一、差异集锦

　　在建表的时候，只有自增的语法不同。

　　下面给出3种数据库通用的建表与初始化测试语句：

CREATE TABLE Country(
   Id int PRIMARY KEY,
   Name varchar(20)
);

CREATE TABLE Person (  
  Id int PRIMARY KEY,  
  CountryId int,
  Name varchar(20) NOT NULL UNIQUE,  
  Sex int DEFAULT 0,
  CONSTRAINT FK_CID_PID FOREIGN KEY (CountryId) REFERENCES Country(Id)
);

　　插入测试数据：

INSERT INTO Country VALUES(1, '蜀国');
INSERT INTO Country VALUES(2, '魏国');
INSERT INTO Country VALUES(3, '吴国');

INSERT INTO Person VALUES(1,1,'刘备',1);
INSERT INTO Person VALUES(2,1,'关羽',1);
INSERT INTO Person VALUES(3,1,'张飞',1);
INSERT INTO Person VALUES(4,2,'曹操',1);
INSERT INTO Person VALUES(5,2,'郭嘉',1);
INSERT INTO Person VALUES(6,2,'典韦',1);
INSERT INTO Person VALUES(7,3,'孙权',1);
INSERT INTO Person VALUES(8,3,'周瑜',1);
INSERT INTO Person VALUES(9,3,'大乔',2);

	SQLServer	MySQL	Oracle
自增	identity(1,1)	AUTO_INCREMENT	SEQUENCE/触发器
自增行插入	INSERT INTO Person VALUES(NULL,1,'魏延',1)	INSERT INTO Person VALUES(1,'魏延',1)	INSERT INTO Person VALUES(SEQID.NEXTVAL,1,'魏延',1)
取前几条	SELECT TOP 5 * FROM Person	SELECT * FROM Person LIMIT 0,5	SELECT * FROM Person WHERE ROWNUM < 5
表别名	SELECT * FROM Person AS P INNER JOIN Country AS C ON P.CountryId = C.Id	SELECT * FROM Person AS P INNER JOIN Country AS C ON P.CountryId = C.Id	SELECT * FROM Person P INNER JOIN Country C ON P.CountryId = C.Id
字段名大小写	与SELECT相同	与SELECT相同	Oracle中，字段名都是大写的，即使SELECT语句中是小写，得到的结果也是大写。
获取当前时间&查询临时值	SELECT now() AS NOWDATE	SELECT getdate() AS NOWDATE	SELECT sysdate AS NOWDATE FROM Dual--需加Dual虚表