How do I write MySQL queries? - mysql

Question

I had to create two tables (students and grades) and then insert the values.
These are the queries-
CREATE TABLE students ( student_id INT(3) NOT NULL PRIMARY KEY AUTO_ INCREMENT,
firstname VARCHAR(30) NOT NULL,
lastname VARCHAR(50) NOT NULL );
CREATE TABLE grades ( student_id INT (3) NOT NULL PRIMARY KEY AUTO_
INCREMENT,
gpa DOUBLE );'
INSERT INTO students (student_id, firstname, lastname) VALUES (‘123’,
‘Some’, ‘One’);
INSERT INTO students (student_id, firstname,lastname) VALUES (‘125’,
‘Another’, ‘One’);
INSERT INTO students (student_id, firstname,lastname) VALUES (‘167’, ‘Good’,
‘Student’);
INSERT INTO grades (student_id, gpa) VALUES (‘123’, ‘2.34);
INSERT INTO grades (student_id, gpa) VALUES (‘125’, ‘3.21’);
INSERT INTO grades (student_id, gpa) VALUES (‘167’, ‘3.86’);
So, now, if I have to use the SQL select query to find specific rows (GPA above 3.5) and have to use the LEFT and the CONCAT function to give me the name Good Student, exactly how do I do that? How do I use the LEFT and the CONCAT functions to generate rows?

Answer 1

I assume that by "left" you meant a left join, but that won't really be appropriate here - every student must have a GPA, and if, for some obscure reason, a certain student doesn't, then by definition he cannot have a GPA of about 3.5. To make a long story short - an inner join would be much more appropriate:
SELECT CONCAT_WS(' ', firstname, lastname)
FROM students s
JOIN grades g ON s.student_id = g.student_id
WHERE gpa > 3.5

Answer 2

Although the option by Mureinik is legit, and correct you don't need a left-join, you can actually think of it in reverse. Don't start with the student, but start with the grades... This is more of a mental perception, but shows how the same results can be obtained with slightly different queries
select
concat_ws( ' ', s.firstname, s.lastname ) studentName,
g.gpa
from
grades g
join students s
on g.student_id = s.student_id
where
g.gpa > 3.5
Also, best to qualify your table (or alias 'g' and 's' in this case) with the column name references to prevent ambiguity for yourself or others to look back at queries in the future.