MySQL development - advanced query - take a good look at how it suits you

One 、 Unit Overview

Through the study of this chapter, I can understand MySQL The meaning of grouping query in database , Master the use of common grouping functions , master GROUP BY Rules for using clauses , Master the conditional filtering of data results after grouping , master SELECT Statement execution procedure , Understand the meaning of subquery , Master the use of single line sub query and multi line sub query

Two 、 Key points and difficulties in teaching

a key ： Master the use of common grouping functions master GROUP BY Rules for using clauses master HAVING Rules for using clauses Master the usage rules of sub query difficulty ： SELECT Statement execution procedure HAVING and WHERE The difference between Single line subquery and multi line subquery

4.1 Why use grouping functions

● Please consider the following questions ？
○ Query the total monthly salary of all employees , Average wage ？
○ Query the maximum and minimum wages ？
○ Check the total number of people in the company ？
○ Query the total number of people with bonuses ？
○ ………..
● Grouping function is to operate on the set of data rows and give a result by group , This result can be output directly , Or used to make judgment conditions

4.2 Overview of grouping functions

4.2.1 Group function

● Grouping function is to operate a group of records in a table , Each group returns only one result , That is, first group the table records , Then the operation summary , Each group returns a result , When grouping, the whole table may be divided into a group , It may also be divided into groups according to conditions .4.2.2 Common grouping functions
● Grouping functions are commonly used in the following five functions ：
○ MIN
○ MAX
○ SUM
○ AVG
○ COUNT

● 4.2.2 Syntax of grouping function

SELECT    [column,] group_function(column)
FROM        table
[WHERE    condition]
[GROUP BY    column]
[HAVING       group_function(column)expression 
[ORDER BY    column |  group_function(column)expression];

4.3 Use of grouping functions

4.3.1 MIN Functions and MAX function

● MIN and MAX The function mainly returns the minimum and maximum values of each group .
○ MIN([DISTINCT | ALL] column | expression)
○ MAX([DISTINCT | ALL] column | expression)

● MIN and MAX Can be used for any data type

● Query the earliest and latest date of employment

SELECT    MIN(hiredate), MAX(hiredate) 
FROM    emp;

4.3.2 SUM Functions and AVG function

● SUM and AVG Function returns the sum and average of each group .
○ SUM([DISTINCT | ALL] column | expression)
○ AVG([DISTINCT | ALL] column | expression)

● SUM and AVG Functions can only operate on columns or expressions of numeric type .

● Query the position with SALES The average salary of all employees at the beginning 、 minimum wage 、 Maximum wage 、 Wages and .

SELECT    AVG(sal), MAX(sal),MIN(sal), SUM(sal)
FROM    emp
WHERE    job LIKE 'SALES%';

4.3.3 COUNT function

1. COUNT Function syntax

● COUNT The main function of the function is to return the number of records in each group that meet the conditions .

COUNT( * | {[DISTINCT | ALL] column | expression})

● COUNT(*)： Returns the number of qualified row records in the table
● Inquiry Department 30 How many employees

SELECT    COUNT(*)
FROM    emp
WHERE    deptno = 30;

2. count Function USES

● COUNT( [DISTINCT | ALL] column | expression)： Return non empty that meets the condition (NULL) The number of lines
● Inquiry Department 30 How many employees receive bonuses

SELECT    COUNT(comm)
FROM    emp
WHERE    deptno = 30;

3. In group functions DISTINCT

● DISTINCT Will eliminate duplicate records before using group functions
● Check the number of departments with employees .

SELECT    COUNT(DISTINCT deptno)
FROM    emp;

4. Group function empty value processing

● except COUNT（*） outside , All other grouping functions ignore null values in columns , And then calculate .

SELECT AVG(comm)
FROM   emp;

5. Use... In grouping functions IFNULL function

● IFNULL Function to force a group function to contain records with null values

SELECT AVG(IFNULL(comm,0))
FROM   emp;

practice

1. Inquiry Department 20 The employees' , The sum and average wage of each month .
2. The inquiry work is in CHICAGO Of employees , The maximum wage and the minimum wage .
3. There are several job types in the employee table .

