Difference between revisions of "Using Null"

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(Teachers and Departments)
 
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List the teachers who have NULL for their department.
 
List the teachers who have NULL for their department.
 
<div class='hint' title='Why we cannot use ='>
 
<div class='hint' title='Why we cannot use ='>
You might think that the phrase dept=NULL would work here. It doesn't. This is because NULL "propogates". Any normal expression that includes NULL is itself NULL, thus the value of the expressions 2+NULL and party || NULL and NULL=NULL for example are all NULL.
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You might think that the phrase dept=NULL would work here but it doesn't - you can use the phrase <code>dept IS NULL</code>
  
 
<div title="That's not a proper explanation." class='hint'>
 
<div title="That's not a proper explanation." class='hint'>
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<div class='qu'>
 
<div class='qu'>
Note the INNER JOIN misses the teacher with no department and the department with no teacher.  
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Note the INNER JOIN misses the teachers with no department and the departments with no teacher.  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
SELECT teacher.name, dept.name
 
SELECT teacher.name, dept.name
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<p>Using the [[COALESCE]] function</p>
 
<p>Using the [[COALESCE]] function</p>
 
<div class='qu'>
 
<div class='qu'>
Use COALESCE to print the mobile number. Use the number '07986 444 2266' there is no number given.
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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''''   
 
'''Show teacher name and mobile number or '07986 444 2266''''   
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<div class='qu'>
 
<div class='qu'>
Use the COALESCE function and a LEFT JOIN to print the '''name''' and department name.
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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.     
 
Use the string 'None' where there is no department.     
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<div class='qu'>
 
<div class='qu'>
Use CASE to show the '''name''' of each teacher followed by 'Sci' if the the teacher is in '''dept''' 1 or 2 and 'Art' otherwise.       
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Use CASE to show the '''name''' of each teacher followed by 'Sci' if the teacher is in '''dept''' 1 or 2 and 'Art' otherwise.       
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
</source>
 
</source>
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<div class='qu'>
 
<div class='qu'>
Use CASE to show the name of each teacher followed by 'Sci' if the the teacher is in dept 1 or 2 show 'Art' if the dept is 3 and 'None' otherwise.       
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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.       
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
</source>
 
</source>
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<div class="lsclear">Clear your results</div>
 
<div class="lsclear">Clear your results</div>
  
[[Using Null Quiz]]
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<p><div class="quizlink">[[Using Null Quiz]]</div></p>

Latest revision as of 18:37, 22 September 2015

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
106 Deadyawn 3345
...
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.

Selecting NULL values.

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 IS 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 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)
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 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

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 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
Clear your results