# Using Null

teacher
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
...
dept
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.

Summary

## 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=NULL`

No it's not, but you can read a better explanation at Wikipedia:NULL.

```SELECT name
FROM teacher
WHERE dept IS NULL
```

Note the INNER JOIN misses the teacher with no department and the department with no teacher.

```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)
```

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)
```

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)
```

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 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
```

Use COUNT to show the number of teachers and the number of mobile phones.

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

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
```

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 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
```