4.4 Create data group

● Find the average salary of each department , Group by Department

4.4.1 use GROUP BY Clause to create a data group

● adopt GROUP BY Clause can satisfy WHERE The records of conditions are divided into several groups according to the specified columns
○ among GROUP BY Clause specifies the columns to group

SELECT    column, group_function(column)
FROM        table
[WHERE    condition]
[GROUP BY    group_by_expression]
[ORDER BY    column];

4.4.2 GROUP BY Use of clauses

● Look up the number of each department , Average wage

SELECT   deptno, AVG(sal)
FROM     emp
GROUP BY deptno;

4.4.3 GROUP BY Rules for using clauses

● stay SELECT Except for those items in the grouping function in the list , All columns must be included in GROUP BY clause .

SELECT   deptno, AVG(sal)
FROM     emp
GROUP BY deptno;

● GROUP BY The specified column does not have to appear in SELECT In the list .

SELECT   AVG(sal)
FROM     emp
GROUP BY deptno;

4.4.4 Group by multiple columns

● Query the total salary of each position in each department .

● Query the total salary of each position in each department

SELECT   deptno, job, sum(sal)
FROM     emp
GROUP BY deptno, job;

practice

1. Query the department number of each department , Department name , Number of departments , Maximum wage , minimum wage , The sum of wages , Average wage .
2. Check every department , Department number of each position , Department name , Job title , Number of departments , Maximum wage , minimum wage , The sum of wages , Average wage .
3. Check the number of people managed by each manager , Manager number , Name of manager , Requirements include information about people who do not have a manager .

4.5 Exclude group results

4.5.1 Illegal queries using group functions

● Can't be in WHERE Restrict group in clause
● Can pass HAVING Clause restriction group

SELECT     deptno, max(sal)
FROM     emp
WHERE     max(sal) > 2900
GROUP BY     deptno;

4.5.2 use HAVING Clause excludes group results

● Use HAVING Clause restriction group
○ The records have been grouped .
○ Used group functions .
○ And HAVING The result of clause matching is output

SELECT    column, group_function
FROM        table
[WHERE    condition]
[GROUP BY    group_by_expression]
[HAVING    group_condition]
[ORDER BY    column];

4.5.3 HAVING Use of clauses

● Query the maximum salary of each department greater than 2900 Department number of , Maximum wage

SELECT   deptno, max(sal)
FROM     emp
GROUP BY deptno
HAVING   max(sal)>2900;
DEPTNO	MAX(SAL)
10	5000
20	3000
SELECT    job, SUM(sal) PAYROLL
FROM      emp
WHERE      job NOT LIKE 'SALES%'
GROUP BY  job
HAVING    SUM(sal)>5000
ORDER BY  SUM(sal);
JOB	PAYROLL
ANALYST	6000
MANAGER	8275

4.5.4 SELECT Statement execution procedure

● Explain through cases SELECT Statement execution .

SELECT    deptno,job,avg(sal)
FROM      emp
WHERE      job in ('SALESMAN','MANAGER','CLERK')
GROUP BY  deptno,job
HAVING avg(sal)>1000
ORDER BY  3 DESC;

● SELECT Statement execution procedure ：

1. adopt FROM Clause to find the table to query ;
2. adopt WHERE Clause to make non grouping function filtering judgment ;
3. adopt GROUP BY Clause complete grouping operation ;
4. adopt HAVING Clause complete group function filter judgment ;
5. adopt SELECT Clause select the displayed column or expression and group function ;
6. adopt ORDER BY Clause to sort .

practice

1. The number of people in the query department is greater than 2 Department number of , Department name , Number of departments .
2. Query Department average wage greater than 2000, And the number of people is greater than 2 Department number of , Department name , Number of departments , Department average wage , And sort them in ascending order according to the number of people in the Department .

