Mysql 存在的SQL查询没有如我所想的那样工作

这两个SQL查询返回相同的结果

SELECT DISTINCT ItemID
FROM Sale INNER JOIN Department
ON Department.DepartmentID = Sale.DepartmentID
WHERE DepartmentFloor = 2
ORDER BY ItemID


SELECT DISTINCT ItemID
FROM Sale
WHERE EXISTS
(SELECT *
FROM Department
WHERE Sale.DepartmentID = Department.DepartmentID
AND DepartmentFloor = 2)
ORDER BY ItemID;

Exists中的子查询返回True，那么为什么secod查询不返回

SELECT DISTINCT ItemID
FROM Sale

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因为查询的UPPER部分相当于

   SELECT DISTINCT ItemID FROM Sale where EXISTS (true)

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上限是唯一真正检查条件的查询

您被

EXISTS（）

弄糊涂了。。它基于表的相关性逐行发生，而不仅仅是一个真/假。子查询的这一行是您的关联子句：

Sale.DepartmentID = Department.DepartmentID

它的意思是“仅显示Sale.ItemIDs，该ItemID的Sale.DepartmentID在Department中的位置。”

它实现了与连接谓词相同的功能，如在第一个查询中：

FROM Sale S
JOIN Department D on S.DepartmentID = D.DepartmentID --here

相反，此查询：

SELECT DISTINCT ItemID
FROM Sale

没有限制因素

另一方面，您还可以通过以下方式进一步限制每个查询的结果：

WHERE DepartmentFloor = 2

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但我不认为这是让你不高兴的部分，我认为这是一个概念，即每个记录都会出现一个相关的子查询。如果要删除correling子句，那么子查询实际上将始终返回true，并且您将返回所有结果。

子查询并不总是返回true。它将对每一行进行评估，并在DepartmentID上连接，其中DepartmentFloor为2

MySQL 5.6架构设置：

CREATE TABLE Sale ( ItemID int, DepartmentID int ) ;

INSERT INTO Sale ( ItemID, DepartmentID )
VALUES (1,1), (2,2), (3,3), (4,1), (5,4), (6,2), (7,3), (8,4) ;

CREATE TABLE Department ( DepartmentID int, DepartmentFloor int ) ;

INSERT INTO Department ( DepartmentID, DepartmentFloor )
VALUES (1,1), (2,1), (3,2), (4,2) ;

SELECT *
FROM Department
WHERE DepartmentFloor = 2 

SELECT *
FROM Sale

SELECT *
FROM Sale
WHERE DepartmentID IN (3,4)

查询1：

CREATE TABLE Sale ( ItemID int, DepartmentID int ) ;

INSERT INTO Sale ( ItemID, DepartmentID )
VALUES (1,1), (2,2), (3,3), (4,1), (5,4), (6,2), (7,3), (8,4) ;

CREATE TABLE Department ( DepartmentID int, DepartmentFloor int ) ;

INSERT INTO Department ( DepartmentID, DepartmentFloor )
VALUES (1,1), (2,1), (3,2), (4,2) ;

SELECT *
FROM Department
WHERE DepartmentFloor = 2 

SELECT *
FROM Sale

SELECT *
FROM Sale
WHERE DepartmentID IN (3,4)

：仅列出部门楼层2上的部门

| DepartmentID | DepartmentFloor |
|--------------|-----------------|
|            3 |               2 |
|            4 |               2 |

查询2：

CREATE TABLE Sale ( ItemID int, DepartmentID int ) ;

INSERT INTO Sale ( ItemID, DepartmentID )
VALUES (1,1), (2,2), (3,3), (4,1), (5,4), (6,2), (7,3), (8,4) ;

CREATE TABLE Department ( DepartmentID int, DepartmentFloor int ) ;

INSERT INTO Department ( DepartmentID, DepartmentFloor )
VALUES (1,1), (2,1), (3,2), (4,2) ;

SELECT *
FROM Department
WHERE DepartmentFloor = 2 

SELECT *
FROM Sale

SELECT *
FROM Sale
WHERE DepartmentID IN (3,4)

：此列表列出了您的所有销售额

| ItemID | DepartmentID |
|--------|--------------|
|      1 |            1 |
|      2 |            2 |
|      3 |            3 |
|      4 |            1 |
|      5 |            4 |
|      6 |            2 |
|      7 |            3 |
|      8 |            4 |

查询3：

CREATE TABLE Sale ( ItemID int, DepartmentID int ) ;

INSERT INTO Sale ( ItemID, DepartmentID )
VALUES (1,1), (2,2), (3,3), (4,1), (5,4), (6,2), (7,3), (8,4) ;

CREATE TABLE Department ( DepartmentID int, DepartmentFloor int ) ;

INSERT INTO Department ( DepartmentID, DepartmentFloor )
VALUES (1,1), (2,1), (3,2), (4,2) ;

SELECT *
FROM Department
WHERE DepartmentFloor = 2 

SELECT *
FROM Sale

SELECT *
FROM Sale
WHERE DepartmentID IN (3,4)

：这一条显示了与您的EXISTS语句等价的内容。它只显示了我的数据中匹配的4行。所以你只能得到条目ID 3、5、7和8

| ItemID | DepartmentID |
|--------|--------------|
|      3 |            3 |
|      5 |            4 |
|      7 |            3 |
|      8 |            4 |

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对不起，我不太明白。如果子查询返回true，那么查询不应该从Sale中选择DISTINCT ItemID吗？如果true的意思是“在每种情况下”（在此上下文中）则与其中的1=1完全相反。子查询是否总是返回true？似乎有些销售部门id不在楼层2上是的，但我认为一旦其中一个返回True，那么如果某个部门仅为DepartmentFloor=1或DepartmentID没有销售记录，则从sale中选择DISTINCT ItemID即可执行，然后EXISTS查询将为Sale中的行返回FALSE，并排除该DepartmentID的记录。