<>Delete% G* A0 V. J, M: R, c
用途: ) U. y4 I. ]2 r+ P/ O& g, Q0 r% c% _
</P><> 删除表中的数据</P><>语法: + a, b8 M- a, }& ~* y
</P><>DELETE FROM table_name WHERE column_name = some_value # m4 K. b: o! ~9 @! S& j3 p3 o
p* |) H; |+ v; B8 g$ V% Z例: 0 X" e# k- X4 Y1 J5 [5 l
</P><>"erson"表中的原始数据: ' }: ^- O3 b( u8 i
</P><>LastName 4 P+ b6 i4 X9 M & b. |9 M% a& o+ y* h4 |2 o2 F FirstName ; O; i* w3 n% T* p . x. g% c; s! `0 { Address 0 h6 h6 @8 y0 I9 v+ ?& d
! g+ c9 Z4 X: S4 N `+ m City 0 N$ T% m+ z. Z9 N' m/ Y! F; V7 g9 ~ 8 y5 |# A. S' j& m* L
2 g* w' T) m5 k {8 N) x* @! g2 zNilsen 7 X6 B q' g1 o* K 0 s+ e# I3 K" m4 a/ Y1 g' M
Fred - D6 X7 V+ k, E# R
<>Create Table " W _" w, d# E用途: : @8 V: `4 T( l. z. P </P><> 建立新的资料表。</P><>语法: 7 J* t+ b; S$ i I/ v </P><>CREATE TABLE table_name + {3 u. e6 O" D( ~
( 4 d- D) g+ o# C9 q4 E6 V
column_name1 data_type, ) ]# J2 J& ?6 u1 W, M- h
column_name2 data_type, ; a! ]3 g' Z( ~: J....... ; @8 Q2 {. O, i6 S8 ?, x) ; s+ [9 V: |4 w7 B! L( D8 K0 A
1 g0 g4 b/ d5 B& |2 z( M
例: ( J0 g% i1 m/ E; ]/ N8 M
</P><>创建一张叫"erson"的表,该表有4个字段"LastName", "FirstName", "Address", "Age": 6 [! Z/ n$ Y& k# V* s6 y" h </P><>CREATE TABLE Person 9 ^7 E( O& D& \" Z( # M! c4 w* h& d* J% O$ t" B
LastName varchar, - b0 K* Q# c( H* B3 b- FFirstName varchar, 1 h. b7 q. {; n8 w# N2 N
Address varchar, ) _+ ~9 W9 e: [1 [" _3 |) I
Age int 1 m# J. ~5 V/ D, t, J
) : F- {4 e! e4 D9 c3 [ </P><>如果想指定字段的最大存储长度,你可以这样:</P><>CREATE TABLE Person 3 Q3 u/ T! z5 m1 w: K( ?$ n" f# a$ p! KLastName varchar(30), . {+ j1 i" B5 ~5 {3 l/ _$ r5 o3 TFirstName varchar(30), $ _6 ~6 J! w& P9 qAddress varchar(120), ! }) H) A+ }0 ~, A+ xAge int(3) 6 s: w4 B8 A- F& \) : Q4 m" z$ B: e) }
</P><>下表中列出了在SQL的一些数据类型:</P><>Data Type ! B& a7 a. h% i1 E2 J4 n5 E0 a . Y7 k: d3 B& }8 \0 i Description / f1 z8 K ?# C5 F4 j + \4 _7 N0 J' O: A! T : [+ N' N: B0 r5 Finteger(size)5 `8 V' ~9 X6 u* Z2 \- V1 |- m$ @
int(size)4 q1 ^6 h4 u2 c( C6 ` L/ t* x. y
smallint(size) 3 x$ l1 f5 t6 x! ptinyint(size) $ n* |6 E4 N5 ?& E2 n
# f8 g# O; d$ h* i( O' r3 M Hold integers only. The maximum number of digits are specified in parenthesis. # W+ C- O: ~1 m ( q& l4 H# e+ g! ? i ( m/ ^* w+ v. a: y) B" xdecimal(size,d) 9 e3 k9 S$ u* F. Cnumeric(size,d) 6 f1 W Q! J% h- k8 l# w + V+ ^: S; ~7 Q( p Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d". , w1 a; o. e5 P2 V0 o( T 5 b0 B1 X1 L: O7 K0 a " A8 t) K* U/ H2 k
char(size) 9 y: a7 |- \9 p; M* r ) S ^" d$ Z1 T+ j9 m& C! c
Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis. 3 c- f; V# ^3 B/ f' G & H$ b% ^1 ~9 C1 y ^2 @5 \. Z 1 r0 [: ^/ _6 X$ \0 I7 p$ A d
varchar(size) 8 v0 H0 \# r! I8 s; d & F" Z* e; o, U$ A- ]- W# b$ d
Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis. ' W7 s& W. G8 h4 q4 E 3 x7 K E3 w" n6 w1 H U& a+ e
, B9 N5 D9 a2 p3 Adate(yyyymmdd) " d$ B) b7 J, S% _& v
; {* |! E4 q" ] Holds a date 8 ]& ^) M3 T t & V& J3 d( d, m; J* I3 P& ]2 m- Q 6 g: h* T5 K! `! j( H8 ? ?
