SQLZOO (NULL) 答案

List the teachers who have NULL for their department.

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)

Use a different JOIN so that all teachers are listed.
all teacher 很明显需要把teacher表都放进去 所以用OUTER

SELECT teacher.name,dept.name FROM teacher LEFT OUTER JOIN dept ON (teacher.dept = dept.id)

Use a different JOIN so that all departments are listed.
与上题同理

SELECT teacher.name,dept.name FROM teacher RIGHT OUTER JOIN dept ON (teacher.dept = dept.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’
COALESCE函数，返回某一列并填补空值

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 OUTER 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 OUTER 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 in (1,2) THEN ‘Sci’
ELSE 'Art’END
FROM teacher LEFT OUTER JOIN dept ON teacher.dept = dept.id

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 in (1,2) THEN ‘Sci’
WHEN teacher.dept = 3 THEN ‘Art’
ELSE ‘None’ END
FROM teacher LEFT OUTER JOIN dept ON teacher.dept = dept.id