牛客网在线编程网址：https://www.nowcoder.com/activity/oj
(默认使用SQLite)
题目1：
删除emp_no重复的记录，只保留最小的id对应的记录
CREATE TABLE IF NOT EXISTS titles_test ( id int(11) not null primary key, emp_no int(11) NOT NULL, title varchar(50) NOT NULL, from_date date NOT NULL, to_date date DEFAULT NULL);
insert into titles_test values (‘1‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘2‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘3‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘4‘, ‘10004‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘5‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘6‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘7‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘);
delete from titles_test where id not in ( select min(id) from titles_test group by emp_no );
题目2：
将所有to_date为9999-01-01的全部更新为NULL,且 from_date更新为2001-01-01。
CREATE TABLE IF NOT EXISTS titles_test ( id int(11) not null primary key, emp_no int(11) NOT NULL, title varchar(50) NOT NULL, from_date date NOT NULL, to_date date DEFAULT NULL);
insert into titles_test values (‘1‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘2‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘3‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘4‘, ‘10004‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘5‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘6‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘7‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘);
update titles_test set to_date = NULL, from_date = ‘2001-01-01‘;
题目3：
将id=5以及emp_no=10001的行数据替换成id=5以及emp_no=10005,其他数据保持不变，使用replace实现。
insert into titles_test values (‘1‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘2‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘3‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘4‘, ‘10004‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘), (‘5‘, ‘10001‘, ‘Senior Engineer‘, ‘1986-06-26‘, ‘9999-01-01‘), (‘6‘, ‘10002‘, ‘Staff‘, ‘1996-08-03‘, ‘9999-01-01‘), (‘7‘, ‘10003‘, ‘Senior Engineer‘, ‘1995-12-03‘, ‘9999-01-01‘);
update titles_test set emp_no = replace(emp_no, ‘10001‘, ‘10005‘) where id = 5;
http://sqlite.org/lang_corefunc.html#replace
replace(X,Y,Z)
The replace(X,Y,Z) function returns a string formed by substituting string Z for every occurrence of string Y in string X. The BINARY collating sequence is used for comparisons. If Y is an empty string then return X unchanged. If Z is not initially a string, it is cast to a UTF-8 string prior to processing.
题目4：
将titles_test表名修改为titles_2017。
ALTER table titles_test rename to titles_2017
题目5：
在audit表上创建外键约束，其emp_no对应employees_test表的主键id。
CREATE TABLE employees_test( ID INT PRIMARY KEY NOT NULL, NAME TEXT NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(50), SALARY REAL );
CREATE TABLE audit( EMP_no INT NOT NULL, create_date datetime NOT NULL );
Drop table audit; CREATE TABLE audit( EMP_no INT NOT NULL, create_date datetime NOT NULL, foreign key(emp_no) references employees_test(id) );
如果是在mysql中，可以直接用
Alter table audit add foreign key(emp_no) references employees_test(id);
见w3school.
原文：http://www.cnblogs.com/RRRRecord/p/7820517.html