exists 的简单介绍

准备数据：

 CREATE TABLE Books(
     BookID number,
     BookTitle VARCHAR2(20) NOT NULL,
     Copyright varchar2(20)
 )
 INSERT INTO Books VALUES (12786, 'Java',1934);
 INSERT INTO Books VALUES (13331, 'MySQL',1919);
  INSERT INTO Books VALUES (14356, 'PHP',1966);
  INSERT INTO Books VALUES (15729, 'PERL',1932);
  INSERT INTO Books VALUES (16284, 'Oracle',1996);
  INSERT INTO Books VALUES (17695, 'Pl/SQL',1980);
  INSERT INTO Books VALUES (19264, 'JavaScript',1992);
  INSERT INTO Books VALUES (19354, 'www.manongjc.com',1993);

 CREATE TABLE AuthorBook(
     AuthID NUMBER,
     BookID NUMBER
     )

 INSERT INTO AuthorBook VALUES (1006, 14356);
21 INSERT INTO AuthorBook VALUES (1008, 15729);
 INSERT INTO AuthorBook VALUES (1009, 12786);
 INSERT INTO AuthorBook VALUES (1010, 17695);
 INSERT INTO AuthorBook VALUES (1011, 15729);
 INSERT INTO AuthorBook VALUES (1012, 19264);
 INSERT INTO AuthorBook VALUES (1012, 19354);
 INSERT INTO AuthorBook VALUES (1014, 16284); 

BOOKS表：

AUTHORBOOK表：

查询B表ID在A表中的数据

select * from BOOKS B WHERE EXISTS (SELECT * FROM AUTHORBOOK A WHERE A.BOOKID=B.BOOKID)

EXISTS后面表示的只有两个值。就是“真”或者“假”。 NOT EXISTS 与EXISTS相反。