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.
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练习
- List the teachers who have NULL for their department.
找出dept为空的老师
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
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
full join dept on teacher.dept=dept.id
where teacher.name is not null; -- 方法1
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.
-- 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'
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(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 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.
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.
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
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About case function
case表达式的两种写法
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
