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SQL Tutorial
Dng Thin T 1
Welcome to SQL tutorial
Mc lc
SQL cn bn Gii thiu SQL M t th no l SQL, cch dng SQL. SQL Select
Cch dng pht biu SELECT chn d liu t mt bng trong SQL. SQL Where Cch
dng mnh WHERE ch nh tiu chun chn. SQL And & Or Cch dng AND v OR
kt ni hai hay nhiu iu kin trong mnh WHERE. SQL Between Cch dng
BETWEEN....AND tm d liu trong mt khong gii hn. SQL Distinct Cch dng
t kha DISTINCT ch tr v cc tr khc nhau trong mt ct. SQL Order By Cch
dng t kha ORDER BY tr v cc hng c sp xp theo mt th t nh trc. SQL
Insert Cch dng pht biu INSERT chn hng mi vo trong mt bng. SQL
Update Cch dng pht biu UPDATE cp nht hay thay i cc hng trong mt
bng. SQL Delete Cch dng pht biu DELETE xa cc hng trong mt bng. SQL
Count Gii thch cc hm COUNT to sn trong SQL.
SQL nng cao Cc hm SQL Gii thch cch dng cc hm to sn trong SQL.
SQL Group By Gii thch cch dng hm GROUP BY to sn trong SQL. Cc b
danh SQL Gii thch cch dng cc b danh (alias) cho cc tn ct v cc tn
bng. SQL Join Gii thch cch chn thng tin t nhiu bng. SQL Create Cch
to cc c s d liu v cc bng, v cch xa chng. SQL Alter Cch dng pht biu
ALTER TABLE thm hay loi cc ct trong mt bng cho trc.
Gii thiu SQL
SQL l mt ngn ng theo chun ANSI truy xut cc c s d liu.
SQL l g?
SQL l Structured Query Language Ngn ng Truy vn c Cu trc SQL cho
php bn truy xut mt c s d liu SQL l mt ng theo chun ANSI SQL c th
thc hin cc truy vn n mt c s d liu SQL c th truy tm d liu t mt c s d
liu SQL c th chn cc mu tin mi vo trong mt c s d liu SQL c th xa cc
mu tin trong mt c s d liu
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SQL Tutorial
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SQL c th cp nht cc mu tin trong mt c s d liu SQL rt d hc
SQL l mt chun SQL l mt chun ANSI (American National Standards
Institute - Vin Tiu chun Quc gia M) cho cc h thng truy xut c s d
liu. Cc pht biu SQL dng truy tm v cp nht d liu trong mt c s d liu.
SQL lm vic vi cc trnh qun l c s d liu nh Access, DB2, Informix,
Microsoft SQL Server, Oracle, Sybase, v nhiu trnh khc (ng tic l a s
trong chng c cc phn m rng ngn ng SQL ring).
Cc bng c s d liu C s d liu cha cc i tng gi l cc Bng (Tables). Cc
Mu tin (Records) lu trong cc bng ny. Cc bng c gi theo tn bng (nh
"Persons", "Orders", "Suppliers"). Cc bng cha cc Ct (Columns) v cc
Dng (Rows) d liu. Dng cha cc mu tin (nh mu tin v mt ngi). Ct cha d
liu (nh First Name, Last Name, Address, v City). Mt v d l bng
"Persons" sau: LastName FirstName Address City Hansen Ola Timoteivn
10 Sandnes Svendson Tove Borgvn 23 Sandnes Pettersen Kari Storgt 20
Stavanger LastName, FirstName, Address, v City l cc ct ca bng. Cc
dng cha ba mu tin ca 3 ngi.
Cc truy vn SQL Vi SQL, chng ta c th truy vn mt c s d liu v nhn c
mt kt qu tr v vi dng bng. Mt truy vn ging nh sau: SELECT LastName
FROM Persons S tr v mt kt qu ging nh sau: LastName Hansen Svendson
Pettersen Ch : Vi h c s d liu cn mt du ; cui pht biu SQL. Chng ta
khng dng du ; trong bi vit ny.
