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我正在使用MySQL.这是我的架构:

供应商(sid:整数,sname:字符串,地址字符串)

零件(pid:整数,pname:字符串,颜色:字符串)

目录(sid:整数,pid:整数,成本:实际)

(主键是粗体)

我正在尝试编写一个查询来选择至少由两个供应商制作的所有零件:

-- Find the pids of parts supplied by at least two different suppliers.
SELECT c1.pid                      -- select the pid
FROM Catalog AS c1                 -- from the Catalog table
WHERE c1.pid IN (                  -- where that pid is in the set:
    SELECT c2.pid                  -- of pids
    FROM Catalog AS c2             -- from catalog
    WHERE c2.pid = c1.pid AND COUNT(c2.sid) >= 2 -- where there are at least two corresponding …