存在与不存在查询误区？

Question

考虑以下产品数据库架构：

Parts (pid, pname)
Suppliers (sid, sname)
Catalog (sid, pid)

Catalog 中的

sid 是外键并引用 Suppliers 和 Catalog 中的 pid 是外键并引用 Parts table。 (s1, p1) 在 Catalog table 显示供应商 s1 生产 p1.

如果我想查找某些供应商未提供的 Parts 的详细信息，我们该怎么做？我读了一些部分如下：

SELECT *
FROM parts P
WHERE ....... (SELECT S.sid 
               FROM suppliers 
                WHERE .....
                       (SELECT * 
                        FROM catalog C 
                        WHERE s.sid = C.sid AND P.pid = C.pid))

我的问题是 - 为什么一开始？我们使用 exist，其次……我们使用 not exist?

Answer 1

是不是就这么简单？如果不是，请使用我的示例表并指定您的预期输出：

DECLARE @Parts TABLE(pid INT, pname VARCHAR(100));
DECLARE @Suppliers TABLE(sid INT, sname VARCHAR(100));
DECLARE @Catalog TABLE(pid INT, sid INT);

INSERT INTO @Parts VALUES(1,'Part 1'),(2,'Part 2'),(3,'Part 3'),(4,'Part 4');
INSERT INTO @Suppliers VALUES(1,'Suppl 1'),(2,'Suppl 2'),(3,'Suppl 3');
INSERT INTO @Catalog VALUES(1,1),(1,3) --part 1 from two suppliers
                          ,(2,1)       --part 2 from one supplier
                                       --part 3 no supplier
                                       --part 4 no supplier

--Get all parts *that not supplied by some suppliers*
SELECT * 
FROM @Parts AS p 
WHERE p.pid NOT IN(SELECT c.pid FROM @Catalog AS c)

结果

pid pname
3   Part 3
4   Part 4

编辑："your" 查询也达到了同样的效果，但这会表现更差：

此查询正在寻找没有供应商，确实在 "catalog" 中有条目的零件。我必须承认，这闻起来有点像家庭作业:-)

SELECT * FROM @Parts P 
WHERE NOT EXISTS (SELECT S.sid 
                  FROM @Suppliers S 
                  WHERE EXISTS (SELECT 1 
                                FROM @Catalog C 
                                WHERE s.sid = C.sid AND P.pid = C.pid
                                )
                  )

Answer 2

If I want find details of Parts that are not supplied by some suppliers, how we can do this?

如果您想要 "details for parts where there exists a supplier that doesn't supply that part"，那么您的查询 EXISTS 然后 NOT EXISTS 是正确的。

My problem is via - why at first? We use exist and in second ... we use not exist?

我会解释为什么"details for parts where there exists a supplier that doesn't supply that part"可以给出你的查询。（使用 EXISTS 然后 NOT EXISTS。）

A table 持有使某些谓词（陈述模板）成为真实的命题（陈述）的行：

Parts "part P.pid is named P.pname"
Suppliers "supplier S.sid is named S.sname"
Catalog"supplier C.sid supplies part C.pid"

您需要为您的结果表达谓词。您希望 P.pid-P.pname 行从以下内容中做出正确的陈述：

    "part P.pid is named P.pname"
AND EXISTS S.sid ["supplier S.sid is named something"
        AND NOT "supplier S.sid supplies part P.pid"]

但是我们必须用给定的谓词（加上条件和逻辑运算符）来表达这个谓词，这样DBMS才能计算出满足它的行：

    "part P.pid is named P.pname"
AND EXISTS S.sid [EXISTS S.sname "supplier S.sid is named S.sname"
        AND NOT EXISTS C.sid, C.pid [
                "supplier C.sid supplies part P.pid" AND C.sid = S.sid AND C.pid = P.pid]]

现在转换为 SQL:

T 的初始谓词变为 FROMT

AND T 的谓词变为 JOINT

AND 条件变为WHEREcondition或ONcondition

最外层的 EXISTS 删除的列 变为 SELECTkept columns

其他 EXISTS T 的所有列 [T] 的谓词变为 EXISTS (T)

SELECT * FROM Parts P WHERE EXISTS (SELECT S.sid FROM Suppliers S WHERE NOT EXISTS (SELECT 1 FROM Catalog C WHERE C.sid = S.sid AND C.pid = P.pid))

PS你的问题原"details of parts that are not supplied by some suppliers"是unclear/ambiguous。这可能意味着 P.id-P.pname 对，其中：

"the parts are not supplied by all the suppliers"（同意你的解释SQL）

"the parts are not not supplied (by any of the suppliers)"（与
中两个SQL版本的解释一致
"the parts don't have multiple suppliers"

SELECT * FROM Parts P WHERE NOT EXISTS (SELECT * FROM Catalog C1 JOIN Catalog C2 ON C1.sid <> C2.sid AND C1.pid = P.pid AND C2.pid = P.pid)

如果你有 Parts 1-4 和 Suppliers 1-3 那么上面给出了三个不同的答案：

INSERT INTO Catalog VALUES (1,1),(2,1),(2,2),(3,1),(3,2),(3,3);

PPS 以上逻辑公式为：

-- 1. "the parts are not supplied by all the suppliers" P(P.pid, P.pname) AND EXISTS S.sid, S.sname [S(S.sid, S.sname) AND NOT C(S.sid, P.pid)] -- 2 "the parts are not supplied (by some of the suppliers)" P(P.pid, P.pname) AND NOT EXISTS C.sid C(C.sid, P.pid) -- 3. "the parts don't have multiple suppliers" P(P.pid, P.pname) AND NOT EXISTS C1.sid, C2.sid [C(C1.sid, P.pid) AND C(C2.sid, P.pid) AND C1.sid <> C2.sid]

存在与不存在查询误区？

Exist and Not Exists Query Misunderstanding?

sql

database

database-design

relational-algebra

relational-database

编辑："your" 查询也达到了同样的效果，但这会表现更差：