SQLZOO練習筆記-USING NULL
- 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.
- Use a different JOIN so that all teachers are listed.
- Use a different JOIN so that all departments are listed.
- 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
這道題開始使用COALESCE函式 用法1： coalesce(col_name) 可以用來返回col_name裡第一個不為NULL的值，如果col_name這一列全為NULL則返回NULL。 用法2：
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
這題答案是把拼接的SCI\ART重新加了一列，我就直接用CONCAT函式拼到NAME列裡了，輸出的內容都是一樣的。
- 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