A　novel　quantum　private　database　query　protocol　is　proposed,　based　on　passive　round-robin　differential　phase-shift　quantum　key　distribution.　Compared　with　previous　quantum　private　database　query　protocols,　the　present　protocol　has　the　following　unique　merits:　(i)　the　user　Alice　can　obtain　one　and　only　one　key　bit　so　that　both　the　efficiency　and　security　of　the　present　protocol　can　be　ensured,　and　(ii)　it　does　not　require　to　change　the　length　difference　of　the　two　arms　in　a　Mach-Zehnder　interferometer　and　just　chooses　two　pulses　passively　to　interfere　with　so　that　it　is　much　simpler　and　more　practical.　The　present　protocol　is　also　proved　to　be　secure　in　terms　of　the　user　security　and　database　security.