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.
SELECTname FROMteacher WHEREdeptISNULL
注意INNER JOIN 不理會沒有學系的老師及沒有老師的學系。
SELECTteacher.name,dept.name FROMteacherINNERJOINdept ON(teacher.dept=dept.id)
SELECTteacher.name,dept.name FROMteacherINNERJOINdept ON(teacher.dept=dept.id)
使用不同的JOIN(外連接),來列出全部老師。
SELECTteacher.name,dept.name FROMteacher LEFTJOINdeptON(teacher.dept=dept.id)
使用不同的JOIN(外連接),來列出全部學系。
SELECTteacher.name,dept.name FROMteacher RIGHTJOINdeptON(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'
SELECTname,COALESCE(mobile,'07986 444 2266') FROMteacher
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.
SELECTteacher.name,COALESCE(dept.name,'None') FROMteacherLEFTJOINdept ONteacher.dept=dept.id
使用COUNT來數算老師和流動電話數目。
SELECTCOUNT(teacher.name),COUNT(mobile) FROMteacher
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。
SELECTdept.name,COUNT(teacher.name) FROMteacherRIGHTJOINdept ONteacher.dept=dept.id GROUPBYdept.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.
SELECTname,CASEWHENdeptIN(1,2) THEN'Sci' ELSE'Art'END FROMteacher
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.
SELECTname,CASEWHENdeptIN(1,2) THEN'Sci' WHENdept=3 THEN'Art' ELSE'None'END FROMteacher