Oracle，MySQL，SQL Server三种数据库用法差异

1、使用连接符连接字段
将学生信息表（t_student）中学生姓名和性别连接起来查询学生信息
Oracle：
SELECT stuId, stuName||sex FROM t_student;
MySQL：
SELECT stuId, CONCAT(stuName,sex) FROM t_student;
SQL Server：
SELECT stuId, stuName+sex FROM t_student;

2、使用ROLLUP关键字统计数据
对教师信息表中的院系和教师职称进行分组，并对分组后的教师工资进行统计
Oracle：
SELECT dept,profession,SUM(salary) FROM t_teacher GROUP BY ROLLUP(dept,profession);
MySQL：
SELECT dept,profession,SUM(salary) FROM t_teacher GROUP BY dept,profession WHIT ROLLUP;
SQL Server：
SELECT dept,profession,SUM(salary) FROM t_teacher GROUP BY dept,profession WHIT ROLLUP;

3、限制结果集行数
查询教师信息表中的教师信息，只显示按照教师编号升序排序后的第4条到第6条记录
Oracle：
SELECT teaId,teaName,dept,profession FROM (SELECT ROWNUM AS rn,teaId,teaName,dept,profession FROM t_teacher WHERE ROWNUM<=6) WHERE rn>=4;
MySQL：
SELECT teaId,teaName,dept,profession FROM t_teacher ORDER BY teaId LIMIT 3,3;
SQL Server：
SELECT teaId,teaName,dept,profession FROM (SELECT TOP 6 teaId AS r,teaName,dept,profession FROM t_teacher) WHERE r>=4;