Thao tc d liu SQL SQL l mt c php thc hin cc truy vn. Nhng ngn ng
SQL cng cha cc c php cp nht cc mu tin (record), chn cc mu tin mi v
xa cc mu tin ang tn ti. Cc lnh truy vn v cp nht ny thuc dng Ngn ng
Thao tc D liu (Data Manipulation Language - DML) mt phn ca SQL:
SELECT trch d liu t mt c s d liu UPDATE cp nht d liu trong mt c
s d liu DELETE xa d liu t mt c s d liu INSERT chn d liu mi vo trong
mt c s d liu
nh ngha d liu SQL Ngn ng nh ngha D liu (Data Definition Language
- DDL) mt phn ca SQL, cho php to hay xa cc bng c s d liu. Chng ta
cng c th nh ngha cc ch mc (cc kha - key), ch nh lin kt gia cc bng,
v rng but gia cc bng c s d liu. Cc pht biu DDL quan trng nht trong
SQL l::
CREATE TABLE to mt bng c s d liu mi ALTER TABLE thay i (alters)
mt bng c s d liu
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SQL Tutorial
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DROP TABLE xa mt bng c s d liu CREATE INDEX to mt ch mc (kha tm
kim) DROP INDEX xo mt ch mc
SQL v ASP SQL l mt phn quan trng ca ASP (Active Server Pages), v
ADO (Active Data Object) c dng trong ASP truy xut c s d liu, ADO da
trn SQL truy xut d liu.
Pht biu SQL Select
Pht biu SELECT chn cc ct d liu t mt c s d liu.
Kt qu dng bng c lu trong mt bng kt qu (gi l tp kt qu - result
set).
Pht biu SELECT Pht biu SELECT chn cc ct d liu t mt c s d liu.
Dng pht biu ny chn (SELECT) thng tin t (FROM) mt bng nh sau: SELECT
column_name(s) FROM table_name
V d: Chn cc ct t mt bng chn cc ct c tn "LastName" v "FirstName",
dng mt pht biu SELECT nh sau: SELECT LastName,FirstName FROM
Persons Bng "Persons": LastName FirstName Address City Hansen Ola
Timoteivn 10 Sandnes Svendson Tove Borgvn 23 Sandnes Pettersen Kari
Storgt 20 Stavanger Kt qu: LastName FirstName Hansen Ola Svendson
Tove Pettersen Kari
V d: Chn tt c cc ct chn tt c cc ct t bng "Person", dng mt k hiu
* thay th cho tn cc ct nh sau: SELECT * FROM Persons Kt qu:
LastName FirstName Address City Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes Pettersen Kari Storgt 20
Stavanger
Bng kt qu Kt qu t mt truy vn SQL c lu tr trong mt tp kt qu. Tp
kt qu c th xem nh mt bng kt qu. a s cc
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SQL Tutorial
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trnh qun l c s d liu cho php duyt tp kt qu vi cc hm lp trnh nh:
Move-To-First-Record, Get-Record-Content,
Move-To-Next-Record......
Mnh SQL Where
Mnh WHERE dng ch nh mt tiu chun (criteria) chn.
Mnh WHERE chn c iu kin d liu t mt bng, mt mnh WHERE c th thm vo
pht biu SELECT vi c php sau: SELECT column FROM table WHERE column
condition value Vi mnh WHERE, cc iu kin sau c th c dng: Operator
Condition = Bng Khng bng > Ln hn < Nh hn >= Ln hn hoc bng
1965 Vit sai: SELECT * FROM Persons WHERE Year>'1965'
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iu kin LIKE iu kin LIKE dng ch nh vic tm mt mu trong mt ct. C
php: SELECT column FROM table WHERE column LIKE pattern Mt du "%" c
th dng nh k t i din (wildcards) c trc ln sau mu.
