중첩 질의문

subquery.sql

@@ -0,0 +1,49 @@
+USE Study
+GO
+
+SELECT DNAME
+FROM DEPARTMENT
+WHERE DEPARTMENT.DNO = (SELECT DNO
+						FROM EMPLOYEE
+						WHERE EMPLOYEE.ENO = 103)
+
+-- 위 쿼리와 동일
+SELECT DNAME
+FROM DEPARTMENT
+	JOIN EMPLOYEE ON EMPLOYEE.DNO = DEPARTMENT.DNO
+WHERE EMPLOYEE.ENO = 103
+
+
+-- 단일행 서브쿼리
+SELECT ENO,DNO
+FROM EMPLOYEE
+WHERE EMPLOYEE.DNO = (SELECT DNO FROM EMPLOYEE WHERE EMPLOYEE.ENO = 110)
+
+-- 다중행 서브쿼리
+SELECT ENAME, ENO,DNO, SALARY
+FROM EMPLOYEE
+WHERE EMPLOYEE.DNO IN (SELECT DNO FROM EMPLOYEE WHERE SALARY >= 500)
+
+SELECT ENAME, ENO,DNO, SALARY
+FROM EMPLOYEE
+WHERE SALARY >= ANY (SELECT SALARY FROM EMPLOYEE WHERE DNO = 20)
+
+SELECT ENAME, ENO,DNO, SALARY
+FROM EMPLOYEE
+WHERE SALARY >= ALL (SELECT SALARY FROM EMPLOYEE WHERE DNO = 10)
+
+SELECT ENAME
+FROM EMPLOYEE
+WHERE EXISTS (SELECT * FROM EMPLOYEE WHERE SALARY + COMMISSION > 500)
+
+SELECT ENO, ENAME, DNO, SALARY
+FROM EMPLOYEE
+WHERE DNO IN (SELECT DNO FROM EMPLOYEE WHERE ENO = 101)
+	AND SALARY IN (SELECT SALARY FROM EMPLOYEE WHERE ENO = 101)
+
+
+-- 상호 연관 서브쿼리: 일반적으로 성능이 떨어지며, 조인을 쓰는게 좋음
+-- (메인 쿼리에 따라 서브쿼리 결과가 달라지는 상호 연관된 서브쿼리는 빈번한 조회를 유발)
+SELECT ENO, ENAME, SALARY, DNO
+FROM EMPLOYEE AS E
+WHERE DNO IN (SELECT DNO FROM EMPLOYEE AS M WHERE E.MANAGER = M.ENO)