Earlier quoted context omitted.
> The amalgamation is a single C code file, named "sqlite3.c", that contains all C code for the core SQLite library and the FTS3, FTS5, RTREE, DBSTAT, JSON1, and RBU extensions. This file contains about 184K lines of code (113K if you omit blank lines and comments) and is over 6.4 megabytes in size. Though the the various extensions are included in the "sqlite3.c" amalgamation file, they are disabled using #ifdef sta…
In other words, it's stretching the definition of "single file" a bit... but then again, I don't know any other SQL databases anywhere near as small as SQLite.
Smallest commercially viable SQL database is probably the latest k5/kdb (http://kparc.com/), which is implemented in something like 9KB of code (source: http://kparc.com/o.htm).