SQL (Structures Query Language) là ngôn ngữ được sử dụng cho các hệ quản trị cơ sở dữ liệu quan hệ. Ngôn ngữ SQL chuẩn được chia ra bởi ANSI (American National Standards Institude) và ISO (International Standards Organization) với phiên bản mới nhất hiện nay là phiên bản SQL - 92 (phiên bản được đưa ra năm 1992). Mặc dù có nhiều ngôn ngữ khác nhau được đưa ra cho các hệ quản trị CSDL quan hệ, SQL là ngôn ngữ được sử dụng rộng rãi hiện nay trong rất nhiều hệ thống CSDL thương mại như Oracle, SQL Server, DB2, Microsoft Access... Thông qua SQL, người sử dụng có thể dễ dàng định nghĩa được dữ liệu, thao tác với dữ liệu,... Mặt khác, đây là ngôn gnwx có tính khai báo nên nó dễ sử dụng và cũng vì vậy mà trở nên phổ biến.
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hoten char(20) not null,
ngaysinh datetime null,
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dienthoai char(6) check(dienthoai like '[0-9][0-9][0-9]
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CREATE VIEW thongtin_nv(manv,hoten,tuoi,donvi)
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AS
SELECT manv,hoten,datediff(year,ngaysinh,getdate()),tendv
FROM nhanvien,donvi
WHERE nhanvien.madv=donvi.madv
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Tráön Nguyãn Phong
Chæång 2: NGÄN NGÆÎÎ THAO TAÏÏC DÆÎÎ LIÃÛÛU
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§Ó truy xuÊt d÷ liÖu tõ c¸c dßng vµ c¸c cét cña mét hay nhiÒu b¶ng, khung nh×n,
ta sö dông c©u lÖnh SELECT. C©u lÖnh nµy cã thÓ dïng ®Ó thùc hiÖn phÐp chän (tøc lµ
truy xuÊt mét tËp con c¸c dßng trong mét hay nhiÒu b¶ng), phÐp chiÕu (tøc lµ truy
xuÊt mét tËp con c¸c cét trong mét hay nhiÒu b¶ng) vµ phÐp nèi (tøc lµ liªn kÕt c¸c
dßng trong hai hay nhiÒu b¶ng ®Ó truy xuÊt d÷ liÖu).
Có ph¸p chung cña c©u lÖnh SELECT cã d¹ng nh sau:
SELECT [ ALL | DISTINCT ] select_list
[ INTO [ newtable_name ]]
FROM { table_name | view_name }
................
[,{table_name | view_name }]
[WHERE clause ]
[GROUP BY clause ]
[HAVING BY clause ]
[ORDER BY clause ]
[COMPUTE clause ]
Chó ý: C¸c thµnh phÇn trong mét c©u lÖnh SELECT ph¶i ®îc sö dông theo thø tù
®îc nªu trªn.
1.1 X¸c ®Þnh b¶ng b»ng mÖnh ®Ò FROM
MÖnh ®Ò FROM trong c©u lÖnh SELECT ®îc sö dông nh»m chØ ®Þnh c¸c b¶ng
vµ khung nh×n cÇn truy xuÊt d÷ liÖu. Sau mÖnh ®Ò FROM lµ danh s¸ch tªn c¸c b¶ng
vµ khung nh×n tham gia vµo truy vÊn (tªn cña c¸c b¶ng vµ khung nh×n ®îc ph©n c¸ch
nhau bëi dÊu phÈy).
SELECT select_list
FROM {table_nameview_name list}
Giaïo trçnh thæûc haình SQL
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§Ó ®¬n gi¶n ho¸ c©u hái, ta cã thÓ sö dông c¸c bÝ danh (alias) cho c¸c b¶ng hay
khung nh×n. BÝ danh ®îc g¸n trong mÖnh ®Ò FROM b»ng c¸ch chØ ®Þnh bÝ danh sau
tªn b¶ng. VÝ dô c©u lÖnh sau g¸n bÝ danh n1 cho b¶ng nhanvien.
SELECT ten, diachi FROM nhanvien n1
1.2 MÖnh ®Ò WHERE
MÖnh ®Ò WHERE trong c©u lÖnh SELECT x¸c ®Þnh c¸c ®iÒu kiÖn ®èi víi viÖc truy
xuÊt d÷ liÖu. Sau mÖnh ®Ò WHERE lµ mét biÓu thøc logic vµ chØ nh÷ng dßng d÷ liÖu
nµo tho¶ m∙n biÓu thøc sau WHERE míi ®îc hiÓn thÞ