I have an Access 2010 database with a relationship between parent and child tables. I would like to be able to query the database from an external application and show values from the child table as a concatenated list of values in a single column, similar to what MySQL can produce with its GROUP_CONCAT() function.
This has been asked here many times before, e.g., here:
Combine values from related rows into a single concatenated string value
but those solutions rely on a custom VBA function that is not available to external queries.
Is there a way to make such a concatenated list available to external queries without having to manually build the list in the other application?
Issue
Historically, the Access solution for a GROUP_CONCAT()-type query has been to use a VBA function like Allen Browne's ConcatRelated() (ref: here). However, custom VBA functions are only available to queries run from within Microsoft Access itself, so this is not a viable solution for queries against an Access database from some other application (e.g., a .NET application using OLEDB or ODBC).
Solution
With an Access 2010 (or newer) database we can emulate the behaviour of a MySQL GROUP_CONCAT() query by adding a Long Text ("Memo") field to the parent table and using data macros on the child table to maintain the concatenated list.
For example, for tables [Parents] ...
ParentID ParentName
-------- -------------
1 Homer Simpson
2 Ned Flanders
... and [Children] ...
ChildID ParentID ChildName DisplayOrder
------- -------- ----------------- ------------
1 1 Lisa 2
2 1 Bart 1
3 2 Rod, the elder 1
4 1 Maggie 3
5 2 Todd, the younger 2
... we can add a new Memo/Long Text field named [ChildList] to the [Parents] table and then add the following data macros to the [Children] table:
[Named Macro: UpdateChildList]
<?xml version="1.0" encoding="UTF-16" standalone="no"?>
<DataMacros xmlns="http://schemas.microsoft.com/office/accessservices/2009/11/application">
<DataMacro Name="UpdateChildList">
<Parameters>
<Parameter Name="prmParentID"/>
</Parameters>
<Statements>
<Action Collapsed="true" Name="SetLocalVar">
<Argument Name="Name">newList</Argument>
<Argument Name="Value">Null</Argument>
</Action>
<ForEachRecord>
<Data Alias="c">
<Query>
<References>
<Reference Source="Children" Alias="c"/>
</References>
<Results>
<Property Source="c" Name="ChildName"/>
</Results>
<Ordering>
<Order Source="c" Name="DisplayOrder"/>
</Ordering>
</Query>
<WhereCondition>[c].[ParentID]=[prmParentID] And [c].[ChildName] Is Not Null</WhereCondition>
</Data>
<Statements>
<ConditionalBlock>
<If>
<Condition>Not IsNull([newList])</Condition>
<Statements>
<Action Collapsed="true" Name="SetLocalVar">
<Argument Name="Name">newList</Argument>
<Argument Name="Value">[newList] & ";" & Chr(160)</Argument>
</Action>
</Statements>
</If>
</ConditionalBlock>
<Action Collapsed="true" Name="SetLocalVar">
<Argument Name="Name">newList</Argument>
<Argument Name="Value">[newList] & [c].[ChildName]</Argument>
</Action>
</Statements>
</ForEachRecord>
<LookUpRecord>
<Data>
<Reference>Parents</Reference>
<WhereCondition>[Parents].[ParentID]=[prmParentID]</WhereCondition>
</Data>
<Statements>
<EditRecord>
<Data/>
<Statements>
<Action Collapsed="true" Name="SetField">
<Argument Name="Field">Parents.ChildList</Argument>
<Argument Name="Value">[newList]</Argument>
</Action>
</Statements>
</EditRecord>
</Statements>
</LookUpRecord>
</Statements>
</DataMacro>
</DataMacros>
[After Insert]
<?xml version="1.0" encoding="UTF-16" standalone="no"?>
<DataMacros xmlns="http://schemas.microsoft.com/office/accessservices/2009/11/application">
<DataMacro Event="AfterInsert">
<Statements>
<Action Name="RunDataMacro">
<Argument Name="MacroName">Children.UpdateChildList</Argument>
<Parameters>
<Parameter Name="prmParentID" Value="[ParentID]"/>
</Parameters>
</Action>
</Statements>
</DataMacro>
</DataMacros>
[After Update]
<?xml version="1.0" encoding="UTF-16" standalone="no"?>
<DataMacros xmlns="http://schemas.microsoft.com/office/accessservices/2009/11/application">
<DataMacro Event="AfterUpdate">
<Statements>
<ConditionalBlock>
<If>
<Condition>Updated("ParentID") Or Updated("ChildName")</Condition>
<Statements>
<Action Name="RunDataMacro">
<Argument Name="MacroName">Children.UpdateChildList</Argument>
<Parameters>
<Parameter Name="prmParentID" Value="[ParentID]"/>
</Parameters>
</Action>
<ConditionalBlock>
<If>
<Condition>Updated("ParentID")</Condition>
<Statements>
<Action Name="RunDataMacro">
<Argument Name="MacroName">Children.UpdateChildList</Argument>
<Parameters>
<Parameter Name="prmParentID" Value="[Old].[ParentID]"/>
</Parameters>
</Action>
</Statements>
</If>
</ConditionalBlock>
</Statements>
</If>
</ConditionalBlock>
</Statements>
</DataMacro>
</DataMacros>
[After Delete]
<?xml version="1.0" encoding="UTF-16" standalone="no"?>
<DataMacros xmlns="http://schemas.microsoft.com/office/accessservices/2009/11/application">
<DataMacro Event="AfterDelete">
<Statements>
<Action Name="RunDataMacro">
<Argument Name="MacroName">Children.UpdateChildList</Argument>
<Parameters>
<Parameter Name="prmParentID" Value="[Old].[ParentID]"/>
</Parameters>
</Action>
</Statements>
</DataMacro>
</DataMacros>
Results
As changes are made to the child table the list in the parent table will automatically be updated:
ParentID ParentName ChildList
-------- ------------- ---------------------------------
1 Homer Simpson Bart; Lisa; Maggie
2 Ned Flanders Rod, the elder; Todd, the younger
Notes
The [ChildList] field is for display purposes only. Editing the values in that field will not change the values in the child table.
The list is separated with
";" & Chr(160)to differentiate it from any";" & Chr(32)pairs that may be in the actual data. If the non-breaking space (Chr(160)) characters mess up wrapping of the list then we could use theReplace()function in our query to convert";" & Chr(160)to";" & Chr(32)or"," & Chr(32)or whatever would be most appropriate.To populate the lists with existing child data we simply need to "update" one child record for each parent, like so
UPDATE Children SET ChildName=ChildName
WHERE ChildID IN (SELECT MIN(ChildID) AS m FROM Children GROUP BY ParentID)
来源：https://stackoverflow.com/questions/30076586/create-a-concatenated-list-of-child-table-values-from-an-access-database-without