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System tables are used by Access to make databases function correctly. With a few exceptions, there is very little information available online about most of the system tables.
The purpose of this article is to summarise known information about Access system tables.
It is also an invitation to other developers to add to this pool of knowledge
Some system tables can be viewed & a few can be edited
However you should only do so ....IF YOU ARE ABSOLUTELY SURE WHAT YOU ARE DOING
Incorrectly editing system tables may corrupt your database or prevent you opening it
This article describes various tests done to compare different approaches to coding:
1. Handling nulls: Trim / Len / Nz
2. CurrentDB vs DBEngine(0)(0)
3. DoEvents vs DBIdle.RefreshCache
4. HAVING vs WHERE
5. Conditional Updates: If..ElseIf..End If / Select Case / Nested IIf / Switch / Lookup Table
6. Query vs SQL vs QueryDef
7. Check Record Exists
8. Optimise Queries
Example databases are provided so the tests can be done on your own workstations
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This page contains links to some longer articles discussing specific issues in Access in greater detail.
It is intended that the number of articles will be significantly increased in the coming months so please check back again regularly.
There are many things that developers can do to improve the security of their applications (both design & data). However, no Access database can EVER be made 100% secure. A capable and determined hacker can break any Access database given sufficient time and determination.
The older MDB / MDE file format is FAR LESS SECURE than the newer ACCDB / ACCDE format. Whilst this is hardly news to many developers, I’ve had a few requests to provide more details to justify this statement.
During development work, all of us will want to ensure that tasks are completed as quickly as possible. However, it is not always obvious that the current design is inefficient until performance slows to a crawl and clients start to complain.
Where queries or VBA SQL statements are concerned, help is available by making use of the ShowPlan feature which is available with both the JET database engine (up to A2003) and the newer ACE engine (A2007 onwards). The ShowPlan option prints the query's plan to a text file so you can review and, if possible, improve the design.
The purpose of this article is to provide more details about the ShowPlan feature for which little documentation is available online
The first part of this article summarises many of the actions that all Access developers should do when deploying databases in a multi-user environment.
These will significantly reduce the risk of data corruption as well as minimising the possibility of data being misused or stolen
The second part gives a detailed explanation about each of the methods involved together with an example application containing all the code required
Multivalued fields (MVFs) were a new feature introduced with Access 2007
This article explains what MVFs are and how they can be used.
Although often attractive to new users, most experienced developers avoid them completely.
This article explains why this feature is really not a good idea
Access allows you to use lookup fields at table level.
Doing so, allows you to have combo boxes to guide end users with a list of available choices
Although at first sight this may seem an attractive option, experienced developers normally advise against their use. Arvin Meyer described several of the reasons against their use in The Evils of Lookup Fields in Tables
This article describes another issue - exporting lookup field data to Excel
SECTION BELOW NOT CURRENTLY IN USE ...
This article explores some of the causes of the 'dreaded' write conflict error and how to deal with those issues.
Possible causes include:
a) Two or more users are trying to edit the same data at the same time
b) Editing data that uses multiple forms that have the same record source
c) Boolean fields in linked SQL Server tables with no default value
This article explores some of the methods that can be used to synchronise data with an external source. The advantages and disadvantages of each method are also discussed.
This article explores the different types of select query joins available in Access and how these affect the query output in each case:
a) Inner Join
b) Left Outer Join
c) Right Outer Join
d) Union Query (AKA Full Outer Join)
e) Cartesian Join (AKA No Join)
In addition it discusses the causes of ambiguous join errors and how to solve these
This article explains how the column history property can be used to show a history of changes made to memo / long text fields.
The article also explains how this data is stored in a deep hidden system table and how to deal with issues associated with editing the column history
This article explains how Access uses the MSysQueries system table in conjunction with other system tables to display the query structure in the query design window.
This article describes the differences between query joins and table relationships and explains the important use of table relationships to enforce referential integrity.
Three methods of adding relationships are discussed. In addition, the article explains how Access uses the MSysRelationships system table to store the relationships that have been applied
The example application demonstrates some complex processes including:
• Moving a popup form such as a customised zoom box to a precise position over
another form irrespective of form settings
• Highlighting and 'selecting' a listbox record without clicking on the listbox.
This is done by accurately detecting the record underneath the mouse cursor based
on the height of each row in the listbox.
This means the record can be used e.g. to open a filtered form / view an image without
actually selecting the listbox record!
• Determining the x-y coordinates of a form object and nudging the object by a
specified amount in any direction
This requires an understanding of the relationship between the various units of measurement used in Windows applications: pixels, points and twips
The application also makes use of the little known and undocumented VBA Wizhook function
This article compares 6 different timing methods with a view to determining the reliablility and consistency of each method. The 6 methods compared are:
Timer / GetSystemTime / GetTickCount / TimeGetTime
Stopwatch class / High Resolution Timer
Version 1.6 Updated 27/02/2019
This article discusses the benefits of automatic form resizing when developing applications for use with different monitor sizes and resolutions. It also covers an alternative approach based on layout guides and anchoring.
The latest version also include code to zoom forms in/out according to user preference.
This may be particularly useful for those whose eyesight is less than perfect
The article explains how the code works and provides tips for using the code most effectively in your own applications. It also covers some potential issues you may experience and possible solutions for each of these.
Version 2.6 Updated 24/01/2020
This is an updated version of the article discussing ways of helping to keep Access data secure against both hackers and theft whilst still allowing full functionality to authorised users.
This example application should behave exactly as any split database BUT there are no linked tables and therefore no connection strings visible in the MSysObjects system table.
In addition, the data is protected using a strong 128-bit RC4 encryption cipher.
Version 5.25 Updated 26/12/2020