MySQL multi table query

                    **mysql Multi-table query **

# How to realize multi table query
SELECT employee_id,department_name
FROM employees,departments
# I found out 2889 Bar record

SELECT * FROM employees; # Query out 107 Bar record

SELECT 2889/107
FROM DUAL; # The result is 27

SELECT * FROM departments; # You can find that there are 27 Departments

# The correct way to query multiple tables ： There should be connection conditions for multiple tables
SELECT employee_id,department_name,employees.department_id
FROM employees,departments
WHERE employees.department_id=departments.department_id;

# Check the employee's employee_id,last_name,department_name,city
SELECT e.employee_id,e.last_name,d.department_name,l.city
FROM employees e,departments d,locations l
WHERE e.department_id=d.department_idAND d.location_id=l.location_id;

# Examples of non equivalent connections
SELECT *
FROM job_grades;

# Query the employee's name and the name of its manager
SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp,employees mgr
WHERE emp.manager_id=mgr.employee_id;

# practice ： Check the information of all employees last_name,department_name Information
SELECT employee_id,department_name
FROM employees,departments
WHERE employees.department_id=departments.department_id;
#SQL92 The syntax implements the inner connection as above
#sql92 Realize external connection in mysql Can't run on , have access to oracle To achieve the following statement
#SQL92 The syntax implements the left outer connection : Use on the right + as follows
SELECT employee_id,department_name
FROM employees,departments
WHERE employees.department_id=departments.department_id（+）;

#SQL92 The syntax implements the right outer connection : Use on the left + as follows
SELECT employee_id,department_name
FROM employees,departments
WHERE employees.department_id（+）=departments.department_id;

#sql99 Syntax to implement inner connection
SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id=d.department_id;

SELECT last_name,department_name,city
FROM employees e
JOIN departments d
ON e.department_id=d.department_id
JOIN locations l
ON d.location_id=l.location_id;

SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id=d.department_id
JOIN locations l
ON d.location_id=l.location_id

#sql99 Syntax implementation external connection +
# The left outer join
SELECT last_name,department_name
FROM employees e
LEFT OUTER JOIN departments d
ON e.department_id=d.department_id;

# Right connection
SELECT last_name,department_name
FROM employees e
RIGHT OUTER JOIN departments d
ON e.department_id=d.department_id;

# Various connections
# Internal connection
SELECT employee_id,last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id=d.department_id;

# The left outer join
SELECT employee_id,last_name,department_name
FROM employees e
LEFT JOIN departments d
ON e.department_id=d.department_id;

# Right connection
SELECT employee_id,last_name,department_name
FROM employees e
RIGHT JOIN departments d
ON e.department_id=d.department_id;

# Only on the left : Use the fields of the table on the right is null
SELECT employee_id,last_name,department_name
FROM employees e
LEFT JOIN departments d
ON e.department_id=d.department_id
WHERE d.department_id IS NULL;

# Only the right ： Use the fields of the table on the left is null
SELECT employee_id,last_name,department_name
FROM employees e
RIGHT JOIN departments d
ON e.department_id=d.department_id
WHERE e.department_idIS NULL;

# Full outer join : Left outer link union all There is only the right
SELECT employee_id,last_name,department_name
FROM employees e
LEFT JOIN departments d
ON e.department_id=d.department_id
UNION ALL
SELECT employee_id,last_name,department_name
FROM employees e
RIGHT JOIN departments d
ON e.department_id=d.department_id
WHERE e.department_idIS NULL;

# Full outer join : Right outer company union all There is only left
SELECT employee_id,last_name,department_name
FROM employees e
RIGHT JOIN departments d
ON e.department_id=d.department_id
UNION ALL
SELECT employee_id,last_name,department_name
FROM employees e
LEFT JOIN departments d
ON e.department_id=d.department_id
WHERE d.department_idIS NULL;

#sqL 99 New characteristics ： Natural join （natural join） , Generally do not use , It must meet the equivalent content of the same field before it is displayed
SELECT last_name,department_name
FROM employees e
NATURAL JOIN departments d

#sqL 99 New characteristics ：using join
SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id=d.department_id;