Using Null/zh
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 | ||
... |
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。
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