Why some decimal places truncated when insert 1234567890.12345678 into Decimal(18, 8) column in SQLite? - sqlite

Question

Below is my table create in SQLite database,
CREATE TABLE MyData(
Code VARCHAR(20),
Amount DECIMAL(18, 8)
);
then I insert 2 rows into the table.
INSERT INTO MyData
VALUES('A', 1.12345678);
INSERT INTO MyData
VALUES('B', 1234567890.12345678);
After that, execute a SELECT statement,
SELECT * FROM MyData;
SQLite returns the following result:
A|1.12345678
B|1234567890.12346
The DECIMAL(18, 8) suppose means precision=18 and scale=8, why some decimal places are truncated?

Answer 1

The details of how sqlite stores its data is described here. When you specify the DECIMAL column type, the storage for the column has NUMERIC affinity.
Section 2.0 has the following description about type affinity:
A column with NUMERIC affinity may contain values using all five
storage classes. When text data is inserted into a NUMERIC column, the
storage class of the text is converted to INTEGER or REAL (in order of
preference) if such conversion is lossless and reversible. For
conversions between TEXT and REAL storage classes, SQLite considers
the conversion to be lossless and reversible if the first 15
significant decimal digits of the number are preserved. If the
lossless conversion of TEXT to INTEGER or REAL is not possible then
the value is stored using the TEXT storage class. No attempt is made
to convert NULL or BLOB values.
This indicates that sqlite will attempt conversions between types, and if the first 15 digits of the number can be converted and reversed, the numbers are deemed to be equal. This effectively puts a limit on the available precision with which a number can be stored to 15 significant digits.
The wikipedia article on double precision floating point numbers has additional information which is useful when dealing with floating point numbers.