sqlzoo练习14-using-null

sqlzoo练习14-using NULL

本文中讲解的是当数据库的表中的数据存在缺失值NULL的时候，该如何进行处理。下面的数据含有两个表teacher和dept

The school includes many departments. Most teachers work exclusively for a single department. Some teachers have no department.

using null

练习

List the teachers who have NULL for their department.

找出dept为空的老师

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select name
from teacher
where dept in NULL;   -- 注意不能使用dept=NULL

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

inner join能够排除老师为空的系或者没有系的老师

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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.

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select teacher.name. dept.name
from teacher
full join dept on teacher.dept=dept.id
where teacher.name is not null;  -- 方法1

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select teacher.name, dept.name
from teacher
left join dept on teacher.dept=detp.id;  -- 方法2

Use a different JOIN so that all departments are listed.

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-- 方法1
select teacher.name, dept.name
from dept
full join teacher on dept.id=teacher.dept
where dept.name is not null;

-- 方法2
select teacher.name, dept.name
from teacher
right join dept on teacher.dept=detp.id;

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’

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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.

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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.

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select count(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.

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select dept.name, count(teacher.name)
from teacher
right join dept on dept.id=teacher.dept
group by dept.name;

Use CASE to show the name of each teacher followed by ‘Sci’ if the teacher is in dept 1 or 2 and ‘Art’ otherwise.

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select teacher.name,
   case when (teacher.dept=1 or teacher.dept=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.

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select teacher.name,
  case when teacher.dept=1 or teacher.dept=2 then 'Sci'
       when teacher.dept=3 then 'Art'
  else 'None' end
from teacher

About coalesce函数

COALESCE takes any number of arguments and returns the first value that is not null.

笔记：取第一个不是NULL值的数据

About case function

case表达式的两种写法

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case sex   -- 1. 简单表达式
   when '1' then '男'
   when '2' then '女'
else 'other' end


case when sex='1' then '男'  -- 2. 搜索表达式
     when sex='2' then '女'
else 'other' end

function join mobile null using

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