V d: Chn trong bng Persons vi mu tn Pht biu SQL s tr v nhng ngi
c firstname bt u vi mt k t 'O'. SELECT * FROM Persons WHERE
FirstName LIKE 'O%' Pht biu SQl s tr v nhng ngi c firstname kt thc
vi mt k t 'a'. SELECT * FROM Persons WHERE FirstName LIKE '%a' Pht
biu SQL s tr v nhng ngi c firstname cha mu 'la'. SELECT * FROM
Persons WHERE FirstName LIKE '%la%' Tt c cc v d trn s tr v kt qu
sau: LastName FirstName Address City Year Hansen Ola Timoteivn 10
Sandnes 1951
SQL And & Or
AND & OR AND v OR kt ni hai hay nhiu iu kin trong mt mnh
WHERE. Ton t AND hin th mt ct nu TT C cc iu kin lit k u ng. Ton t
OR hin th mt ct nu MT TRONG cc iu kin lit k l ng.
Bng gc (dng trong cc v d) LastName FirstName Address City Hansen
Ola Timoteivn 10 Sandnes Svendson Tove Borgvn 23 Sandnes Svendson
Stephen Kaivn 18 Sandnes
V d Dng AND hin th ngi c firstname l "Tove", v lastname l
"Svendson": SELECT * FROM Persons WHERE FirstName='Tove' AND
LastName='Svendson' Kt qu: LastName FirstName Address City Svendson
Tove Borgvn 23 Sandnes
V d Dng OR hin th ngi c firstname l "Tove", hoc c lastname l
"Svendson": SELECT * FROM Persons WHERE firstname='Tove' OR
lastname='Svendson' Kt qu:
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LastName FirstName Address City Svendson Tove Borgvn 23 Sandnes
Svendson Stephen Kaivn 18 Sandnes
V d Bn cng c th dng phi hp AND v OR (dng du ngoc n bao cc biu
thc phc tp): SELECT * FROM Persons WHERE (FirstName='Tove' OR
FirstName='Stephen') AND LastName='Svendson' Kt qu: LastName
FirstName Address City Svendson Tove Borgvn 23 Sandnes Svendson
Stephen Kaivn 18 Sandnes
SQL Between...And
BETWEEN ... AND Ton t BETWEEN ... AND chn tt c cc tr trong khong
gii hn gia hai tr. Cc tr ny c th l cc s, vn bn, hay ngy thng.
SELECT column_name FROM table_name WHERE column_name BETWEEN value1
AND value2
Bng gc (dng trong cc v d) LastName FirstName Address City Hansen
Ola Timoteivn 10 Sandnes Svendson Tove Borgvn 23 Sandnes Nordmann
Anna Neset 18 Sandnes Pettersen Kari Storgt 20 Stavanger
V d 1 hin th cc tn theo th t alphabet gia hai tn (k c hai tn ny)
"Hansen" v "Pettersen", dng SQL sau: SELECT * FROM Persons WHERE
LastName BETWEEN 'Hansen' AND 'Pettersen' Kt qu: LastName FirstName
Address City Hansen Ola Timoteivn 10 Sandnes Nordmann Anna Neset 18
Sandnes Pettersen Kari Storgt 20 Stavanger
V d 2 hin th cc tn ngoi cc tn trong v d trn, dng ton t NOT:
SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Hansen' AND
'Pettersen' Kt qu:
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LastName FirstName Address City Svendson Tove Borgvn 23
Sandnes
SQL Select Distinct
T kha DISTINCT dng tr v ch cc tr khc bit (distinct).
T kha DISTINCT Pht biu SQL SELECT tr v thng tin t cc ct ca bng.
Nhng lm th no nu chng ta ch mun chn cc kt qu khng trng nhau? Vi
SQL, chng ta ch cn thm vo mt t kha DISTINCT cho pht biu SELECT vI c
php sau: SELECT DISTINCT column-name(s) FROM table-name
V d: Chn tn cng ty t bng Orders V d: Bng t hng n gin: Company
OrderNumber Sega 3412 W3Schools 2312 Trio 4678 W3Schools 6798 Pht
biu SQL sau: SELECT Company FROM Orders S tr v kt qu: Company Sega
W3Schools Trio W3Schools Ch rng cng ty W3Schools xut hin hai ln
trong kt qu. i lc chng ta khng mun iu ny.
