TFDQuery and SQLite: Type mismatch for field, expe

Question

Using Delphi 10.2, SQLite and Teecharts. My SQLite database has two fields, created with:

CREATE TABLE HistoryRuntime ('DayTime' DateTime, Device1 INTEGER DEFAULT (0));

I access the table using a TFDQuery called qryGrpahRuntime with the following SQL:

SELECT DayTime AS TheDate, Sum(Device1) As DeviceTotal
FROM HistoryRuntime 
WHERE  (DayTime >= "2017-06-01") and (DayTime <= "2017-06-26") 
Group by Date(DayTime)

Using the Field Editor in the Delphi IDE, I can add two persistent fields, getting TheDate as a TDateTimeField and DeviceTotal as a TLargeIntField.

I run this query in a program to create a TeeChart, which I created at design time. As long as the query returns some records, all this works. However, if there are no records for the requested dates, I get an EDatabaseError exception with the message:

qryGrpahRuntime: Type mismatch for field 'DeviceTotal', expecting: LargeInt actual: Widestring

I have done plenty of searching for solutions on the web on how to prevent this error on an empty query, but have had not luck with anything I found. From what I can tell, SQLite defaults to the wide string field when no data is returned. I have tried using CAST in the query and it did not seem to make any difference.

If I remove the persistent fields, the query will open without problems on an empty return set. However, in order to use the TeeChart editor in the IDE, it appears I need persistent fields.

Is there a way I can make this work with persistent fields, or am I going to have to throw out the persistent fields and then add the TeeChart Series at runtime?

Answer 1

This behavior is described in Adjusting FireDAC Mapping chapter of the FireDAC's SQLite manual:

For an expression in a SELECT list, SQLite avoids type name information. When the result set is not empty, FireDAC uses the value data types from the first record. When empty, FireDAC describes those columns as dtWideString. To explicitly specify the column data type, append ::<type name> to the column alias:

SELECT count(*) as "cnt::INT" FROM mytab

So modify your command e.g. this way (I used BIGINT, but you can use any pseudo data type that maps to a 64-bit signed integer data type and is not auto incrementing, which corresponds to your persistent TLargeIntField field):

SELECT
   DayTime AS "TheDate",
   Sum(Device1) AS "DeviceTotal::BIGINT"
FROM
   HistoryRuntime 
WHERE
   DayTime BETWEEN {d 2017-06-01} AND {d 2017-06-26}
GROUP BY
   Date(DayTime)

P.S. I did a small optimization by using BETWEEN operator (which evaluates the column value only once), and used an escape sequence for date constants (which, in real you replace by parameter, I guess; so just for curiosity).

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This data type hinting is parsed by the FDSQLiteTypeName2ADDataType procedure that takes and parses column name in format <column name>::<type name> in its AColName parameter.