[TIL] SQL - JOIN

SQL - JOIN 정리

예시 테이블

Table: employees
+--------+--------------+------------+---------+
| emp_id | emp_name     | hire_date  | dept_id |
+--------+--------------+------------+---------+
|      1 | Ethan Hunt   | 2001-05-01 |       4 |
|      2 | Tony Montana | 2002-07-15 |       1 |
|      3 | Sarah Connor | 2005-10-18 |       5 |
|      4 | Rick Deckard | 2007-01-03 |       3 |
|      5 | Martin Blank | 2008-06-24 |    NULL |
+--------+--------------+------------+---------+

Table: departments
+---------+------------------+
| dept_id | dept_name        |
+---------+------------------+
|       1 | Administration   |
|       2 | Customer Service |
|       3 | Finance          |
|       4 | Human Resources  |
|       5 | Sales            |
+---------+------------------+

1. Inner Join

1-1) Cross Join (교차 조인)

• 특별한 조건없이 테이블 A의 각 행과 테이블 B의 각행을 다 조합한 결과(곱하기)

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
CROSS JOIN departments AS t2;

Cross Join Result
+--------+--------------+------------+------------------+
| emp_id | emp_name     | hire_date  | dept_name        |
+--------+--------------+------------+------------------+
|      1 | Ethan Hunt   | 2001-05-01 | Administration   |
|      2 | Tony Montana | 2002-07-15 | Administration   |
|      3 | Sarah Connor | 2005-10-18 | Administration   |
|      4 | Rick Deckard | 2007-01-03 | Administration   |
|      5 | Martin Blank | 2008-06-24 | Administration   |
|      1 | Ethan Hunt   | 2001-05-01 | Customer Service |
|      2 | Tony Montana | 2002-07-15 | Customer Service |
|      3 | Sarah Connor | 2005-10-18 | Customer Service |
|      4 | Rick Deckard | 2007-01-03 | Customer Service |
|      5 | Martin Blank | 2008-06-24 | Customer Service |
|      1 | Ethan Hunt   | 2001-05-01 | Finance          |
|      2 | Tony Montana | 2002-07-15 | Finance          |
|      3 | Sarah Connor | 2005-10-18 | Finance          |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      5 | Martin Blank | 2008-06-24 | Finance          |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      2 | Tony Montana | 2002-07-15 | Human Resources  |
|      3 | Sarah Connor | 2005-10-18 | Human Resources  |
|      4 | Rick Deckard | 2007-01-03 | Human Resources  |
|      5 | Martin Blank | 2008-06-24 | Human Resources  |
|      1 | Ethan Hunt   | 2001-05-01 | Sales            |
|      2 | Tony Montana | 2002-07-15 | Sales            |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
|      4 | Rick Deckard | 2007-01-03 | Sales            |
|      5 | Martin Blank | 2008-06-24 | Sales            |
+--------+--------------+------------+------------------+

1-2) Inner Join

• 가장 많이 사용된다.
• 2개의 테이블의 컬럼을 합쳐 새로운 테이블을 생성한다.

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
INNER JOIN departments AS t2
ON t1.dept_id = t2.dept_id 
ORDER BY emp_id;

Inner Join Result
+--------+--------------+------------+-----------------+
| emp_id | emp_name     | hire_date  | dept_name       |
+--------+--------------+------------+-----------------+
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources |
|      2 | Tony Montana | 2002-07-15 | Administration  |
|      3 | Sarah Connor | 2005-10-18 | Sales           |
|      4 | Rick Deckard | 2007-01-03 | Finance         |
+--------+--------------+------------+-----------------+

2. Outer Join

• 조건문에 만족하지 않는 행도 표시해주는 조인
• 조인했을 때, 한쪽의 테이블에 데이터가 없어도 조인 결과에는 포함된다.

2-1) Left Outer Join

• 테이블 A의 모든 데이터와 테이블 B와 매칭 되는 레코드를 포함하는 조인

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
LEFT JOIN departments AS t2
ON t1.dept_id = t2.dept_id 
ORDER BY emp_id;

Left Outer Join Result
+--------+--------------+------------+-----------------+
| emp_id | emp_name     | hire_date  | dept_name       |
+--------+--------------+------------+-----------------+
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources |
|      2 | Tony Montana | 2002-07-15 | Administration  |
|      3 | Sarah Connor | 2005-10-18 | Sales           |
|      4 | Rick Deckard | 2007-01-03 | Finance         |
|      5 | Martin Blank | 2008-06-24 | NULL            |
+--------+--------------+------------+-----------------+

2-2) Right Outer Join

• 오른쪽 외부 조인은 테이블 B의 모든 데이터와 테이블 A와 매칭이 되는 레코드를 포함하는 조인

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
RIGHT JOIN departments AS t2
ON t1.dept_id = t2.dept_id 
ORDER BY dept_name;

Right Outer Join Result
+--------+--------------+------------+------------------+
| emp_id | emp_name     | hire_date  | dept_name        |
+--------+--------------+------------+------------------+
|      2 | Tony Montana | 2002-07-15 | Administration   |
|   NULL | NULL         | NULL       | Customer Service |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
+--------+--------------+------------+------------------+

2-3) Full Outer Join

• Oracle, MySQL은 지원하지 않음
• Left, Right Join, Union, Union All 등을 같이 써야함

## Union All
SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
LEFT JOIN departments AS t2
ON t1.dept_id = t2.dept_id
UNION ALL
SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 
RIGHT JOIN departments AS t2
ON t1.dept_id = t2.dept_id ORDER BY emp_name;

+--------+--------------+------------+------------------+
| emp_id | emp_name     | hire_date  | dept_name        |
+--------+--------------+------------+------------------+
|   NULL | NULL         | NULL       | Customer Service |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      5 | Martin Blank | 2008-06-24 | NULL             |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
|      2 | Tony Montana | 2002-07-15 | Administration   |
|      2 | Tony Montana | 2002-07-15 | Administration   |
+--------+--------------+------------+------------------+

참조 : https://velog.io/@kdh10806/DataBase-Join
https://www.tutorialrepublic.com/sql-tutorial/