V d: Chn tn cng ty (khng trng tn) t bng Orders Pht biu SQL sau:
SELECT DISTINCT Company FROM Orders S tr v kt qu: Company Sega
W3Schools Trio By gi tn cng ty W3Schools ch xut hin mt ln trong kt
qu.
SQL Order By
T kha ORDER BY dng sp xp kt qu th t kt qu.
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Sp xp cc Dng Mnh ORDER BY dng sp xp cc dng. Mt s cch sp xp:
Company OrderNumber Sega 3412 ABC Shop 5678 W3Schools 2312
W3Schools 6798
V d hin th tn cng ty (Company) theo th t alphabet: SELECT
Company, OrderNumber FROM Orders ORDER BY Company Kt qu: Company
OrderNumber ABC Shop 5678 Sega 3412 W3Schools 6798 W3Schools
2312
V d hin th tn cng ty (Company) theo th t alphabet, nu tn cng ty
ging nhau th sp xp theo s th t (OrderNumber): SELECT Company,
OrderNumber FROM Orders ORDER BY Company, OrderNumber Kt qu:
Company OrderNumber ABC Shop 5678 Sega 3412 W3Schools 2312
W3Schools 6798
V d hin th tn cng ty (Company) theo th t alphabet o ngc (t Z n
A): SELECT Company, OrderNumber FROM Orders ORDER BY Company DESC
Kt qu: Company OrderNumber W3Schools 6798 W3Schools 2312 Sega 3412
ABC Shop 5678
SQL INSERT INTO
Chn cc dng mi Pht biu INSERT INTO chn cc dng mi vo trong mt bng:
INSERT INTO table_name VALUES (value1, value2,....)
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Bn c th ch nh cc ct bn mun chn chn d liu vo: INSERT INTO
table_name (column1, column2,...) VALUES (value1, value2,....)
Chn mt dng mi Bng "Persons": LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger Pht biu SQL chn vo bng trn:
INSERT INTO Persons VALUES ('Hetland', 'Camilla', 'Hagabakka 24',
'Sandnes') S cho kt qu nh sau: LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger Hetland Camilla Hagabakka 24
Sandnes
Chn d liu vo trong cc ct ch nh Bng "Persons": LastName FirstName
Address City Pettersen Kari Storgt 20 Stavanger Hetland Camilla
Hagabakka 24 Sandnes Pht biu SQL chn d liu vo cc ct ch nh: INSERT
INTO Persons (LastName, Address) VALUES ('Rasmussen', 'Storgt 67')
S cho kt qu nh sau:: LastName FirstName Address City Pettersen Kari
Storgt 20 Stavanger Hetland Camilla Hagabakka 24 Sandnes Rasmussen
Storgt 67
SQL Update
Update Rows Pht biu UPDATE cp nht hoc thay i cc dng: UPDATE
table_name SET column_name = new_value WHERE column_name =
some_value
Bng Person: LastName FirstName Address City Nilsen Fred Kirkegt
56 Stavanger Rasmussen Storgt 67
Cp nht mt ct trong mt dng Chng ta s thm mt first name Nina n ngi
c lastname="Rasmussen":
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UPDATE Person SET FirstName = 'Nina' WHERE LastName =
'Rasmussen'
Cp nht vi ct trong mt dng Chng ta s thay i a ch (Address) v thm
tn thnh ph. UPDATE Person SET Address = 'Stien 12', City =
'Stavanger' WHERE LastName = 'Rasmussen' Kt qu LastName FirstName
Address City Nilsen Fred Kirkegt 56 Stavanger Rasmussen Nina Stien
12 Stavanger
SQL Delete
Xa cc ct Pht biu DELETE dng xa mt hay nhiu dng trong mt bng.
