mysql - SQL IN 工作困难

Question

我的理解是，IN 就像一个 bool 函数，为使用 IN 时外部查询中的一行返回 true 的每个实例返回一个值。因此，结果集不应该只包含与外部查询一样多的行吗？

表 1:

   DNUMBER DLOCATION
---------- ---------------
         1 Houston
         4 Stafford
         5 Bellaire
         5 Houston
         5 Sugarland

表 2:

DNAME                   DNUMBER
-------------------- ----------
Research                      5
Administration                4
Headquarters                  1

那么这个查询不应该只返回 3 行吗？

SELECT D.dname, DL.dlocation
FROM department D, dept_locations DL
WHERE D.dnumber IN
(SELECT dnumber FROM dept_locations)

结果:

DNAME                DLOCATION
-------------------- -------------
Headquarters         Houston
Administration       Houston
Research             Houston
Headquarters         Stafford
Administration       Stafford
Research             Stafford
Headquarters         Bellaire
Administration       Bellaire
Research             Bellaire
Headquarters         Houston
Administration       Houston
Research             Houston
Headquarters         Sugarland
Administration       Sugarland
Research             Sugarland

Answer 1

SELECT D.dname, DL.dlocation
FROM department D, dept_locations DL
WHERE D.dnumber IN
(SELECT dnumber FROM dept_locations)

没有以任何方式限制要匹配的行的连接条件(显式或隐式)，因此执行交叉连接(左侧行与右侧行的所有组合)。