这里的 where 100 in 的100怎么理解丫？

USE pubs
SELECT DISTINCT au_lname, au_fname
FROM authors
WHERE 100 IN
  (SELECT royaltyper
  FROM titleauthor
  WHERE titleauthor.au_id = authors.au_id)

没见过这种用法,除非100是个列名,那么应该用[]括号括起来.
WHERE [100] IN

SQL code

USE pubs
SELECT DISTINCT au_lname, au_fname
FROM authors
WHERE exists
  (SELECT 1
  FROM titleauthor
  WHERE titleauthor.au_id = authors.au_id and royaltyper=100)

就等这
authors表的au_id 

的titleauthor表同一au_id 的royaltyper=100的记录

没有见过这种用法，观看！

等价于:
SQL code

select distinct a.au_lname,a.au_fname
from authors a inner join titleauthor b on a.au_id=b.au_id where b.royaltyper=100
/*
au_lname                                 au_fname
---------------------------------------- --------------------
Blotchet-Halls                           Reginald
Carson                                   Cheryl
del Castillo                             Innes
Green                                    Marjorie
Locksley                                 Charlene
Panteley                                 Sylvia
Ringer                                   Albert
Straight                                 Dean
White                                    Johnson

(9 行受影响)
*/

一样的理解 100包含在子查询里面啊

这样更容易理解:
SQL code

SELECT DISTINCT au_lname, au_fname
FROM authors
WHERE exists
  (SELECT 1
  FROM titleauthor
  WHERE titleauthor.au_id = authors.au_id and royaltyper=100)