在数据库设计中，有两种方法可设定自动化的资料处理规则，一种是条件约束，一种是触发器,一般而言，条件约束比触发器较容易设定及维护，且执行效率较好，但条件约束只能对资料进行简单的栏位检核，当涉及到多表操作等复杂操作时，就要用到触发器了。
一个数据库系统中有两个虚拟表用于存储在表中记录改动的信息，分别是：
虚拟表Inserted 虚拟表Deleted
在表记录新增时 存放新增的记录 不存储记录
修改时 存放用来更新的新记录 存放更新前的记录
删除时 不存储记录 存放被删除的记录
触发器的种类及触发时机
After触发器：触发时机在资料已变动完成后，它将对变动资料进行必要的善后与处理，若发现有错误，则用事务回滚（Rollback Transaction)将此次操作所更动的资料全部回复。
Istead of 触发器：触发时机在资料变动前发生，且资料如何变动取决于触发器现在介绍一下创建触发器的编写格式：
After类型:
Create Trigger 触发器名称
on 表名
after 操作(insert,update)
as
Sql语句
Instead类型
Create Trigger 触发器名称
on 表名
Instead of 操作(update,delete)
as
Sql语句
实例1:
在订单(表orders)中的订购数量(列名为num)有变动时，触发器会先到客户(表Customer)中
取得该用户的信用等级(列名为Level),然后再到信用额度(Creit)中取出该等级
许可的订购数量上下限，最后比较订单中的订购数量是否符合限制。
代码：
Create Trigger num_check
on orders
after insert,update
as
if update(num)
begin
if exists(select a.* from orders a join customer b on a.customerid=b.customerid
join creit c on b.level=c.level
where a.num between c.up and c.down)
begin
rollback transaction
exec master..xp_sendmail 'administrator','客户的订购数量不符合限制'
end
end
实例2:
有工资管理系统中，当公司对某员工甲的月薪进行调整时，通常会先在表员工中修改薪资列,然后在表员工记录中修改薪资调整时间与薪资
Create trigger compensation
on 员工
after update
as
if @@rowcount=0 return
if update(薪资)
begin
insert 员工记录
select 员工遍号,薪资,getdate()
from inserted
end
ez=application/andrew-inset
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cpt=application/mac-compactpro
doc=application/msword
bin=application/octet-stream
dms=application/octet-stream
lha=application/octet-stream
lzh=application/octet-stream
exe=application/octet-stream
class=application/octet-stream
so=application/octet-stream
dll=application/octet-stream
oda=application/oda
pdf=application/pdf
ai=application/postscript
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ps=application/postscript
smi=application/smil
smil=application/smil
mif=application/vnd.mif
xls=application/vnd.ms-excel
ppt=application/vnd.ms-powerpoint
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wmlsc=application/vnd.wap.wmlscriptc
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vcd=application/x-cdlink
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dir=application/x-director
dxr=application/x-director
dvi=application/x-dvi
spl=application/x-futuresplash
gtar=application/x-gtar
hdf=application/x-hdf
js=application/x-javascript
skp=application/x-koan
skd=application/x-koan
skt=application/x-koan
skm=application/x-koan
latex=application/x-latex
nc=application/x-netcdf
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sh=application/x-sh
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swf=application/x-shockwave-flash
sit=application/x-stuffit
sv4cpio=application/x-sv4cpio
sv4crc=application/x-sv4crc
tar=application/x-tar
tcl=application/x-tcl
tex=application/x-tex
texinfo=application/x-texinfo
texi=application/x-texinfo
t=application/x-troff
tr=application/x-troff
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man=application/x-troff-man
me=application/x-troff-me
ms=application/x-troff-ms
ustar=application/x-ustar
src=application/x-wais-source
xhtml=application/xhtml+xml
xht=application/xhtml+xml
zip=application/zip
au=audio/basic
snd=audio/basic
mid=audio/midi
midi=audio/midi
kar=audio/midi
mpga=audio/mpeg
mp2=audio/mpeg
mp3=audio/mpeg
aif=audio/x-aiff
aiff=audio/x-aiff
aifc=audio/x-aiff
m3u=audio/x-mpegurl
ram=audio/x-pn-realaudio
rm=audio/x-pn-realaudio
rpm=audio/x-pn-realaudio-plugin
ra=audio/x-realaudio
wav=audio/x-wav
pdb=chemical/x-pdb
xyz=chemical/x-xyz
bmp=image/bmp
gif=image/gif
ief=image/ief
jpeg=image/jpeg
jpg=image/jpeg
jpe=image/jpeg
png=image/png
tiff=image/tiff
tif=image/tiff
djvu=image/vnd.djvu
djv=image/vnd.djvu
wbmp=image/vnd.wap.wbmp
ras=image/x-cmu-raster
pnm=image/x-portable-anymap
pbm=image/x-portable-bitmap
pgm=image/x-portable-graymap
ppm=image/x-portable-pixmap
rgb=image/x-rgb
xbm=image/x-xbitmap
xpm=image/x-xpixmap
xwd=image/x-xwindowdump
igs=model/iges
iges=model/iges
msh=model/mesh
mesh=model/mesh
silo=model/mesh
wrl=model/vrml
vrml=model/vrml
css=text/css
html=text/html
htm=text/html
asc=text/plain
txt=text/plain
rtx=text/richtext
rtf=text/rtf
sgml=text/sgml
sgm=text/sgml
tsv=text/tab-separated-values
wml=text/vnd.wap.wml
wmls=text/vnd.wap.wmlscript
etx=text/x-setext
xsl=text/xml
xml=text/xml
mpeg=video/mpeg
mpg=video/mpeg
mpe=video/mpeg
qt=video/quicktime
mov=video/quicktime
mxu=video/vnd.mpegurl
avi=video/x-msvideo
movie=video/x-sgi-movie
ice=x-conference/x-cooltalk
为什么一个结果是0，一个是10？
第一段：
Private Sub Form_Click()
Dim a As Integer
For i = 1 To 10
Call abcd(a)
Next i
Print a
End Sub
Sub abcd(ByVal x)
x = x + 1
End Sub
第二段：
Private Sub Form_Click()
Dim a As Integer
For i = 1 To 10
Call abcd(a)
Next i
Print a
End Sub
Sub abcd(x)
x = x + 1
End Sub
byval是把内存数值的拷贝给程序，所以改变的只是拷贝，内存原来的值是不会改变的。
第一个a的值并没有变化。