DELETE FROM table_name WHERE column_name = some_value
Bng Person: LastName FirstName Address City Nilsen Fred Kirkegt
56 Stavanger Rasmussen Nina Stien 12 Stavanger
Xa mt dng "Nina Rasmussen" s b xa: DELETE FROM Person WHERE
LastName = 'Rasmussen' Kt qu LastName FirstName Address City Nilsen
Fred Kirkegt 56 Stavanger
Cc hm SQL Count
SQL c cc hm to sn m cc mu tin c s d liu.
C php hm Count C php ca cc hm COUNT to sn nh sau: SELECT
COUNT(column) FROM table
Hm COUNT(*) Hm COUNT(*) tr v s hng chn c trong mt php chn. Vi
bng "Persons" sau:
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Name Age Hansen, Ola 34 Svendson, Tove 45 Pettersen, Kari 19 V d
ny tr v s hng trong bng: SELECT COUNT(*) FROM Persons Kt qu: 3 V d
ny tr v s ngi ln hn 20 tui: SELECT COUNT(*) FROM Persons WHERE
Age>20 Kt qu: 2
Hm COUNT(column) Hm COUNT(column) tr v s hng (ngoi tr hng c gi
tr NULL) trong ct ch nh. Vi bng "Persons": Name Age Hansen, Ola 34
Svendson, Tove 45 Pettersen, Kari V d ny tm s ngi c ghi tui ti
field Age trong bng "Persons": SELECT COUNT(Age) FROM Persons Kt
qu: 2 Hm COUNT(column) cng dng tnh s hng khng cha tr. Ch kt qu s nh
hn s hng trong bng.
COUNT DISTINCT T kha DISTINCT vi COUNT c th dng m s kt qu khc
nhau (khng trng nhau). C php nh sau: SELECT DISTINCT
COUNT(column(s)) FROM table Vi bng "Orders": Company OrderNumber
Sega 3412 W3Schools 2312 Trio 4678 W3Schools 6798 Vi pht biu SQL
sau: SELECT COUNT(Company) FROM Orders S tr v kt qu: 4 Vi pht biu
SQL sau: SELECT DISTINCT COUNT(Company) FROM Orders S tr v kt qu:
3
Cc hm SQL
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SQL c mt s hm to sn m v tnh ton.
C php dng hm C php cho cc hm SQL to sn nh sau:: SELECT
function(column) FROM table
Bng gc (dng trong cc v d) Name Age Hansen, Ola 34 Svendson, Tove
45 Pettersen, Kari 19
Hm AVG(column) Hm AVG tr trung bnh ca d liu trong mt ct c u7c nh
php chn. Cc tr NULL s khng c tnh ton. V d V d ny tr v tui trung bnh
ca nhng ngi trong bng "Persons": SELECT AVG(Age) FROM Persons Kt qu
32.67 V d V d ny tr v tui trung bnh ca nhng ngi c tui ln hn 20 tui:
SELECT AVG(Age) FROM Persons where Age>20 Kt qu 39.5
Hm MAX(column) Hm MAX tr v tr ln nht trong mt ct. Cc tr NULL s
khng c tnh ton. V d SELECT MAX(Age) FROM Persons Kt qu: 45
Hm MIN(column) Hm MIN tr v tr ln nht trong mt ct. Cc tr NULL s
khng c tnh ton. V d SELECT MIN(Age) FROM Persons Kt qu: 19 Ch : Cc
hm MIN v MAX cng c th dng trn cc ct vn bn, tm tr ln nht v nh nht
theo th t alphabet.
Hm SUM(column) Hm SUM tng ca mt ct c c nh php chn. Cc tr NULL s
khng c tnh ton. V d V d ny tr v tng s tui ca nhng ngi trong bng
"Persons": SELECT SUM(Age) FROM Persons Kt qu:
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98 V d V d ny tr v tng s tui ca nhng ngi ln hn 20 tui. SELECT
SUM(Age) FROM Persons where Age>20 Kt qu: 79
SQL Group By v SQL Having
Cc hm tng (nh SUM) thng km theo chc nng GROUP BY.
