Database field type
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Overview
FCL-DB database fields can have different data types. In datasets, a subset of these types is available. What types are supported differ per dataset type.
Types
Currently, the following field types are defined. See FCL field type documentation. To do: add assignment information (e.g. can you use .AsString) and additional information below.
| Field type | Type | Size | Description |
|---|---|---|---|
| ftADT | Abstract data type (user defined types created on the server); not supported yet (?). | ||
| ftArray | Represents an Interbase 6 / Firebird array datatype (array of simple data-types like varchar or integer). However, SQLDB does not support the array datatype currently. | ||
| ftAutoInc | An auto-incrementing integer field. | ||
| ftBCD | packed record | A binary coded decimal floating point value. | |
| ftBlob | A binary large object (BLOB), meant to store arbitrary binary data. | ||
| ftBoolean | Boolean | 1 byte | A boolean value (yes/no). |
| ftBytes | Presumably a fixed number of bytes stored as-is. Needs to have its Size property set to work. | ||
| ftCurrency | A format to precisely store currency values. | ||
| ftCursor | |||
| ftDataSet | Presumably meant to store an entire dataset (possibly to implement master/detail table). | ||
| ftDate | A date without time information. | ||
| ftDateTime | A date and time information. | ||
| ftDBaseOle | Presumably meant to store OLE objects in a DBase database. Needs to have its Size property set to work. | ||
| ftFMTBcd | A form of Binary Coded Decimal (BCD) number field. Needs to have its Size property set to work. | ||
| ftFixedChar | A fixed width character field, similar to a Pascal shortstring. Needs to have its Size property set to work. | ||
| ftFixedWideChar | A fixed width multibyte character field. Needs to have its Size property set to work. | ||
| ftFloat | Double | 8 bytes | A floating point numeric type. |
| ftFmtMemo | Needs to have its Size property set to work. | ||
| ftGraphic | Needs to have its Size property set to work. | ||
| ftGuid | A field used to store a GUID (Globally Unique Identifier). With the current code, this field needs to have its Size property set to 38. | ||
| ftIDispatch | |||
| ftInteger | Longint | 4 bytes | An integer field. |
| ftInterface | |||
| ftLargeint | 8 bytes | A field that contains an integer, stores more bytes than an integer and therefore has a larger range. | |
| ftMemo | Stores a variable amount of string/text data; needs no size set. | ||
| ftOraBlob | Presumably stores Oracle BLOB. | ||
| ftOraClob | Presumably stores Oracle CLOB: an Oracle data type that can hold up to 4 GB of data [1]. | ||
| ftParadoxOle | Presumably meant to store OLE objects in a Paradox database. | ||
| ftReference | |||
| ftSmallint | 2 bytes, signed | An integer type field with less bytes than ftInteger. | |
| ftString | Stores string data; needs to have its Size property set to the maximum number of characters possible in that field. | ||
| ftTime | Stores time-only data. | ||
| ftTimeStamp | Stores date/time data. Probably equivalent to ftDateTime. | ||
| ftTypedBinary | Some kind of BLOB-like field? | ||
| ftUnknown | |||
| ftVarBytes | Presumably a variant with byte/binary data? | ||
| ftVariant | Presumably meant to store variant data. | ||
| ftWideMemo | An ftMemo with widestring (UTF-16) characters. | ||
| ftWideString | An ftString with widestring (UTF-16) characters. | ||
| ftWord | 2 bytes, unsigned | Presumably stores an integer value. |
Size, DataSize and Unicode
Note that for string type fields, Size indicates the number of characters that can be stored. As indicated in FPC Unicode support#Introduction, FPC up to and including 2.6 only deals with ANSI/ASCII single byte characters; it does not support Unicode/UTF8/UTF16/Unicodestring characters.
The read-only property DataSize indicates the field size in bytes.
If you use multibyte characters (e.g. UTF8 or UTF16/Unicodestring encoded), DataSize and Size do not mean the same thing. If you use only ANSI/ASCII characters, DataSize and Size are effectively the same thing.
Defining types in your dataset
Todo: write me. Explain various ways of doing things (TFieldDef, TFields) and which dataset supports which methods.
Assigning and retrieving values
Once you have the fields in your dataset defined, you can assign and retrieve data like this - suppose you have a dataset called FTestDataset:
FTestDataset.Open; //Open for viewing/editing/inserting
FTestDataset.Append;
FTestDataset.FieldByName('YourFieldName').AsString := 'This is my field data'; //Suppose field YourFieldName is a memo field
FTestDataset.Post; //"Commit"/save changes in the record to the dataset.
Writeln('YourFieldName has data:' + FTestDataset.FieldByName('YourFieldName').AsString);
{ Retrieve the field value of the current record. Because we haven't moved the record, we should get what we just entered }
For text/memo fields, use the AsString method. For date/time fields, use the AsDateTime method. For binary fields, I suppose you can use the AsString method - but there must be another way, too
