MySQL - composite query external connection

Composite query

The basic query

Be careful ：MySQL in ,select What comes out is also a watch , You can use... In a query statement

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There is an employee list emp：

Example ：

• Check that the salary is higher than 500 Or position is MANAGER Of employees , At the same time, their initials should be capitalized J
  
• Sort by department number in ascending order and employee's salary in descending order
  
• Use annual salary to sort in descending order
  
• Displays the name and position of the highest paid employee
  
• Displays information about employees whose wages are higher than the average
  
• Displays the average wage and maximum wage for each department
  
• It shows that the average wage is lower than 2000 Your department number and its average salary
  Be careful ： Not here select deptno, avg(sal) as 'avgsal' from emp group by deptno where avg(sal)<2000;,where It must be group by Before
  and where Is the first operation , Obviously not in line with the meaning of the question , You should use having
  
• Displays the total number of employees for each position , Average wage
  

Multi-table query

There's a database scott There are three tables in it ：dept emp salgrade
select count(*) from dept;
select count(*) from emp;
select count(*) from salgrade;

Their Cartesian product ：

The so-called Cartesian product , Just at the mathematical level , Enumerate all the data combinations of multiple tables , But it doesn't mean , All new data records , It all makes sense , The next step is to screen
Finish the Cartesian product , The essence is that we put all relevant data , Aggregate to a table , The next question , It becomes a single table query , According to the problem surface , Which tables are the data that need to be traced + Correlation field

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Example ：

• Show employee name 、 The employee's salary and the name of the Department
  
• The display department number is 10 The name of your department , Employee name and salary
  
• Show the name of each employee , Wages , And salary level
  

Self join

Self join ： It refers to the connection query in the same table
Example ：

• Show employees FORD The number and name of the superior
  

Subquery

Subquery ： Embedded in other sql Statement select sentence , Also called nested query

Single line sub query

Same as employee table
Example ：

• Show SMITH Employees in the same department
  

Multi line sub query

in keyword ： In which

• Query and 10 The name of the employee with the same position in department No , Position , Wages , Department number , But it doesn't include 10 Their own
  

all keyword ： Must satisfy any

• Show salary ratio Department 30 The names of all the employees with high salaries 、 Salary and department number
  

any keyword ： Satisfy any one of them

• Show salary ratio Department 30 Name of any employee with a higher salary 、 Salary and department number （ Including employees in their own department ）
  

Multi column subquery

There are more than 1 Column names , Such as sub query where (ename, sal)=(select ename,sal from emp)
Example ：

• Query and SMITH All employees in the same department and position , Not included SMITH I am
  

from Clause

Use a subquery as a temporary table （ Be careful , First, according to the requirements of the topic , First confirm the table of sub query ）
Example ：

• Displays the name of each employee whose salary is higher than the average salary of his / her department 、 department 、 Wages 、 Average wage

• • 1. List the average salary of all departments ： select deptno, avg(sal) from emp;
• • 2. Filter out two from the Cartesian product deptno The same data is valid data
• • 3.

• Find the name of the highest paid person in each department 、 Wages 、 department 、 Maximum wage
  

• Display information for each department （ Department name , Number , Address ） And the number of people

• • 1. Subquery
    
• • 2. Multiple tables
    Pay attention to the pit here , front select There are specific column names ,group by Also be sure to bring

Merge query

Merge multiple select The results of the implementation of , You can use the set operator union,union all

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union： Used to obtain the union of two result sets . When using this operator , Duplicate lines in the result set will be automatically removed

• Example ： Pay more than 2500 Or position is MANAGER Find out
  

union all： This operator is used to get the union of two result sets . When using this operator , Duplicate rows in the result set are not removed

• Example ： Pay more than 25000 Or position is MANAGER Find out
  

Inside and outside connection

Internal connection

utilize where Clause to filter the Cartesian product formed by the two tables , That is, the writing method used before

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Standard writing ：select Field from surface 1 inner join surface 2 on Connection condition and Other conditions

External connection

There are two tables ：

The left outer join

If the union query , The table on the left shows that it is a left outer join
Standard writing ： select Field name from Table name 1 left join Table name 2 on Connection condition

• Example ： Check the grades of all the students , If the student has no grades , Also display the student's personal information
  

Right connection

If the union query , The table on the right shows that we call it right outer join
Standard writing ：select Field from Table name 1 right join Table name 2 on Connection condition ;
Example ：

• Yes stu Table and exam Table union query , Show all your grades , Even if there are no students corresponding to this grade , Show it, too
  
• List Department names and employee information for these departments , Also list departments without employees
  right join Empathy , Just exchange the positions of two meters