Difference between revisions of "Using Null"

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==Teachers and Departments==
 
<p>The school includes many departments. Most teachers work exclusively for a single department.
 
Some teachers have no department.</p>
 
  
<table class='zoo' style='' border='1'>
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<div class='ref_section'>
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<table class='zoo db_ref'>
 
<caption>teacher</caption>
 
<caption>teacher</caption>
 
<tr>
 
<tr>
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<td align='right'>3345</td>
 
<td align='right'>3345</td>
 
<td></td>
 
<td></td>
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</tr>
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<tr>
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<td colspan='5'>...</td>
 
</tr>
 
</tr>
 
</table>
 
</table>
<table class='zoo' style='' border='1'>
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<table class='zoo db_ref'>
 
<caption>dept</caption>
 
<caption>dept</caption>
 
<tr>
 
<tr>
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<td align='right'>3</td>
 
<td align='right'>3</td>
 
<td>Engineering</td>
 
<td>Engineering</td>
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</tr>
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<tr>
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<td colspan='2'>...</td>
 
</tr>
 
</tr>
 
</table>
 
</table>
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</div>
  
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==Teachers and Departments==
 +
<p>The school includes many departments. Most teachers work exclusively for a single department.
 +
Some teachers have no department.</p>
  
 
[[Selecting NULL values.]]
 
[[Selecting NULL values.]]
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<div class="progress_panel"><div>
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  <div class="summary">Summary</div>
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    <div class="progressbar"></div>
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  </div>
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</div></div>
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<div class='extra_space' style='width:1em; height:2.5em;'></div>
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<div class='extra_space' style='width:1em; height:2.5em;'></div>
  
 
<h2>NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN</h2>
 
<h2>NULL, INNER JOIN, LEFT JOIN, RIGHT JOIN</h2>
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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.
+
You might think that the phrase dept=NULL would work here. It doesn't. This is because NULL propagates. 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.
  
 
<div title="That's not a proper explanation." class='hint'>
 
<div title="That's not a proper explanation." class='hint'>
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</source>
 
</source>
 
</div>
 
</div>
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<div class="lsclear">Clear your results</div>
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<p><div class="quizlink">[[Using Null Quiz]]</div></p>

Latest revision as of 06:52, 8 September 2012

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
...

[edit] 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. It doesn't. This is because NULL propagates. 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.

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' 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 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 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
Clear your results

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