SQL語句允許將一個查詢語句做為一個結果集供其他SQL語句使用，就像使用普通的表一樣，被當作結果集的查詢語句被稱為子查詢。所有可以使用表的地方几乎都可以使用子查詢來代替，比如SELECT FROM T 中就可以用子查詢來代替表T，比如SELECTFROM(SELECT FROM T2 where FAge<30)，這裡的“SELECT FROM T2 where FAge<30”就是子查詢，可以將子查詢看成一張暫態的資料表，這張表在查詢開始時被創造，在查詢結束時被刪除。子查詢大大簡化了複雜SQL語句的編寫，不過使用不當也容易造成效能問題。

子查詢的語法與普通的SELECT語句語法相同，所有可以在普通SELECT語句中使用的特性都可以在子查詢中使用，比如WHERE子句過濾、UNION運算子、HAVING子句、GROUPBY子句、ORDER BY子句，甚至在子查詢中還可以包含子查詢。同時，不僅可以在SELECT語句中使用子查詢，還可以在UPDATE、DELETE 等語句中使用子查詢。

為了更容易的執行本章中的例子，必須首先建立所需要的資料表，因此下面列出本章中要用到資料表的建立SQL語句：

MYSQL,MSSQLServer,DB2：

CREATE TABLE T_Reader (FId INT NOT NULL ,FName VARCHAR(50),FYearOfBirth INT,FCity VARCHAR(50),FProvince VARCHAR(50),FYearOfJoin INT) Oracle： CREATE TABLE T_Reader (FId NUMBER (10) NOT NULL ,FName VARCHAR2(50),FYearOfBirth NUMBER (10),FCity VARCHAR2(50),FProvince VARCHAR2(50), FYearOfJoin NUMBER (10)) MYSQL,MSSQLServer,DB2： CREATE TABLE T_Book (FId INT NOT NULL ,FName VARCHAR(50),FYearPublished INT,FCategoryId INT) Oracle： CREATE TABLE T_Book (FId NUMBER (10) NOT NULL ,FName VARCHAR2(50),FYearPublished NUMBER (10),FCategoryId NUMBER (10)) MYSQL,MSSQLServer,DB2： CREATE TABLE T_Category (FId INT NOT NULL ,FName VARCHAR(50)) Oracle： CREATE TABLE T_Category (FId NUMBER (10) NOT NULL ,FName VARCHAR2(50)) MYSQL,MSSQLServer,DB2： CREATE TABLE T_ReaderFavorite (FCategoryId INT,FReaderId INT) Oracle： CREATE TABLE T_ReaderFavorite (FCategoryId NUMBER (10),FReaderId NUMBER (10)) 
 

請在不同的資料庫系統中執行相應的SQL語句。其中表T_Reader儲存的是讀者資訊，FId為主鍵、FName為讀者姓名、FYearOfBirth為讀者出生年份、FCity為讀者所在城市、FProvince為讀者所在省份、FYearOfJoin為讀者入會年份；表T_Book儲存的是書籍資訊，FId為主鍵、FName為書名、FYearPublished為出版年份、FCategoryId為所屬分類；表T_Category儲存的是分類資訊，FId為主鍵、FName為分類名；表T_ReaderFavorite儲存的是讀者和讀者喜愛的類別之間的對應關係，FCategoryId為分類主鍵、FReaderId為讀者主鍵。

為了更加直觀的驗證本章中函式使用方法的正確性，我們需要在這幾張表中預置一些初始資料，請在資料庫中執行下面的資料插入SQL語句：

INSERT INTO T_Category(FId,FName)VALUES(1,"Story"); INSERT INTO T_Category(FId,FName)VALUES(2,"History"); INSERT INTO T_Category(FId,FName)VALUES(3,"Theory"); INSERT INTO T_Category(FId,FName)VALUES(4,"Technology"); INSERT INTO T_Category(FId,FName)VALUES(5,"Art"); INSERT INTO T_Category(FId,FName)VALUES(6,"Philosophy"); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(1,"Tom",1979,"TangShan","Hebei",2003); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(2,"Sam",1981,"LangFang","Hebei",2001); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(3,"Jerry",1966,"DongGuan","GuangDong",1995); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(4,"Lily",1972,"JiaXing","ZheJiang",2005); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(5,"Marry",1985,"BeiJing","BeiJing",1999); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(6,"Kelly",1977,"ZhuZhou","HuNan",1995); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(7,"Tim",1982,"YongZhou","HuNan",2001); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(8,"King",1979,"JiNan","ShanDong",1997); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(9,"John",1979,"QingDao","ShanDong",2003); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(10,"Lucy",1978,"LuoYang","HeNan",1996); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(11,"July",1983,"ZhuMaDian","HeNan",1999); INSERT INTO T_Reader(FId,FName,FYearOfBirth,FCity,FProvince,FYearOfJoin)VALUES(12,"Fige",1981,"JinCheng","ShanXi",2003); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(1,"About J2EE",2005,4); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(2,"Learning Hibernate",2003,4); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(3,"Two Cites",1999,1); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(4,"Jane Eyre",2001,1); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(5,"Oliver Twist",2002,1); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(6,"History of China",1982,2); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(7,"History of England",1860,2); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(8,"History of America",1700,2); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(9,"History of TheWorld",2008,2); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(10,"Atom",1930,3); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(11,"RELATIVITY",1945,3); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(12,"Computer",1970,3); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(13,"Astronomy",1971,3); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(14,"How To Singing",1771,5); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(15,"DaoDeJing",2001,6); INSERT INTO T_Book(FId,FName,FYearPublished,FCategoryId)VALUES(16,"Obedience toAuthority",1995,6); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(1,1); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(5,2); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(2,3); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(3,4); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(5,5); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(1,6); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(1,7); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(4,8); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(6,9); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(5,10); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(2,11); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(2,12); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(1,12); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(3,1); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(1,3); INSERT INTO T_ReaderFavorite(FCategoryId,FReaderId)VALUES(4,4); 

初始資料預置完畢以後執行SELECT * FROM T_Reader 檢視T_Reader 表中的資料。