* d0 P" U* b P9 k7 ] </P><> 5 ^9 Y# H$ P2 s U) E& X, m1 F </P><>Alter Table & i& s- Y* M% G1 ]6 ^用途: 4 C; [- S. T" V; x% _- z7 M
</P><> 在已经存在的表中增加后者移除字段</P><>语法: " C( H$ ~) E- U. h </P><>ALTER TABLE table_name 8 {) \0 Q# D* n; FADD column_name datatype </P>
<>sum) a( \# a, @3 y
用途: % T! B, `7 q# Y2 F f
</P><> 以表达式传回所有值的总和,或仅 DISTINCT 值。SUM 仅可用于数值资料行。已忽略 Null 值。 % I' ~8 g% h: F5 w+ |
</P><>语法: 2 H& l5 R* s3 q
</P><>SELECT SUM(column_name) FROM table_name 6 y+ S0 @. M) V: u' i: ] </P><>例:</P><>"ersons"表中原始数据如下:</P><>Name 3 F0 ?" t0 D3 d4 u. g/ E7 A
1 T% P7 o: K* t9 h1 `9 C& ~% @. j
Age % @# _% \9 q' y5 H# g- }! o
3 B7 |! G1 {; a" G8 O
% f+ y4 b$ b* B1 @1 l5 X3 H& `# \. }
Hansen, Ola 4 {- z* D! Q% w, M% R4 a5 ]1 P$ j! X
/ z! V) F4 z5 ^9 z( Y1 W
34 ; h- |: m. j3 b( y3 y0 G - K: x+ o" L/ F G4 x
3 |, t+ E5 d R1 G1 R5 r- r: oSvendson, Tove 0 t, k% l- ~$ o, j: l( H& E + \+ y4 [( J+ _ 45 . z2 v5 a) h, }* i" p ! c; Q, i# g% k % _0 H- j; Q x# Z
Pettersen, Kari ! R8 p- W' } m/ X ( f# j5 L8 g) a5 T- P: ~) e 19 - y$ a2 ]. e! u# G " P3 p/ Z! f( x! ^ ) Z p& n2 W$ @选取"ersons"表中所有人的年龄总和: 6 ~& ?8 e' b. E7 r0 S; J2 l0 A" B6 M" l </P><>SELECT SUM(Age) FROM Persons - p$ u X a0 s( ] </P><>执行结果:</P><>98 / \$ K0 U3 h: v* {' X) } @. S7 \
0 b% J) }, T- p- P4 M选取"ersons"表中年龄超过20岁的人的年龄总和: ) Q6 X0 o" E A- \7 Y
</P><>SELECT SUM(Age) FROM Persons WHERE Age>20 b, g9 q M7 d6 a, S. n </P><>执行结果:</P><>79 & p a" K7 M8 A5 F3 m
* B6 l# Y8 P: \! x & T/ y) _4 }' L/ ~+ R
</P><>avg / X Y2 S6 {/ @% E* E用途: 3 q+ c' P* m3 |" ?
</P><> 传回选取的结果集中值的平均值。已忽略 Null 值。 - J, O) ^/ j/ h+ C/ p
</P><>语法: 5 r" C# F) k7 l1 x6 U </P><>SELECT AVG(column_name) FROM table_name ) L7 j9 g. k9 k </P><>例:</P><>"ersons"表中原始数据如下:</P><>Name $ `6 \, ]/ u9 _$ l7 T/ S6 p8 y " P+ I1 Q2 K/ u7 {, s Age 2 S; X3 R0 q) X' Z1 r: ~ 3 [' h; l5 u# l3 g% e * G9 ?3 E" T7 C6 L7 J2 R5 G: kHansen, Ola % A/ s& K N! v% ^8 B5 a
, o3 |5 w5 Q- Z# B
34 9 Q5 R2 m" A) q' U1 h; ]" z* o
7 O5 S; A3 p. }! G6 Z5 W ! p! k6 j+ E0 S9 q- {0 z, F; _% T
Svendson, Tove , b2 `/ P7 ]) S$ s5 m0 X+ W! V
% o0 @( N) I& C; D" ~- o5 @8 l( {
45 ! v3 w& b+ x+ l % ^ a, y: t8 |+ R$ e1 l' s8 q - y1 n% ~# J* J- A- f2 j8 X o
Pettersen, Kari : l0 F9 ?5 s; q5 j
0 o1 Z) h+ \: S. ~* M+ w+ d
19 $ R; ?; x; J# [/ i # e, d% X! o. \8 `0 ^ 4 W" q4 X1 C/ P6 A5 ?% i6 h% s
选取"ersons"表中所有人的平均年龄: 6 q- }& j+ z* E& d$ P' L
</P><>SELECT AVG(Age) FROM Persons 3 K0 B3 f6 _; v" L4 | </P><>执行结果:</P><>32.67 5 V3 F- u+ v) Z" I! H# A: c+ P / d3 J' u8 k( g& S/ _选取"ersons"表中年龄超过20岁的人的平均年龄: 3 P% b+ \& j7 i1 P7 y9 y </P><>SELECT AVG(Age) FROM Persons WHERE Age>20 # ~* ]! C4 f: H( Y) Q# I% i </P><P>执行结果:</P><P>39.5 ! O# I7 t1 o7 D( b% e
% R5 ]- l6 S' ^ . G6 A4 F, \3 e# h( r0 \7 f5 m
</P>