如何从多个表组合一个记录

Question

I have two tables lesson and subjects. It is a one to many ratio as in each lesson can be categorized under 1 or more subjects resulting in something like "Name" subject: math, science, social studies

This would return 3 records:
Name, math
Name, science
name, social studies

This results in my looping through and combining and messes up my search count
How can I get one record with all three instead? So my search count is accurate and I don't need extra PHP code to check if name is the same and gather the extra subject?

I've tried basic

SELECT * FROM lesson INNER JOIN subjects ON subject.id = lesson.subject

But this results in 3 entries

EDIT
My query is more complicated than I lead on. I have a middle table keeping tracking of the two tables above and their relation
This query is for searching. Here's what I have.

SELECT name, subject        
FROM lesson As l
INNER JOIN lesson_sub As ls
  ON ls.lesson_id = l.id
INNER JOIN subjects As s
  ON s.id = ls.subject_id
WHERE CONCAT(l.name, s.subject) LIKE '%KEYWORD%' AND s.id = SUBJECT_ID

Answer 1

You can use the GROUP_CONCAT() function with your JOIN query:

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat

GROUP_CONCAT(expr)

This function returns a string result with the concatenated non-NULL values from a group. It returns NULL if there are no non-NULL values. The full syntax is as follows:

GROUP_CONCAT([DISTINCT] expr [,expr ...] [ORDER BY {unsigned_integer | col_name | expr} [ASC | DESC] [,col_name ...]] [SEPARATOR str_val])

mysql> SELECT student_name, -> GROUP_CONCAT(test_score) -> FROM student -> GROUP BY student_name;

Or:

mysql> SELECT student_name, -> GROUP_CONCAT(DISTINCT test_score -> ORDER BY test_score DESC SEPARATOR ' ') -> FROM student -> GROUP BY student_name;

So:

SELECT lesson.name, GROUP_CONCAT(subjects.name SEPARATOR ', ')
    FROM lesson JOIN subjects ON (subject.id = lesson.subject)
    GROUP BY lesson.name;

TEST

CREATE TABLE lesson ( name varchar (20), subject integer );
CREATE TABLE subjects ( id integer, name varchar(20) );

INSERT INTO subjects VALUES ( 1, 'Math' ), ( 2, 'Physics' ), ( 3, 'Chemistry' );

INSERT INTO lesson VALUES ( 'Lesson A', 1 );
INSERT INTO lesson VALUES ( 'Lesson A', 2 );
INSERT INTO lesson VALUES ( 'Lesson A', 3 );
INSERT INTO lesson VALUES ( 'Lesson B', 2 );
INSERT INTO lesson VALUES ( 'Lesson B', 3 );
INSERT INTO lesson VALUES ( 'Lesson C', 1 );

SELECT lesson.name, GROUP_CONCAT(subjects.name SEPARATOR ', ')
    FROM lesson JOIN subjects ON (subjects.id = lesson.subject)
    GROUP BY lesson.name;

+----------+--------------------------------------------+
| name     | GROUP_CONCAT(subjects.name SEPARATOR ', ') |
+----------+--------------------------------------------+
| Lesson A | Math, Chemistry, Physics                   |
| Lesson B | Chemistry, Physics                         |
| Lesson C | Math                                       |
+----------+--------------------------------------------+
3 rows in set (0.00 sec)

MORE COMPLICATED TEST (with intermediate table)

CREATE TABLE lesson ( id integer, name varchar (20) );
CREATE TABLE subjects ( id integer, name varchar(20) );
CREATE TABLE lesson_sub ( lesson_id integer, subject_id integer );

INSERT INTO subjects VALUES ( 1, 'Math' ), ( 2, 'Physics' ), ( 3, 'Chemistry' );
INSERT INTO lesson VALUES ( 1, 'Lesson A' ), ( 2, 'Lesson B' ), ( 3, 'Lesson C' );

INSERT INTO lesson_sub VALUES (1,1), (1,2),(1,3),(2,2),(2,3),(3,1);

SELECT lesson.name, GROUP_CONCAT(subjects.name SEPARATOR ', ') AS subjects
    FROM lesson_sub JOIN lesson ON ( lesson.id = lesson_sub.lesson_id )
                    JOIN subjects ON (subjects.id = lesson_sub.subject_id)
    WHERE CONCAT(lesson.name, subjects.name) LIKE '%Chem%'
    GROUP BY lesson.name;

SELECT name, subjects FROM (
    SELECT lesson.name, GROUP_CONCAT(subjects.name SEPARATOR ', ') AS subjects
    FROM lesson_sub JOIN lesson ON ( lesson.id = lesson_sub.lesson_id )
                    JOIN subjects ON (subjects.id = lesson_sub.subject_id)
    GROUP BY lesson.name ) AS lesson_clear
    WHERE CONCAT(name, subjects) LIKE '%Chem%';

+----------+--------------------------------------------+
| name     | GROUP_CONCAT(subjects.name SEPARATOR ', ') |
+----------+--------------------------------------------+
| Lesson A | Chemistry                                  |
| Lesson B | Chemistry                                  |
+----------+--------------------------------------------+
2 rows in set (0.00 sec)

+----------+--------------------------+
| name     | subjects                 |
+----------+--------------------------+
| Lesson A | Physics, Math, Chemistry |
| Lesson B | Physics, Chemistry       |
+----------+--------------------------+
2 rows in set (0.00 sec)

Answer 2

Why the inner join, if you just want the subject names?

SELECT subject
FROM lesson

Now, if you want subjects that have at least one lesson, then you could simply do

SELECT DISTINCT subject
FROM lesson
INNER JOIN subjects ON subject.id = lesson.subject

Answer 3

It looks like you are hoping for one record per person, that is per name .

Here's a way to do that:

      SELECT name, count(*) subject_count
        FROM lesson 
  INNER JOIN subjects ON subject.id = lesson.subject
    GROUP BY name
    ORDER BY name

If you need the subjects shown in the record by name, try this:

      SELECT name, 
             count(*) subject_count,
             group_concat(subject) subjects
        FROM lesson 
  INNER JOIN subjects ON subject.id = lesson.subject
    GROUP BY name
    ORDER BY name