# x1 r1 Z: L/ U2 @5 i5 B) M8 h% z Hold integers only. The maximum number of digits are specified in parenthesis. * F- J2 A5 M( ~' _
% q; j( }5 T7 v0 p4 p 2 O) E- n, b+ _; _1 ]decimal(size,d)1 P) ?+ s c" I7 Y4 x- N# C
numeric(size,d) , F; e1 I( p9 P/ k; U6 v
3 q2 d0 F( ~( Y3 { 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". $ O) [& x" r( m, G9 d5 I! ? " x3 ~+ @/ ?+ u 2 @/ i f; Q6 `char(size) 0 ?: y% \" N( l7 v, P$ U 6 l5 x- o9 ~9 o/ ?9 X5 G8 ` Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis. ' I& I2 R6 x5 ^4 Z, w% `5 t 3 M" \. r" f5 P5 @1 K- j% |
|$ m& u ^ U' y! S6 H/ D
varchar(size) * W( U* S7 z' w7 x6 a- A & u, b# p, n6 l- P% X Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis. ' f. u( E0 u. H7 i( I5 |% t; b4 f' B1 }
9 J% ]6 v, Z6 R4 n - s6 ^# U2 I+ x# w
date(yyyymmdd) % D; A+ e5 ^% C3 A2 K
; {: X5 c1 e2 W* M. S7 E% o# V4 ~0 B& u Holds a date 5 \ i9 l0 I: R : @( T& _( o' ^
3 u5 ~. n( q3 t5 `5 k* P" M
<>Alter Table0 @% k$ t) n( D" X
用途: 1 w% U* d$ R0 { [+ v1 c
</P><> 在已经存在的表中增加后者移除字段</P><>语法: 4 O7 O0 g3 r" h4 M" A* H p/ c
</P><>ALTER TABLE table_name f4 u; _1 ` k& D
ADD column_name datatype 4 y" K( Q5 l' E/ t
ALTER TABLE table_name ; {" w! H% _2 y' S
DROP COLUMN column_name 1 m' |9 I( A- W/ c1 F/ B8 ?' D " z2 Q- M& K0 S2 M. u4 G u2 S# `
注意:某些数据库管理系统不允许移除表中的字段 : C4 c6 [ U* f3 } </P><> / K: T* S( h2 {+ d </P><>例: # `7 c/ l8 w! P7 l: d. g </P><>"erson"表中的原始数据: $ x- c( `% b1 b7 M
</P><>LastName , g& d; G; n8 m" _+ p 5 Y/ k8 r: I0 K3 X- q1 N FirstName ( r/ x, Q0 Z5 `1 X) v + q: }! }- s9 `, o" C7 v S- e- Q Address ( @4 m3 o& N+ T2 f 5 D, {) u( t x" k6 L$ c# b6 v . Y B0 O5 u* O1 ]; D8 @. A) zPettersen 5 l2 M7 j; O( o 2 u2 i9 D# Q9 A Kari ! D) k W3 z/ t, }6 ?
# S7 m9 R: |. o' S Storgt 20 # C7 n% x% V1 n9 H1 \6 o1 b, S
% n/ c& K# n* H( O: x1 |# M& f4 ?8 I
& u1 V7 S. p7 @5 j' q
8 Y$ C3 M$ Q, [5 Q2 o2 b& G </P><>在Person表中增加一个名为City的字段:</P><>ALTER TABLE Person ADD City varchar(30) + _: A) [/ e/ U; h6 k1 O
</P><>增加后表中数据如下:</P><>LastName 6 j9 ]/ R: e8 m5 e( M) ^( V0 c: z 7 {8 f( }- Q. R! j3 s5 Z7 d FirstName ( u3 W5 d2 X+ ]. G 9 ~0 j* j6 M. B* V; W
Address 4 \! T [! ~; \+ M; Y' L / G# P& c- y5 g* p4 I5 Y/ Z) b
City / g+ e3 [3 U. U/ A- H
6 c" E2 M7 y2 i9 l! A : y* F3 W- f( r' W
Pettersen # D, Q8 s8 x4 H( q " }! _* R% S, e Kari " f( ^: y1 d C' l" A+ N! G
- o$ M( w2 o, o' B) E- p/ B; h Storgt 20 . \9 u- }# K$ @8 L5 q & e7 f' v" w# Q9 B - i0 Z# W5 u* K" l
8 K2 B% Y+ I9 ]
0 M% U7 S! f: ^2 V& L1 E
3 G& `8 O+ N0 h# G) [ </P><>移除Person表中原有的Address字段:</P><>ALTER TABLE Person DROP COLUMN Address 5 b0 c% k0 w) ]' h8 w2 L </P><>移除后表中数据如下:</P><>LastName 8 M, m4 O1 Q/ {, p. b; g+ c' k 4 r r) _" W& B3 P