T kha GROUP BY T kha GROUP BY c thm vo SQL v cc hm tng (nh SUM)
tr v tng ca tt c cc tr trong ct mi khi chng ta gi n. Thiu chc nng
GROUP BY, khng th tm tng ca mi nhm tr ring trong ct. C php ca GROUP
BY nh sau: SELECT column,SUM(column) FROM table GROUP BY column
V d GROUP BY Bng "Sales": Company Amount W3Schools 5500 IBM 4500
W3Schools 7100 Vi SQL: SELECT Company, SUM(Amount) FROM Sales Tr v
kt qu nh sau: Company SUM(Amount) W3Schools 17100 IBM 17100
W3Schools 17100 SQL trn khng tr v tng ring bit ca tng cng ty. Dng
mnh GROUP BY nh sau: SELECT Company,SUM(Amount) FROM Sales GROUP BY
Company S tr v kt qu ng: Company SUM(Amount) W3Schools 12600 IBM
4500
T kha The HAVING T kha HAVING c thm vo SQL v t kha WHERE khng th
dng vi cc hm tng (nh hm SUM). Thiu t kha HAVING s khng th kim tra
cc iu kin dng hm tng. C php ca HAVING nh sau: SELECT
column,SUM(column) FROM table GROUP BY column HAVING SUM(column)
condition value
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Bng "Sales": Company Amount W3Schools 5500 IBM 4500 W3Schools
7100 Vi SQL: SELECT Company,SUM(Amount) FROM Sales GROUP BY Company
HAVING SUM(Amount)>10000 Tr v kt qu Company SUM(Amount)
W3Schools 12600
Cc b danh (Alias) SQL
Vi SQL, cc b danh (alias) c th dng thay cc tn ct v cc tn
bng.
B danh tn Ct C php nh sau: SELECT column AS column_alias FROM
table
B danh tn Bng C php nh sau: SELECT column FROM table AS
table_alias
V d: Dng b danh tn Ct Bng Persons: LastName FirstName Address
City Hansen Ola Timoteivn 10 Sandnes Svendson Tove Borgvn 23
Sandnes Pettersen Kari Storgt 20 Stavanger Vi SQL sau: SELECT
LastName AS Family, FirstName AS Name FROM Persons S tr v kt qu
sau: Family Name Hansen Ola Svendson Tove Pettersen Kari
V d: Dng b danh tn Bng Bng Persons: LastName FirstName Address
City Hansen Ola Timoteivn 10 Sandnes Svendson Tove Borgvn 23
Sandnes
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Pettersen Kari Storgt 20 Stavanger Vi SQL sau: SELECT LastName,
FirstName FROM Persons AS Employees S tr v kt qu sau: Bng
Employees: LastName FirstName Hansen Ola Svendson Tove Pettersen
Kari
SQL Join
Joins and cc Kha (Key) i khi chng ta chn d liu t hai bng to kt
qu, Chng ta thc hin mt kt ni (join). Cc bng trong c s d liu c th
lin h vi cc bng khc thng qua cc kha. Mt kha chnh (primary key) l mt
ct vi cc tr duy nht cho mi hng. Mc tiu l rng buc d liu, tham chiu
cho cc bng, khng cn lp li tt c d liu trong tng bng. Trong bng
"Employees" pha di, ct "ID" l kha chnh, ngha l ct ny khng c hai hng
cng ID. ID dng phn bit hai ngi nu c hai c cng tn. Khi bn xem bng v
d pha di, ch rng:
Ct "ID" l kha chnh ca bng "Employees" Ct "ID" trong bng "Orders"
dng tham chiu cc tn trong bng "Employees" khng cn a cc tn ny vo
bng Orders
Employees: ID Name 01 Hansen, Ola 02 Svendson, Tove 03 Svendson,
Stephen 04 Pettersen, Kari Orders: ID Product 01 Printer 03 Table
03 Chair
Tham chiu n hai Bng Chng ta c th chn d liu t hai bng bng cch
tham chiu n hai bng, nh sau:
V d Ai ng k mt sn phm v ng k sn phm no? SELECT Employees.Name,
Orders.Product FROM Employees, Orders WHERE Employees.ID =
Orders.ID Kt qu Name Product Hansen, Ola Printer Svendson, Stephen
Table
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Svendson, Stephen Chair
V d Ai ng k mt my in? SELECT Employees.Name FROM Employees,
Orders WHERE Employees.ID = Orders.ID AND Orders.Product =
'Printer' Kt qu Name Hansen, Ola
Dng cc Kt ni (Join) HOC, chng ta c th chn d liu t hai bng vi t
kha JOIN, ging nh sau:
V d INNER JOIN C php SELECT field1, field2, field3 FROM
first_table INNER JOIN second_table ON first_table.keyfield =
second_table.foreign_keyfield Ai ng k mt sn phm v ng k sn phm no?
