SQLμμ UNION μ°μ°μλ λ κ° μ΄μμ SELECT λ¬Έμ κ²°κ³Όλ₯Ό νλμ μ§ν©μΌλ‘ κ²°ν©νλ λ° μ¬μ©λ©λλ€. κ° SELECT λ¬Έμ κ²°κ³Όλ λμνλ νλμ μ΄λ¦κ³Ό νμ μ΄ κ°μμΌ ν©λλ€.
# μμ 쿼리
SELECT employee_id, first_name, last_name
FROM employees
WHERE department_id = 1
UNION
SELECT employee_id, first_name, last_name
FROM employees
WHERE department_id = 2;
μ΄ μμ μλ employees ν μ΄λΈμμ employee_id, first_name λ° last_name μ΄μ μ νν©λλ€. 첫 λ²μ§Έ SELECT λ¬Έμ department_id 1μ μ§μλ§ μ ννκ³ , λ λ²μ§Έ SELECT λ¬Έμ department_id 2μ μ§μλ§ μ νν©λλ€.
UNION μ°μ°μλ μ΄ λ SELECT λ¬Έμ κ²°κ³Όλ₯Ό νλμ μ§ν©μΌλ‘ κ²°ν©ν©λλ€. μ΄ κ²°κ³Ό μ§ν©μλ λ SELECT λ¬Έμμ μ νν λͺ¨λ νμ΄ μ€λ³΅ μ κ±°λ μνλ‘ ν¬ν¨λ©λλ€. UNIONμ λ€μ DISTINCTκ° μλ΅λ κ²κ³Ό κ°μ΅λλ€. κ²°κ³Ό μ§ν©μ μ€λ³΅λ λ°μ΄ν°λ ν¬ν¨νλ €λ©΄ UNION ALLμ ν©λλ€.
# κ²°κ³Ό
employee_id | first_name | last_name
-------------|--------------|------------
1 | John | Smith
2 | Jane | Doe
3 | Tom | Jones
4 | Bob | Johnson
5 | Mary | Smith
6 | Jim | Brown
μ°Έκ³