The third normal form (3NF) which was introduced by E. F. Code is an important normal form for relation in the relation database. In this paper, we put forward a method for normalizing a relation (that is a data file) from the first normal form to the third normal form. T6m tll.t. Dang chu[n 3 d6ng vai tro quan trong trong mo hlnh duoli~u quan h~. Trong bai bao nay, chiing toi d"exu St phiro-ng phap chua'n h6a m9t quan h~, ma thu-c chfit Ill. mot t~p dir lieu, tir dang chua n 1 v"edang chu[n 3.

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... Ma hlnh dir lieu quan h~diro'c E. F. Codd de xuat la mo hlnh dir li~u ph5 bien nhat hi~n nay. Hat nhan chfnh cua mf hlnh nay la quan h~ma thuc chat la m9t t~p dir lieu (dai khi goi la bing). M9t van de quan trong la viec chuan h6a cac t~p dii' li~u. Thirc chat cua van de nay la vi~c chuydn m9t t~p dii' lieu bat ky ve t~p dir li~u 0-nhirng dang chu~n quen thuoc nhir dang chu~n 2 (2NF) va dang chu~n 3 (3NF). Muc tieu chinh cu a bai bao nay la de iuat m9t phtro'ng phap chu~n h6a m9t t~p dir li~u ve dang 3NF. 2. MQT s6 D!NH NGHIA Trong phan nay, cluing tai trlnh bay nhfmg dinh nghia CO" ban nhat can dung cho viec trinh bay bai bao (c6 thg d9C trong [1-8]). D!nh nghia 2.1. [quan h~) M9t quan h~r xac dinh tren t~p hfru han va khOng ding cac thu9C tinh 0= {aI, az, ... , an} la m9t t~p hop m b9 c6 dang: hj = (AI, Az, ... , An), J' = 1, ... , m, Al E Dom (ad, Az E Dom (az)' ... , An E Dom (an), trong d6 Dom (ai) la mien gia tri cua thuoc tinh ai. N6i each kh ac m9t quan h~r tren t~p thuoc tinh 0 = {aI, az, ... , an} la m9t t~p con cd a tich Descarts: Dom (ad X Dom (az) X ... X Dom (an). Dirong nhien quan h~r c6 thg bi thay d5i theo thai gian do viec tlnrc hi~n cac phep toan c~p nh~t tren cac b9 cu a quan h~r (them vao, IO,!-i bo, stl.-ad5i). Trong khi do, ngir nghia cii a m9t b9 thuoc r la bat bien va di"eu d6 lien quan t6i md ta cau true cua t~p cac b9 ma ta goi la hrcc do quan h~. Dlnh nghia 2.2. [phu thuoc ham tren quan h~) Cho 0 = {aI, az, ... , an} la t~p hiru han va khong rt)ng cac thuoc tinh, r = {hI, hz, ... , hm} la m9t quan h~tren 0 va A, B 111, cac t~p con cua 0 (A < B <;;; 0). Khi d6, chung ta n6i A xac dinh ham cho B hay B phu thuoc ham vao A trong r (ky hi~u 111,: A --.L.. B) neu: r