使用JOIN进行SQL查询计数

您需要先CROSS JOIN确定不同的严重性和状态值集，然后再LEFT JOIN对表进行设置，以允许您计算每个严重性/状态组合的值。没有示例数据，很难确定，但是这样的方法应该可以工作：

SELECT sv.vseverity, st.vulnstatus, COUNT(v.vseverity) AS count
FROM (
    SELECT DISTINCT vseverity
    FROM datasets
) sv
CROSS JOIN (
    SELECT DISTINCT vulnstatus
    FROM vulnsummary
) st
LEFT JOIN (
    SELECT d.vseverity, v.vulnstatus 
    FROM vulnsummary v 
    JOIN project p ON v.projid = p.projid 
                  AND v.stagename = p.currentstage 
    JOIN datasets d ON v.vtitleid = d.datasetid
) v ON v.vseverity = sv.vseverity AND v.vulnstatus = st.vulnstatus
GROUP BY sv.vseverity, st.vulnstatus