SELECT Employees.Name, Orders.Product FROM Employees INNER JOIN
Orders ON Employees.ID = Orders.ID INNER JOIN tr v tt c cc hng t
hai bng khi iu kin c so trng. Nu cc hng trong bng Employees khng so
trng trong bng Orders, hng s khng c lit k ra. Kt qu Name Product
Hansen, Ola Printer Svendson, Stephen Table Svendson, Stephen
Chair
V d LEFT JOIN C php SELECT field1, field2, field3 FROM
first_table LEFT JOIN second_table ON first_table.keyfield =
second_table.foreign_keyfield Lit k tt c nhn vin, v cc ng k mua ca
h nu c. SELECT Employees.Name, Orders.Product FROM Employees LEFT
JOIN Orders ON Employees.ID = Orders.ID LEFT JOIN tr v tt c cc hng
t bng th nht (Employees), cho d n khng c so trng trong bng th hai
(Orders). Nu cc hng trong bng Employees khng so trng trong bng
Orders, nhng hng ny cng c lit k. Kt qu Name Product Hansen, Ola
Printer Svendson, Tove Svendson, Stephen Table Svendson, Stephen
Chair Pettersen, Kari
V d RIGHT JOIN
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C php SELECT field1, field2, field3 FROM first_table RIGHT JOIN
second_table ON first_table.keyfield =
second_table.foreign_keyfield Lit k tt c nhn vin, v cc ng k mua ca
h nu c. SELECT Employees.Name, Orders.Product FROM Employees RIGHT
JOIN Orders ON Employees.ID = Orders.ID RIGHT JOIN tr v tt c cc hng
t bng th hai (Orders), cho d n khng c so trng trong bng th nht
(Employees). Nu c bt k hng no trong bng Orders khng c so trng trong
bng Employees, cc hng ny cng c lit k. Kt qu Name Product Hansen,
Ola Printer Svendson, Stephen Table Svendson, Stephen Chair
V d Ai ng k mt my in? SELECT Employees.Name FROM Employees INNER
JOIN Orders ON Employees.ID = Orders.ID WHERE Orders.Product =
'Printer' Kt qu Name Hansen, Ola
SQL To C s d liu v Bng
To mt C s d liu to mt c s d liu: CREATE DATABASE
database_name
To mt bng to mt bng trong mt c s d liu: CREATE TABLE table_name
( column_name1 data_type, column_name2 data_type, ....... )
V d V d ny minh ha cc bn to mt bng tn "Person", vi bn ct
tn:"LastName", "FirstName", "Address", v "Age": CREATE TABLE Person
( LastName varchar, FirstName varchar, Address varchar, Age int ) V
d ny minh ha cch bn ch nh kch thc ti a ca vi ct:
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CREATE TABLE Person ( LastName varchar(30), FirstName varchar,
Address varchar, Age int(3) ) Kiu d liu c ch nh l kiu d liu cha
trong ct. Bng di cha cc kiu d liu thng gp nht trong SQL: Kiu d liu
M t integer(size) int(size) smallint(size) tinyint(size)
Ch cha s nguyn. S k t s ti a c ch nh trong du ngoc n
decimal(size, d) numeric(size,d)
Cha s vi phn s. S k t s ti a c ch nh trong "size". S k t s ti a
bn phi (phn phn s) c ch nh trong "d"
char(size) Cha chui c kch thc c nh (c th cha k t ch, s, v cc k t
c bit). Kch thc c nh c ch nh trong du ngoc n
