This is a newsgroup post from OneDayWhen. He’s an expert (in my opinion) on External Data. The issue he discusses is that of mixed data types. If you have, for instance, both strings and numerics in one external data column, you can get unexpected results – like some of your data doesn’t show up.
This is a common problem in the newsgroups and this is the best post I’ve ever seen on the subject. As OneDayWhen says himself, it needs looking after. Therefore, I’m posting it here in its entirety for all time.
Here my notes on the subject: (look after them; as my granddad used to say, ‘I won’t be around forever, there’ll be one day when you’ll have to fend for yourself, sonny’):
The relevant registry keys (for Jet 4.0) are in:
The ImportMixedTypes registry key is always read (whether it is honored is discussed later). You can test this by changing the key to ImportMixedTypes=OneDayWhen and trying to use the ISAM: you get the error, ‘Invalid setting in Excel key of the Engines section of the Windows Registry.’ The only valid values are:
Data type is determined column by column. ‘Majority Type’ means a certain number of rows (more on this later) in each column are scanned and the data types are counted. Both a cell’s value and format are used to determine data type. The majority data type (i.e. the one with the most rows) decides the overall data type for the entire column. There’s a bias in favor os numeric in the event of a tie. Rows from any minority data types found that can’t be cast as the majority data type will be returned with a null value.
For ImportMixedTypes=Text, the data type for the whole column will be:
Jet (MS Access UI): ‘Text’ data type
ADO: adWChar (‘a null-terminated Unicode character string’)
Note that this is distinct from:
Jet (MS Access UI): ‘Memo’ data type
ADO: adLongVarWChar (‘a long null-terminated Unicode string value’)
ImportMixedTypes=Text will curtail text at 255 characters as ‘Memo’ is cast as ‘Text’. For a column to be recognized as ‘Memo’, majority type must be detected, meaning the majority of rows detected must contain 256 or more characters.
But how many rows are scanned for each column before is decided that mixed and/or what the majority type is? There is a second registry Key, TypeGuessRows. This can be a value from 0-16 (decimal). A value from 1 to 16 inclusive is the number of rows to scan. A value of zero means all rows will be scanned.
There is one final twist. A setting of IMEX=1 in the connection string’s extended property determines whether the ImportMixedTypes value is honored. IMEX refers to IMport EXport mode. There are three possible values. IMEX=0 and IMEX=2 result in ImportMixedTypes being ignored and the default value of ‘Majority Types’ is used. IMEX=1 is the only way to ensure ImportMixedTypes=Text is honored. The resulting connection string might look like this:
Data Source=C:\ db.xls;
Extended Properties=’Excel 8.0;HDR=Yes;IMEX=1′
Finally, although it is mentioned in MSDN articles that MAXSCANROWS can be used in the extended properties of the connection string to override the TypeGuessRows registry keys, this seems to be a fallacy. Using MAXSCANROWS=0 in this way never does anything under any circumstances. Put another way, is has just the same effect as putting ONEDAYWHEN=0 in the extended properties, being none (not even an error!). The same applied to ImportMixedTypes i.e. can’t be used in the connection string to override the registry setting.
In summary, use TypeGuessRows to get Jet to detect whether a ‘mixed types’ situation exists or use it to ‘trick’ Jet into detecting a certaint data type as being the majority type. In the event of a ‘mixed types’ situation being detected, use ImportMixedTypes to tell Jet to either use the majority type or coerce all values as ‘Text’ (max 255 characters).