Mysql 无法想象使用唯一谓词而不使用其相关形式的原因。这意味着什么？ where unique (select title from course) SELECT DISTINCT c.title FROM COURSE c SELECT

无法想象使用唯一谓词而不使用其相关形式的原因。这意味着什么？

where unique (select title from course)

SELECT DISTINCT 
       c.title
  FROM COURSE c

SELECT DISTINCT 
       c.title,
       c.instructor
  FROM COURSE c

  SELECT c.title
    FROM COURSE c
GROUP BY c.title

1) Let T be the result of the <table subquery>. 2) If there are no two rows in T such that the value of each column in one row is non-null and is equal to the value of the cor- responding column in the other row according to Subclause 8.2, "<comparison predicate>", then the result of the <unique predi- cate> is true; otherwise, the result of the <unique predicate> is false.

Where Not Exists    (
                    Select 1
                    From Course
                    Where Title Is Not Null
                    Group By Title
                    Having Count(*) > 1
                    )

WHERE 1 = ALL(SELECT COUNT(*) FROM course GROUP BY title)

sameera___ 20

john_______ 22

mac_______26

john_______22

sameera___20

mac______26

sameera___20

john______22

mac______26