4.6 Overview of subquery

4.6.1 Why use subqueries

● Think about the following ？
○ Check the wage ratio Jones High paid employee information ？
○ Check the name of the lowest paid employee ？
● “ Whose salary is better than Jones It's still high ？”

● Subquery Syntax

SELECT    select_list
FROM    table
WHERE    expr operator
         (SELECT    select_list
           FROM        table);

● Queries in parentheses are called subqueries , Also called internal query , Execute before the main query .
● The results of the sub query are by the main query （ External query ） Use
● expr operator Including comparison operators
○ Single line operators ：>、=、>=、<、<>、<=
○ Multiline operator ： IN、ANY、ALL
● Subqueries can be embedded in the following SQL clause ：
○ WHERE Clause
○ HAVING Clause
○ FROM Clause
4.6.2 Use subquery
● Find out more than JONES For other employees with high wages

SELECT ename
FROM   emp
WHERE  sal > 
        (SELECT sal
         FROM   emp   
         WHERE  ename='JONES');

4.6.3 Subquery type

According to the number of rows and columns returned by the sub query , It is divided into ：

4.6.4 Subquery instructions

● Subqueries should be enclosed in brackets
● Place the subquery to the right of the comparison operator
● For single line subqueries, use single line operators
● For multi row subqueries, use multi row operators

4.7 Single line sub query

4.7.1 Overview of single line sub query

● Subquery only returns one row and one column
● Use the single line operator | Operator | meaning | | ---- | ----- | | = | be equal to | | > | Greater than | | >= | Greater than or equal to | | < | Less than | | <= | Less than or equal to | | <> | It's not equal to |
4.7.2 Single row subquery statement
● Display and employees 7369 Engaged in the same job and paid more than employees 7876 The name and job of the employee .

SELECT   ename, job  
FROM     emp
WHERE    job = 
    (SELECT job    FROM  emp WHERE empno = 7369)  
    AND  sal > 
    (SELECT sal FROM  emp WHERE    empno = 7876);

4.7.3 Group functions are used in subqueries

● Check the name of the lowest paid employee , Position and salary

SELECT    ename, job, sal
FROM    emp
WHERE    sal = 
        (SELECT    MIN(sal) FROM emp);

4.7.4 HAVING Clause using subqueries

● Check the minimum wage ratio of the Department 20 Department number with high minimum wage and minimum wage

SELECT    deptno, MIN(sal)
FROM    emp
GROUP BY    deptno
HAVING    MIN(sal) >
        (SELECT    MIN(sal)
        FROM    emp
        WHERE    deptno = 20);

practice

1. Query the name of the employee with the earliest entry date , Date of entry
2. Check the wage ratio SMITH The salary is high and the place of work is CHICAGO Of employees , Wages , Department name
3. Query the employment date ratio 20 The name of the employee with the earliest entry date in the Department , Date of entry

4.8 Multi line sub query

4.8.1 Overview of multiline subqueries

● The number of records returned by the sub query It can be one or more .
● When comparing with multi row subqueries , You need to use multiline operators , Multiline operators include ：
○ IN
○ ANY
○ ALL
● IN The function of the operator is the same as that described before , Determine whether it is the same as any return value of the sub query .

4.8.2 IN Use

SELECT    ename, sal
FROM    emp
WHERE     empno IN (SELECT mgr
                   FROM  emp);

4.8.3 ANY Use

● ANY： Represents a comparison with any row result of a subquery , There is a condition to be satisfied .
○ < ANY： Indicates that it is smaller than any one in the sub query result set , That is, less than the maximum value .
○ > ANY： Indicates that it is larger than any one in the sub query result set , That is, if it is greater than the minimum value .
○ = ANY： Indicates that it is equal to any one of the sub query results , That is, anyone can , amount to IN.

● Case study 1 The query is the name of the manager's employee , Wages .

SELECT    ename, sal
FROM    emp
WHERE     empno = ANY (SELECT mgr
                   FROM   emp);

● Case study 2 The query department number is not 10, And the wage ratio 10 The employee number of any employee in the Department with high salary , full name , Position , Wages .

