MySQL connection query

Mysql Link query

Mysql Link query

Database data preparation

--  Create department table 
CREATE TABLE dept (
  id INT PRIMARY KEY AUTO_INCREMENT,
  NAME VARCHAR(20)
);

INSERT INTO dept (NAME) VALUES (' Development Department '),(' The Marketing Department '),(' Finance Department ');

--  Create an employee table 
CREATE TABLE emp (
  id INT PRIMARY KEY AUTO_INCREMENT,
  NAME VARCHAR(10),
  gender CHAR(1),   --  Gender 
  salary DOUBLE,   --  Wages 
  join_date DATE,  --  Date of entry 
  dept_id INT
);

INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' The Monkey King ',' male ',7200,'2013-02-24',1);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Pig eight quit ',' male ',3600,'2010-12-02',2);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Tang's monk ',' male ',9000,'2008-08-08',2);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Bones jing ',' Woman ',5000,'2015-10-07',3);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Spider essence ',' Woman ',4500,'2011-03-14',1);

Internal connection

Implicit inner join

# Implicit inner join 
SELECT * FROM emp,dept WHERE emp.id = dept.id;

Explicit inner connection

# Explicit inner connection 
SELECT * FROM emp INNER JOIN dept ON emp.id = dept.id;

Summary ：

• The internal connection query is the public part , Meet the connection conditions ( Master foreign key relationship ) Part of
• Use the records of the left table to match the records of the right table , If the conditions are met, then

External connection

The left outer join

# The left outer join 
SELECT * FROM emp LEFT JOIN dept ON emp.id = dept.id;

Right connection

# Right connection 
SELECT * FROM emp RIGHT JOIN dept ON emp.id = dept.id;

Summary ：

• The left outer join ： On the basis of inner connection, ensure that all the data in the left table are displayed
• Right connection ： On the basis of inner connection, ensure that all the data in the table on the right are displayed

Subquery

What is subquery ？
A query contains at least 2 individual select

Three cases of sub query results

The subquery result is a value

problem 1： Find out who the highest paid employees are
decompose : Query maximum wage

SELECT MAX(salary) FROM emp;

Check salary is 9000 The employees' （ That is to query the highest paid employees ）

SELECT * FROM emp WHERE salary=(SELECT MAX(salary) FROM emp);

When the sub query result is single column and multiple rows

problem 1： Query the employees whose salary is less than the average salary
decompose : Query average wage

 SELECT AVG(salary) FROM emp;

Query employees whose salary is less than the average salary

SELECT * FROM emp WHERE salary < (SELECT AVG(salary) FROM emp);

The result of subquery is multi row and multi column

problem 1： Query salary greater than 5000 The employees' , Names from which departments

decompose ： Query salary greater than 5000 The employees'

  SELECT dept_id FROM emp WHERE salary > 5000;

Query the Department name according to the sub query results

SELECT dept.name FROM dept WHERE dept.id IN (SELECT dept_id FROM emp WHERE salary > 5000);