MS Access Function : Function to create SQL union queries

Another small function that can speed up the text required to be written for large union queries.

Typically this can be used with
MS Access Function : Scan through a directory and write list of files to a table.

There are a number of data providers that provide data files broken down into different geographical areas. In previous posts I have outlined how we can get these all into Postgis. But once they are in postgis (or any other database) you may wish to get these separate tables into one single global table. Clearly a union query will do this, however it can be time consuming to write the union query out as it simply has so many tables in it.

I used the code in the link to scan a directory and get all the filenames (in this case shape files of the UK road network) into a table that I called UKRoadLinks which had two fields PKID (primary long integer autonumber) and Filen text field where Filen were the filenames.

I then wrote the following function to write a text file that on completion will contain an sql union of all the tables listed in your recordset – I then copied and pasted this into Postgis database within which I had already imported all the sub tables to union the tables into a single copy. Alter the recordset source if for instance if you wish to use only a subset. The nice thing about this is if you have hundreds of tables to amalgamate there should be less likelyhood of you accidentally missing or misspelling table names.

Public Function createunionsqllinks()

Dim rst As DAO.Recordset
Set rst = CurrentDb.OpenRecordset("UKRoadLinks")

Dim fs, TextFile
Set fs = CreateObject("Scripting.FileSystemObject")
Set TextFile = fs.CreateTextFile("c:\data\sqlmerge.txt", True)
TextFile.WriteLine ("CREATE TABLE sqltomergetables AS ")
Do Until rst.EOF = True
TextFile.WriteLine (Chr$(40) & "select * from " & rst!Filen & Chr$(41) & " UNION ")
rst.MoveNext
Loop
rst.Close
TextFile.WriteLine (";")
TextFile.Close

MsgBox "Created"

End Function

SQL SERVER – Create TSQL Trigger to add date to existing record after INSERT or UPDATE

I came across an interesting issue with a web application I have been trying to create. DEFAULTS in SQL Server didn’t seem to be registering when new records were inserted through the web application.

How come? Surely the web application is not bypassing requirements set out by the database.

What I believe is happening is that the web application is explicitly supplying a null value to the dateupdated field and because of this the default value is NOT applied by SQL Server. You are therefore in the strange position of creating a trigger on a column after insert. Normally you would never really do this as that’s specifically what default is for.

Here’s code to create the trigger on the SQL Server that will do just this.

CREATE TRIGGER TR_moddate
On T0001PersonData
AFTER INSERT, UPDATE
AS
BEGIN
UPDATE T0001PersonData SET DATEUPDATED=GETDATE()
WHERE PKID IN (SELECT PKID FROM INSERTED)
END

Some explanation – This updates the field DateUpdated within the table T0001PersonData after an update or an insert. If you have default on a column this may mean that it updates to default then to null then to trigger so you may wish to remove the default on the column. Alternatively you may wish to get the front end to update to the current date. I guess this could have performance issues at scale but my application is pretty small beer so I am happy with this at the moment. I think I prefer to have it at database level as well. It should be noted that INSERTED is actually a ALIAS table created by SQL Server and held in memory that allows for reference to pull out items before they are registered in the database. Leaving out the where clause may have resulted in the application updating the dateupdated column for all records anytime an update or insert was performed. BE WARNED