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  • mysql学习08--使用NULL

    teacher
    iddeptnamephonemobile
    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
    idname
    1 Computing
    2 Design
    3 Engineering
    ...

    1.List the teachers who have NULL for their department.

    select name from teacher
    where dept is null

    2.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)

    3.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)

    4.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)

    5.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') AS mobile
    from teacher

    6.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)

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

    select count(teacher.id),count(teacher.mobile)
    from teacher

    8.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.id)
    from teacher right join dept on teacher.dept=dept.id
    group by dept.name

    9.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 dept=1 or dept=2 then 'Sci'
    else 'Art' end
    from teacher left join dept 
    on teacher.dept=dept.id

    10.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 dept=1 or dept=2 then 'Sci'
    when dept=3 then 'Art'
    else 'None' end
    from teacher left join dept 
    on teacher.dept=dept.id

    题目来源:SQLZOO 

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  • 原文地址:https://www.cnblogs.com/asenyang/p/14351952.html
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