In recent years, query-based static application security testing (Q-SAST) tools such as CodeQL have gained popularity due to their ability to codify vulnerability knowledge into SQL-like queries and search for vulnerabilities in the database derived from software. The industry has made considerable progress in building Q-SAST tools, facilitating their integration into the continuous integration (CI) pipeline, and sustaining an active community. However, we do not have a systematic understanding of their vulnerability detection capability in comparison to conventional SAST tools. We conduct the first in-depth study of Q-SAST to demystify their C/C++ vulnerability detectability. Our study is conducted from three complementary aspects. We first use a synthetic CWE test suite and a real-world CVE test suite, totaling almost 30K programs with known CWE/CVE, to assess popular (commercial) Q-SAST and industry-leading SAST (requiring no queries). Then, we gather defect-fixing pull requests (PRs) since the release dates of three popular Q-SAST tools, characterizing historically-fixed detects and comparing them to pitfalls exposed in our CWE/CVE study. To enhance vulnerability detection, we design SAST-MT, a metamorphic testing framework to detect false positives (FPs) and false negatives (FNs) of Q-SAST. Findings of SAST-MT can be used to easily expose root causes of Q-SAST’s FPs and FNs. We summarize lessons from our study that can benefit both users and developers of Q-SAST.