AVG() can work with a join : AVG « Aggregate Functions « MySQL Tutorial

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MySQL Tutorial » Aggregate Functions » AVG 
16.2.4.AVG() can work with a join
mysql>
mysql> CREATE TABLE Employee(
    ->     id            int,
    ->     first_name    VARCHAR(15),
    ->     last_name     VARCHAR(15),
    ->     start_date    DATE,
    ->     end_date      DATE,
    ->     salary        FLOAT(8,2),
    ->     city          VARCHAR(10),
    ->     description   VARCHAR(15)
    -> );
Query OK, rows affected (0.03 sec)

mysql>
mysql> create table job (
    ->   id         int,
    ->   title      VARCHAR(20)
    -> );
Query OK, rows affected (0.05 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->              values (1,'Jason',    'Martin',  '19960725',  '20060725', 1234.56'Toronto',  'Programmer');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(2,'Alison',   'Mathews',  '19760321', '19860221', 6661.78'Vancouver','Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(3,'James',    'Smith',    '19781212', '19900315', 6544.78'Vancouver','Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(4,'Celia',    'Rice',     '19821024', '19990421', 2344.78'Vancouver','Manager');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(5,'Robert',   'Black',    '19840115', '19980808', 2334.78'Vancouver','Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(6,'Linda',    'Green',    '19870730', '19960104', 4322.78,'New York',  'Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(7,'David',    'Larry',    '19901231', '19980212', 7897.78,'New York',  'Manager');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(8,'James',    'Cat',     '19960917',  '20020415', 1232.78,'Vancouver', 'Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into job (id, titlevalues (1,'Tester');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (2,'Accountant');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (3,'Developer');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (4,'Coder');
Query OK, row affected (0.02 sec)

mysql> insert into job (id, titlevalues (5,'Director');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (6,'Mediator');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (7,'Proffessor');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (8,'Programmer');
Query OK, row affected (0.00 sec)

mysql> insert into job (id, titlevalues (9,'Developer');
Query OK, row affected (0.00 sec)

mysql>
mysql> select from job;
+------+------------+
| id   | title      |
+------+------------+
|    | Tester     |
|    | Accountant |
|    | Developer  |
|    | Coder      |
|    | Director   |
|    | Mediator   |
|    | Proffessor |
|    | Programmer |
|    | Developer  |
+------+------------+
rows in set (0.00 sec)

mysql> select from Employee;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id   | first_name | last_name | start_date | end_date   | salary  | city      | description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
|    | Jason      | Martin    | 1996-07-25 2006-07-25 1234.56 | Toronto   | Programmer  |
|    | Alison     | Mathews   | 1976-03-21 1986-02-21 6661.78 | Vancouver | Tester      |
|    | James      | Smith     | 1978-12-12 1990-03-15 6544.78 | Vancouver | Tester      |
|    | Celia      | Rice      | 1982-10-24 1999-04-21 2344.78 | Vancouver | Manager     |
|    | Robert     | Black     | 1984-01-15 1998-08-08 2334.78 | Vancouver | Tester      |
|    | Linda      | Green     | 1987-07-30 1996-01-04 4322.78 | New York  | Tester      |
|    | David      | Larry     | 1990-12-31 1998-02-12 7897.78 | New York  | Manager     |
|    | James      | Cat       | 1996-09-17 2002-04-15 1232.78 | Vancouver | Tester      |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
rows in set (0.00 sec)

mysql>
mysql>
mysql>
mysql> SELECT title, ROUND(AVG(salary),1)
    -> FROM employee
    -> LEFT JOIN job
    -> USING (ID)
    -> GROUP BY employee.city;
+------------+----------------------+
| title      | ROUND(AVG(salary),1|
+------------+----------------------+
| Mediator   |               6110.3 |
| Tester     |               1234.6 |
| Accountant |               3823.8 |
+------------+----------------------+
rows in set (0.00 sec)

mysql>
mysql>
mysql> drop table job;
Query OK, rows affected (0.00 sec)

mysql> drop table Employee;
Query OK, rows affected (0.00 sec)

mysql>
16.2.AVG
16.2.1.AVG([DISTINCT] expr): Returns the average value of expr.
16.2.2.AVG() can work out more that one value at a time when used with a GROUP BY clause
16.2.3.AVG() can work with other functions
16.2.4.AVG() can work with a join
16.2.5.AVG with NULL value
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