delphi 连接DBF
使用 DBF 文件时，文件夹表示数据库，单个 DBF 文件表示表
1、BDE，已淘汰
table1.Databasename:=dbdir;//设置库路径
table1.TableName:=dbname;
2、ADO
1）用Microsoft jet4.0驱动字符串，大多数机器都安装了，可用
2）Microsoft Visual FoxProDriver驱动，需要下载安装
https://docs.microsoft.com/en-us/sql/odbc/microsoft/visual-foxpro-odbc-driver
https://support.microsoft.com/en-us/help/277772/visual-foxpro-odbc-driver-not-included-in-mdac-2.6-and-later
连接字符串格式如下：
PROVIDER=MSDASQL;DRIVER={Microsoft Visual Foxpro Driver};SourceDB=%S;SourceType=DBF
3）Visual FoxPro OLE DB provider
ADOConstr =
"Provider=VFPOLEDB.1;Data Source=%s;Mode=Share Deny None;Extended Properties=;Collating Sequence=MACHINE";
Microsoft OLE DB Provider for Visual FoxPro 9.0 下载地址2017.4.20
https://www.microsoft.com/en-us/download/details.aspx?id=14839
VFPOLEDBSetup.msi，安装后DBF问题都可以解决，突破dbase里的单行4000字节的限制。
setup path
C:Program Files (x86)Microsoft Visual FoxPro OLE DB Provider
3、ADS（Advantage Database Server）
DriverID=ADS Database=c:adsdata TableType=VFP
http://docs.embarcadero.com/products/rad_studio/firedac/frames.html?frmname=topic&frmfile=Connect_to_Advantage_Database_Server.html
微软官方不建议用Visual FoxPro ODBC Driver，官方声明如下。代替的是Visual FoxPro OLE DB provider
https://msdn.microsoft.com/en-us/library/mt490121
Visual FoxPro ODBC Driver
The VFPODBC driver is no longer supported.
We strongly recommend using the Visual FoxPro OLE DB provider as a
replacement. Please refer to the following article for more information
and related links to issues when using the VFPODBC driver: http://support.microsoft.com/kb/277772.
ADO连接DBF的连接字符串
Provider = Microsoft.Jet.OLEDB.4.0 ;Data Source ={0};Extended Properties=dBASE IV
Provider=Microsoft.Jet.OLEDB.4.0;Password="";Data Source=%s;Extended Properties=dbase 5.0;Persist Security Info=True
每行记录中字符的最大数目 65,500
每行记录中字段的最大数目 255
每个字段中字符数的最大值 254
create table 的时候，80个字段成功，81个字段就报错。有36个字段的类型varchar(200)。是不是太宽了。
字段将不能置于记录中。
某个字段类型宽度太大的原因？字段没有超过255的varchar(255)，这个原因不存在.
20个varchar(200)可以创建成功，第21个字段即使是1个字节varchar(1)也不成功。
把第20个改成varchar(201)，不要第21个字段，也是报错。20个varchar(200)成功建表。证明200*200=4000是上限。这是为何？
如果4000个字节是上限，那么版本是什么dbase？
实验200个字段可以创建成功char(1)，那就说明字段数上限不是128.
DBF规范dBASE III
最大字段数为 128
每个字段最多包含 254 个字符
每个记录最多包含 4000 个字节
http://www.dof.ca.gov/Accounting/CALSTARS/Monarch/Monarch_System_Specifications/
http://johnbrown.com.au/approach/webfaq04040210.html
- maximum file size = 2 gigabytes (per type .dbf or .dbt file)
- maximum records per database = 1,000,000,000 (1 billion)
- maximum of 30 open dBaseIV databases on a single computer
- maximum number of fields per record = 255
- maximum size of a record = 4000 characters (= all the field-lengths added together)
BDE DBF
http://www.delphigroups.info/2/81/365949.html
dBASE Limitations
256 Open dBASE tables per system (16 Bit)
350 Open dBASE tables per system (BDE 3.0 - 4.0, 32 Bit)
512 Open dBASE tables per system (BDE 4.01, 32 Bit) (NEW BDE 4.01)
100 Record locks on one dBASE table (16 and 32 Bit)
100 Records in transactions on a dBASE table (32 Bit)
1 Billion records in a table
2 Billion bytes in .DBF (Table) file
4000 Size in bytes per record (dBASE 4)
32767 Size in bytes per record (dBASE for Windows)
255 Number of fields per table (dBASE 4)
1024 Number of fields per table (dBASE for Windows)
47 Number of index tags per .MDX file
254 Size of character fields
10 Open master indexes (.MDX) per table
220 Key expression length in characters
http://edn.embarcadero.com/article/15159
DBASE LIMITS
256 Open dBASE tables per system (16 Bit)
350 Open dBASE tables per system (BDE 3.0 - 4.0,32 Bit)
512 Open dBASE tables per system (BDE 4.01, 32 Bit)
100 Record locks on one dBASE table (16 and 32 Bit)
100 Records in transactions on a dBASE table (32 Bit)
1 Billion records in a table
2 Billion bytes in .DBF (Table) file
4000 Size in bytes per record (dBASE 4)
32767 Size in bytes per record (dBASE for Windows)
255 Number of fields per table (dBASE 4)
1024 Number of fields per table (dBASE for Windows)
47 Number of index tags per .MDX file
254 Size of character fields
10 Open master indexes (.MDX) per table
220 Key expression length in characters
http://www.dbase.com/help/BDE_Limits/IDH_BDE_LIMITS.htm