sqld#Transaction#A.C.I.D SQLD#Null#제약조건 SQL#이론#DDL#DCL#DML#TCL#기본#시험 SQL#수업정리#서브쿼리부터어질어질#뒷부분은이해안됨#복습중 어렵다#하루가짧다 groupby#orderby#as sql#기초#selwct#where#between#count#like