Retrieve Columns from Relational Database at Once - mysql

Question

In order to simplify the problem, let's assume this is for a school database, and we have 3 tables. In this school the student can participate in multiple courses which have multiple lessons, and we store in the DB the information about the available courses, each course lessons and which courses and lessons the student is currently studying.
Table courses has 2 columns, cs_id and cs_name. Table lessons has 3 columns, ls_course which stores the ID of the course the lesson is from, ls_number —a sequential number that identifies the lesson within the course— and ls_name. The last table students stores the student ID, st_id, the course ID and current lesson in that course, st_course and st_lesson. Some sample data:
courses
+-------+----------+
| cs_id | cs_name |
+-------+----------+
| 1 | Course A |
| 2 | Course B |
+-------+----------+
lessons
+-----------+-----------+--------------+
| ls_course | ls_number | ls_name |
+-----------+-----------+--------------+
| 1 | 1 | Lesson One |
| 1 | 2 | Lesson Two |
| 1 | 3 | Lesson Three |
| 2 | 1 | Lesson One |
| 2 | 2 | Lesson Two |
| 2 | 3 | Lesson Three |
+-----------+-----------+--------------+
students
+-------+-----------+-----------+
| st_id | st_course | st_lesson |
+-------+-----------+-----------+
| 1 | 1 | 2 |
| 2 | 1 | 3 |
| 2 | 2 | 2 |
+-------+-----------+-----------+
As you can see we currently have 2 students, and the student with ID 2 is currently taking 2 courses at once. In Course A, she is in the third lesson and in Course B, in Lesson Two. What I want is to retrieve the listing for the courses one student is currently in. But not as:
SELECT * FROM students WHERE st_id = 2;
This returns the filtered rows, but I'd like to retrieve the course and lesson ID and names for them. To get the course name column I'd do this:
SELECT s.*, c.cs_name
FROM students s, courses c
WHERE s.st_id = 2 AND s.st_course = c.cs_id
Which results in:
+-------+-----------+-----------+----------+
| st_id | st_course | st_lesson | cs_name |
+-------+-----------+-----------+----------+
| 2 | 1 | 3 | Course A |
| 2 | 2 | 2 | Course B |
+-------+-----------+-----------+----------+
The far I can get is:
SELECT s.*, c.cs_name, l.ls_name
FROM students s, courses c, lessons l
WHERE
s.st_id = 2 AND
s.st_course = c.cs_id AND
s.st_lesson = l.ls_number
But how can I retrieve the lesson ID and chapter to check for a match in the lessons table, is it possible at all? The result I want is:
+-------+-----------+-----------+----------+--------------+
| st_id | st_course | st_lesson | cs_name | ls_name |
+-------+-----------+-----------+----------+--------------+
| 2 | 1 | 3 | Course A | Lesson Three |
| 2 | 2 | 2 | Course B | Lesson Two |
+-------+-----------+-----------+----------+--------------+

Answer 1

This query is wrong:
SELECT s.*, c.cs_name, l.ls_name
FROM students s, courses c, lessons l
WHERE
s.st_id = 2 AND
s.st_course = c.cs_id AND
s.st_lesson = l.ls_number
You need also to match st_course to ls_course
SELECT s.*, c.cs_name, l.ls_name
FROM students s, courses c, lessons l
WHERE
s.st_id = 2 AND
s.st_course = c.cs_id AND
(s.st_lesson = l.ls_number AND st.st_course=l.ls_course)
I would also recommend using the current JOIN syntax, i.e.
SELECT s.*, c.cs_name, l.ls_name
FROM students s
JOIN courses c ON c.cs_id=s.st_course
JOIN lessons l ON (s.st_lesson=l.ls_number AND st.st_course=l.ls_course)
WHERE s.st_id = 2