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 2 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。