FirstName ) {; \) l: k2 ~' q4 B5 x2 u
- Y: `. a5 u. o% Z( Y: s
City . `# {& @. c' U* h+ [( p , g8 D! e/ t) y+ S4 U( R3 X
( V5 k4 W$ Z0 f
Pettersen % X. `1 G0 ]8 s9 r/ _ x4 |) ~
5 |& Z+ C+ }$ O4 _8 ~
Kari : R3 n' i/ w8 z; [</P>
<>sum 6 c( ?7 N) y$ n$ x用途: + g2 M. h5 S8 [6 G </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。 ; u- E X* T2 D% @6 s! n
</P><>语法: : i) A0 l. l1 c </P><>SELECT SUM(column_name) FROM table_name ; f r. B( o/ {, A, [2 N </P><>例:</P><>"ersons"表中原始数据如下:</P><>Name 9 Y) r" ~, h' Y; t1 a0 l2 e
Z. `# q$ Q" M f9 D8 P$ m
Age : N6 }9 B$ P( L# m& M' F* ^) S# y2 @ . z/ |8 n6 ^+ i0 B) d
' V0 y" P' j& k: k( n
Hansen, Ola * e" Z5 S. f) d# g" ?- l
2 {4 [+ G# p6 e# V* d: ] 34 . }: P0 z: B! l5 ^; N* F' T 7 V. H: j0 \$ |" D0 `1 d. h" e ' z/ B! U/ Q2 F% K- A% X
Svendson, Tove 8 L9 d" A- E& L0 n
- y1 I2 a- x6 C 45 9 S; f7 } T5 ] L$ {- j 1 d. F2 D' r: |& a % u/ I5 w* X! f7 }' E4 B- `
Pettersen, Kari 4 a% O( Z- D2 ~# [- | 5 s2 G$ L# f8 v H8 ]/ h) v" p 19 4 I, u( d, J" K; ~9 G7 U5 _' D* x $ Z Y4 }- R( t* C, i: x( m 2 X! [4 [4 P( `' l: L选取"ersons"表中所有人的年龄总和: % S( o, K6 R+ @- ^% G1 J
</P><>SELECT SUM(Age) FROM Persons ) R8 V t+ E- L
</P><>执行结果:</P><>98 8 V1 S: r9 b N: _
1 Z( E3 B+ |2 x2 m2 u8 Y+ _选取"ersons"表中年龄超过20岁的人的年龄总和: / Z8 `5 o! S8 l# Q0 c# T$ Y; i, q0 f
</P><>SELECT SUM(Age) FROM Persons WHERE Age>20 7 D! B# A4 p0 N
</P><>执行结果:</P><>79 , N: v v) s4 K9 ]8 F
+ W% t% j7 Z$ v5 M8 U( Q! @7 R
! G9 n0 H( T) e& y! f </P><>avg 7 \$ w1 l ] ~( ~% h用途: $ N- @' B! V6 S ?
</P><> 传回选取的结果集中值的平均值。已忽略 Null 值。 ) P7 Z. J$ X6 v/ i' o
</P><>语法: 3 ~4 i: ^ x& Y$ ^# A) \ </P><>SELECT AVG(column_name) FROM table_name $ ?: G) ?9 w% M& I$ w2 d </P><>例:</P><>"ersons"表中原始数据如下:</P><>Name ' h- @% T( L" r! K 0 w' f7 q- a, e. Z$ Q Age ( z8 A" k/ j" _. E. b, B& e1 ~' y % E1 _( t+ ~, t; d9 I |# F4 ? . V( N8 ^- c; ?+ N; R
Hansen, Ola # X0 {- [4 {$ G 0 @) q' E/ C# \, Q% R+ m 34 " a2 d4 [ N4 V. E ' _8 _5 n @1 q, w k, S3 }, J
! B, ^2 q' w: G6 g* y. u
Svendson, Tove % ` }. O2 M6 @& f / s0 t! I7 {9 x1 t
45 3 |8 i6 q" m8 W, {5 o) K - V+ O, S4 L3 w' _ 2 d' Z$ t8 C6 m! B. N) Z1 V. z
Pettersen, Kari $ L, B& Y4 b% i( O3 p9 b / `0 r9 ]5 o7 U1 s2 F* ?
19 $ z$ [) y. ~& I2 X y; F * Z9 r. X" V/ _/ h$ t
) p) k% d9 O. v! q/ K选取"ersons"表中所有人的平均年龄: ' q- D! N: I1 J- {8 l7 `- x </P><>SELECT AVG(Age) FROM Persons 0 \6 Q9 {6 H% j2 P0 ~; A+ {
</P><>执行结果:</P><>32.67 6 w# P5 L* s2 P9 f
% _% B3 K% l2 ^' z3 T% g2 k选取"ersons"表中年龄超过20岁的人的平均年龄: # r8 M5 J; G6 b; i6 g/ L; e+ `( d/ K
</P><>SELECT AVG(Age) FROM Persons WHERE Age>20 , ]4 }5 w) G8 `$ ~0 }# t" @
</P><P>执行结果:</P><P>39.5 ; W) I. S4 K0 D* e' S
0 _, f7 `8 l9 M
8 h i) d8 N% x6 q
</P>