SELECT empno, ename, job, sal
FROM   emp
WHERE  sal > ANY (SELECT sal
                FROM   emp
                 WHERE  deptno = 10)
AND    deptno <> 10;

| EMPNO | ENAME | JOB | SAL | | 7499 | ALLEN | SALESMAN | 1600.00 | | 7566 | JONES | MANAGER | 2975.00 | | 7698 | BLAKE | MANAGER | 2850.00 | | 7788 | SCOTT | ANALYST | 3000.00 | | 7844 | TURNER | SALESMAN | 1500.00 | | 7902 | FORD | ANALYST | 3000.00 |

4.8.4 ALL Use

● ALL： Represents a comparison with all row results of a subquery , Each line must meet the conditions .
○ < ALL: Represents all rows smaller than the sub query result set , That is, less than the minimum value .
○ >ALL: Represents all rows larger than the sub query result set , I.e. greater than the maximum .
○ = ALL : Indicates that it is equal to all rows in the sub query result set , Equal to all values , Usually meaningless .
● Case study 1 The query department number is not 20, And the wage ratio 20 The employee number of all employees in the Department with high salary , full name , Position , Wages .

SELECT empno, ename,job, sal
FROM     emp
WHERE     sal > ALL (SELECT sal
                   FROM   emp
                   WHERE  deptno= 20)
AND    deptno <> 20;

● Case study 2 The query department number is not 10, And the wage ratio 10 The employee number of all employees in the Department with low salary , full name , Position , Wages .

SELECT empno, ename,job, sal
FROM     emp
WHERE     sal < ALL (SELECT sal
                   FROM   emp
                   WHERE  deptno= 10)
AND    deptno <> 10;

practice

1. Query the employment date ratio 10 Name of any employee in the Department 、 Date of entry , barring 10 Department staff
2. Query the employment date ratio 10 Name of all employees in the Department 、 Date of entry , barring 10 Department staff
3. Query position and 10 Name of any employee with the same position in the Department , Position , barring 10 Department staff

4.9 Null value in subquery

● Query the names of employees who are not managers .

SELECT    ename
FROM     emp
WHERE     empno NOT IN
    (SELECT mgr
    FROM   emp);

● The result returned by subquery contains null value
● above SQL Statement attempts to find employees who have no subordinates , logically , This SQL The statement should return 8 Bar record , But none of them came back ,why?
● Because there is a null value in the result of the subquery , This null value causes the main query to return no records . This is because all conditions and null value comparison results are null . Therefore, whenever a null value is used, it may become a part of the sub query result set , You can't use it NOT IN Operator

4.10 stay FROM Clause using subqueries

● Query the names of employees with higher average salary than their own department , Wages , Department number , Department average wage

SELECT  a.ename, a.sal, a.deptno, b.salavg
FROM    emp a, (SELECT   deptno, AVG(sal) salavg
                 FROM     emp
                 GROUP BY deptno) b
WHERE   a.deptno = b.deptno  AND    a.sal > b.salavg;

4.11 Summary of this chapter

● MIN Functions and MAX function
● SUM Functions and AVG function
● COUNT function
● Group function null value processing
● adopt GROUP BY Clause to group and summarize
● HAVING Use of clauses
● Single line sub query
● Multi line sub query
● Subquery null value problem

4.12 Homework after class

1. The average salary of the inquiry department is 2500 Department name and average salary above yuan .
2. The query of employee position is not based on “SA” At the beginning, and the average salary is 2500 Positions above RMB yuan and average salary , And in descending order of average wage .
3. The number of people in the inquiry department is 2 Department name with more than people 、 minimum wage 、 Maximum wage .
4. The query position is not SALESMAN, The sum of wages is greater than or equal to 2500 And the salary and .
5. Show manager number and manager name , The minimum wage of the staff managed by this manager , There is no manager KING Also show , Excluding minimum wage less than 3000 Of , Sort the minimum wage from high to low
6. The inquiry salary is higher than the number 7782 The salary of our employees , and 7369 Employee No. the number of the employee who is engaged in the same work 、 Name and salary .
7. Check the name and salary of the highest paid employee .
8. The minimum wage of the inquiry department is higher than 10 Department No. the number of the minimum wage Department 、 Name and department minimum wage .
9. Query the number, name and salary of the employee whose salary is the minimum wage of his department .
10. Show manager is KING Of employees , Wages .
11. Show employees SMITH Names of employees who work late , Wages , Working hours .

Green mountain does not change , Green water flows long ！ Thank you. , I hope it helped you ！