【Tips】

• sqlzoo官网链接：【https://sqlzoo.net/wiki/SQL_Tutorial/zh】
• 每个系列开头介绍所用表及其信息
• 每道题均测试通过，含有题目描述、代码和结果
• 题目为自己简写，最好在官网查看具体题目
• 部分测试结果不完整，仅为一部分截图

NULL值之老师和学系练习链接：【https://sqlzoo.net/wiki/Using_Null/zh】

所用的表teacher老师,dept学系

1.列出学系department是NULL值的老师

SELECT teacher.name
FROM teacher LEFT JOIN dept
ON teacher.dept=dept.id
WHERE dept.name IS NULL

2.INNER JOIN的使用

SELECT teacher.name, dept.name
 FROM teacher INNER JOIN dept
           ON (teacher.dept=dept.id)

3.使用不同的JON（外连接）列出全部老师

SELECT teacher.name,dept.name
FROM teacher LEFT JOIN dept
ON teacher.dept=dept.id              

4.使用外连接，列出全部学系 

SELECT teacher.name,dept.name
FROM dept LEFT JOIN teacher ON
dept.id=teacher.dept

5.列出教师名字和电话，如果没有电话显示07986 444 2266

• COALESCE返回第一个非空值，若全为空，返回NULL

SELECT name,COALESCE(mobile,‘07986 444 2266‘)
FROM teacher
--若mobile为空，返回‘07986 444 2266‘

6.显示教师name和department name，如果没有department的话显示None

SELECT teacher.name,COALESCE(dept.name,‘None‘)
FROM teacher LEFT JOIN dept ON teacher.dept=dept.id

7.使用COUNT算老师和流动电话数目

SELECT COUNT(name),COUNT(mobile)
FROM teacher

8.使用COUNT和GROUP BY dept.name显示每一学系的老师数目，使用RIGHT JOIN

SELECT dept.name,COUNT(teacher.name)
FROM teacher RIGHT JOIN dept ON dept.id=teacher.dept
GROUP BY dept.name

9.用CASE 。如果这个老师在dept1或dept2,则“Sci”，否则Art

SELECT name,
CASE WHEN dept=1 OR dept=2 THEN ‘Sci‘
ELSE ‘Art‘ END
FROM teacher

10.Use CASE to show the name of each teacher followed by ‘Sci‘ if the teacher is in dept 1 or 2, show ‘Art‘ if the teacher‘s dept is 3 and ‘None‘ otherwise

SELECT name,
CASE WHEN dept=1 OR dept=2 THEN ‘Sci‘
     WHEN dept=3 THEN ‘Art‘ 
     ELSE ‘None‘ END
FROM teacher

sqlzoo练习系列（十三）——NULL值之老师和学系