM employees
);

4. Correlation subquery

4.1 Related sub query execution process

If the execution of a subquery depends on an external query , Usually, it is because the tables in the subquery use external tables , Conditional correlation is carried out , So every time you execute an external query , All subqueries have to be recalculated , Such a subquery is called Associated subquery .

Related sub queries are executed row by row , Each row of the main query executes a subquery .


explain ： The columns in the main query are used in the subquery

4.2 Code example

subject ： Query the salary of employees whose salary is greater than the average salary of the Department last_name,salary And its department_id

Mode one ： Correlation subquery

Mode two ： stay FROM Use subqueries in

SELECT last_name,salary,e1.department_id
FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP
BY department_id) e2
WHERE e1.`department_id` = e2.department_id
AND e2.dept_avg_sal < e1.`salary`;

from Subquery of type ： Subquery is used as from Part of , The subquery should use () Lead up , And alias this subquery , Think of it as a “ Temporary virtual table ” To use .

stay ORDER BY Use subqueries in ：

subject ： Check the employee's id,salary, according to department_name Sort

SELECT employee_id,salary
FROM employees e
ORDER BY (
           SELECT department_name
           FROM departments d
           WHERE e.`department_id` = d.`department_id`
);

subject ： if employees In the table employee_id And job_history In the table employee_id The same number is not less than 2, Output these same id Of our employees employee_id,last_name And its job_id

SELECT e.employee_id, last_name,e.job_id
FROM employees e
WHERE 2 <= (SELECT COUNT(*)
            FROM job_history
           WHERE employee_id = e.employee_id);

4.3 EXISTS And NOT EXISTS keyword

• Associated subqueries are usually associated with EXISTS Operators are used together , Used to check whether there are qualified rows in the sub query .

• If there are no qualified rows in the subquery ：

  • Conditional return FALSE
  • Continue to find... In subqueries

• If there are qualified rows in the subquery ：

  • Do not continue to find in subquery
  • Conditional return TRUE

• NOT EXISTS Keyword means that if there is no condition , Then return to TRUE, Otherwise return to FALSE.

subject ： Query company managers employee_id,last_name,job_id,department_id Information

 Mode one ：
SELECT employee_id, last_name, job_id, department_id
FROM employees e1
WHERE EXISTS ( SELECT *
               FROM employees e2
               WHERE e2.manager_id =
                     e1.employee_id);

 Mode two ： Self join 
SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id;

 Mode three ：
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
                      SELECT DISTINCT manager_id
                      FROM employees
                     );

subject ： Inquire about departments In the table , There is no in employees Of the departments in the table department_id and department_name

SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (SELECT 'X'
                  FROM employees
                  WHERE department_id = d.department_id);

4.4 Related updates

UPDATE table1 alias1
SET column = (SELECT expression
              FROM table2 alias2
              WHERE alias1.column = alias2.column);

Use related subqueries to update data from one table to another .

subject ： stay employees Add one department_name Field , The data is the Department name corresponding to the employee

# 1）
ALTER TABLE employees
ADD(department_name VARCHAR2(14));
# 2）
UPDATE employees e
SET department_name = (SELECT department_name
                        FROM departments d
                        WHERE e.department_id = d.department_id);

4.4 Relevant delete

DELETE FROM table1 alias1
WHERE column operator (SELECT expression
                       FROM table2 alias2
                       WHERE alias1.column = alias2.column);

Use related subqueries to delete data from one table based on data from another .

subject ： Delete table employees in , And emp_history Data from all tables

DELETE FROM employees e
WHERE employee_id in
                    (SELECT employee_id
                     FROM emp_history
                     WHERE employee_id = e.employee_id);

5. Throw a question

problem ： Whose pay ratio is Abel The height of ？

answer ：

# The way 1： Self join 
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name = 'Abel'
AND e1.`salary` < e2.`salary`

# The way 2： Subquery 
SELECT last_name,salary
FROM employees
WHERE salary > (
          SELECT salary
          FROM employees
          WHERE last_name = 'Abel'
);

problem ： Are the above two methods good or bad ？

answer ： Good self connection mode ！
Subqueries can be used in the title , You can also use self connection . In general, it is recommended that you use self connection , Because in many DBMS In the process of processing , The processing speed of self join is much faster than sub query .

It can be understood in this way ： Sub query is actually the condition judgment after query through unknown table , The self connection is to judge the conditions through the known self data table , So in most cases DBMS Self join processing has been optimized in .