SQLite Query using nested SELECT
You should always use WHERE xxx IN (SELECT xxx FROM ...), instead of selecting multiple items in the inner select. You can add those in the outer select though, for example:
SELECT DISTINCT mgc_version, test_type_letter, design_index, test_index, cpu_time, multi_thread FROM TestExecutionsWHERE cpu_time IN ( SELECT DISTINCT cpu_time FROM TestExecutions WHERE test_type_letter BETWEEN $testletter AND $testletter)