| 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 |
| ... | |
1、
List the teachers who have NULL for their department.
Why we cannot use =
You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase dept IS NULL
That's not a proper explanation.
No it's not, but you can read a better explanation at Wikipedia:NULL
select name from teacher where dept IS NULL
2、Note the INNER JOIN misses the teachers with no department and the departments with no teacher.
SELECT teacher.name, dept.name FROM teacher INNER JOIN dept ON (teacher.dept=dept.id)
3、Use a different JOIN so that all teachers are listed.
SELECT teacher.name, dept.name FROM teacher left JOIN dept ON (teacher.dept=dept.id)
4、Use a different JOIN so that all departments are listed
SELECT teacher.name, dept.name FROM teacher right JOIN dept ON (teacher.dept=dept.id)
5、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
6、Use the COALESCE function and a LEFT JOIN to print the teacher nameand 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)
7、Use COUNT to show the number of teachers and the number of mobile phones.
SELECT count(name),count(mobile) FROM teacher
8、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.
select dept.name ,count(teacher.name) from teacher right join dept on teacher.dept=dept.id group by dept.name
9、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' endfrom 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 in(1,2) then 'Sci'when dept=3 then 'Art'else 'None' endfrom teacher