Using Null/zh

From SQLZOO
Jump to: navigation, search
teacher老師
id編號 dept學系 name名字 phone內線電話 mobile流動電話
101 1 Shrivell 2753 07986 555 1234
102 1 Throd 2754 07122 555 1920
103 1 Splint 2293
104 Spiregrain 3287
105 2 Cutflower 3212 07996 555 6574
106 Deadyawn 3345
...
dept學系
id編號 name名稱
1 Computing
2 Design
3 Engineering
...

老師和學系

一間學校有很多學系,大多數老師只為一個學系工作。 有些老師沒有學系。

Selecting NULL values.

NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN

列出學系department是NULL值的老師。

你可能會以為 dept=NULL 是可行的,但它不是。你要使用dept IS NULL

不,不是,但你可以看更好的解說,在維基百科:NULL.

SELECT name
FROM teacher
WHERE dept IS NULL

注意INNER JOIN 不理會沒有學系的老師及沒有老師的學系。

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

使用不同的JOIN(外連接),來列出全部老師。

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

使用不同的JOIN(外連接),來列出全部學系。

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

Using the COALESCE function

Use COALESCE to print the mobile number. Use the number '07986 444 2266' if there is no number given. Show teacher name and mobile number or '07986 444 2266'

SELECT name, COALESCE(mobile,'07986 444 2266')
FROM teacher

Use the COALESCE function and a LEFT JOIN to print the teacher name and department name. Use the string 'None' where there is no department.

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

使用COUNT來數算老師和流動電話數目。

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

使用COUNT 和 GROUP BY dept.name來顯示每一學系的老師數目。 使用 RIGHT JOIN 以確保工程系Engineering 是在當中。

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

Using CASE

Use CASE to show the name of each teacher followed by 'Sci' if the teacher is in dept 1 or 2 and 'Art' otherwise.

SELECT name, CASE WHEN dept IN (1,2) 
THEN 'Sci'
ELSE 'Art' END
FROM teacher


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 IN (1,2) 
THEN 'Sci'
WHEN dept = 3 
THEN 'Art'
ELSE 'None' END
FROM teacher
Clear your results