varchar(size) Cha mt chui c chiu di thay i (c th cha k t ch, s,
v cc k t c bit). Kch thc ti a c ch nh trong du ngoc n
date(yyyymmdd) Cha mt ngy
To Ch mc (Index) Ch mc c to ra trn mt bng c sn nh v thm nhanh v
hiu qu cc hng. C th to mt ch mc trn mt hoc nhiu ct ca mt bng, vi mt
ch mc cho mt tn. Ngi dng khng nhn thy cc ch mc, chng ch dng tng tc
truy vn. Ch : Cp nht mt bng cha ch mc cn nhiu thi gian hn cp nht mt
bng khng cha ch mc, v ch mc cng cn cp nht. Tuy nhin, tng tt l to ch
mc ch trn cc ct thng tm kim nht. Mt Ch mc duy nht To mt ch mc duy
nht trn mt bng. mt ch mc duy nht ngha l khng th c hai hng c cng mt
tr ch mc. CREATE UNIQUE INDEX index_name ON table_name
(column_name) "column_name" ch nh ct bn mun ch mc. Mt Ch mc n gin
To mt ch mc n gin trn mt bng. Khi t kha UNIQUE khng c, cc tr trng s
c cho php. CREATE INDEX index_name ON table_name (column_name)
"column_name" ch nh ct bn mun ch mc.
V d V d ny to mt ch mc n gin, c tn "PersonIndex", trn field
LastName ca bng Person: CREATE INDEX PersonIndex ON Person
(LastName) Nu bn mun ch mc cc tr trong mt ct theo th t gim
(descending), bn c th thm t DESC sau tn ct: CREATE INDEX
PersonIndex ON Person (LastName DESC) Nu bn mun ch mc nhiu hn mt ct
bn c th lit k cc tn ct trong du ngoc n, tch chng bng du phy: CREATE
INDEX PersonIndex ON Person (LastName, FirstName)
Xa ch mc Bn c th xa mt ch mc c trong mt bng vi pht biu DROP.
DROP INDEX table_name.index_name
Xa mt c s d liu hoc bng xa mt c s d liu:
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SQL Tutorial
Dng Thin T 19
DROP DATABASE database_name xa mt bng: DROP TABLE table_name xa
ton b d liu trong bng m khng xa bng: DELETE TABLE table_name
SQL Alter Table
Alter Table Pht biu ALTER TABLE dng thm hay loi b cc ct trong mt
bng cho trc. ALTER TABLE table_name ADD column_name datatype ALTER
TABLE table_name DROP column_name
Person: LastName FirstName Address Pettersen Kari Storgt 20
V d thm mt ct tn "City" vo bng "Person": ALTER TABLE Person ADD
City varchar(30) Kt qu: LastName FirstName Address City Pettersen
Kari Storgt 20
V d loi ct "Address" khi bng "Person": ALTER TABLE Person DROP
Address Kt qu: LastName FirstName City Pettersen Kari
Sch SQL
Sch M t
Teach Yourself SQL in 10 Minutes
September 1999
Loi sch tutorial, t chc thnh chui cc bi hc-10 pht n gin.
-
SQL Tutorial
Dng Thin T 20
SQL Queries for Mere Mortals
August 2000
Gip ngi dng mi hc c bn v cc truy vn SQL, v cung cp mt hng dn
tham chiu cn thit vi ngi dng c trnh cao hn.
SQL: The Complete Reference
October 1999
Cung cp tt c nhng g bn cn bit v SQL.
Professional SQL Server 2000 Programming
December 2000
Cung cp mt hng dn ton din lp trnh vi SQL Server 2000.
Professional SQL Server 7.0 Programming
September 1999
Cung cp tng quan v tt c cc b phn ca SQL Server.
