Using Null
| Language: Project:Language policy | English • 日本語 • 中文 |
|---|
| id | dept | name | phone | mobile |
|---|---|---|---|---|
| 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 | name |
|---|---|
| 1 | Computing |
| 2 | Design |
| 3 | Engineering |
| ... | |
Teachers and Departments
The school includes many departments. Most teachers work exclusively for a single department. Some teachers have no department.
NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN
List the teachers who have NULL for their department.
You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase dept IS NULL
No it's not, but you can read a better explanation at Wikipedia:NULL.
SELECTname FROMteacher WHEREdeptISNULL
Note the INNER JOIN misses the teachers with no department and the departments with no teacher.
SELECTteacher.name,dept.name FROMteacherINNERJOINdept ON(teacher.dept=dept.id)
SELECTteacher.name,dept.name FROMteacherINNERJOINdept ON(teacher.dept=dept.id)
Use a different JOIN so that all teachers are listed.
SELECTteacher.name,dept.name FROMteacher LEFTJOINdeptON(teacher.dept=dept.id)
Use a different JOIN so that all departments are listed.
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
Use COUNT to show the number of teachers and the number of mobile phones.
SELECTCOUNT(teacher.name),COUNT(mobile) FROMteacher
Use COUNT and GROUP BY dept.name to show each department and the number of staff. Use a RIGHT JOIN to ensure that the Engineering department is listed.
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