Qt - QMYSQL driver not loaded

The problem occurs when attempting to run a program that uses a MySQL database.

Solution

$ cd ~/Qt5.12.12/5.12.12/Src/qtbase/src/plugins/sqldrivers/mysql
$ ~/Qt5.12.12/5.12.12/gcc_64/bin/qmake
$ make
$ make install

Replace 5.12.12 with the version you have. The libraries will appear in ~/Qt5.12.12/5.12.12/Src/qtbase/src/plugins/sqldrivers/plugins/sqldrivers.

At the end, copy the sqldrivers directory to the location where the binary is located (or copy the individual library).

Possible issues

Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. no
  OCI (Oracle) ........................... no
  ODBC ................................... no
  PostgreSQL ............................. no
  SQLite2 ................................ no
  SQLite ................................. no
    Using system provided SQLite ......... no
  TDS (Sybase) ........................... no

To handle this:

Install libmysqlclient-dev, mysql-client, and libssl-dev (optionally, also libqt5sql5-mysql).
Delete the config.cache file.
Repeat the procedure from Solution.