- 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.
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
- 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
这道题开始使用COALESCE函数
用法1：
coalesce(col_name) 可以用来返回col_name里第一个不为NULL的值，如果col_name这一列全为NULL则返回NULL。
用法2：
coalesce(col_name, [str1])将col_name列里所有NULL值替换为str1.
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 dept = dept.id
7.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
CASE
WHEN dept in (1, 2) THEN CONCAT(name, ‘Sci’)
ELSE CONCAT(name, ‘Art’)
END AS name
FROM teacher
这题答案是把拼接的SCI\ART重新加了一列，我就直接用CONCAT函数拼到NAME列里了，输出的内容都是一样的。
- 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 name,
CASE
WHEN dept = 1 OR dept = 2 THEN ‘Sci’
WHEN dept = 3 THEN ‘Art’
ELSE ‘None’
END
FROM teacher