Question

I have the schema like the following:

CREATE TABLE prop_set (
      id INT UNSIGNED NOT NULL AUTO_INCREMENT,
      name varchar(255),
      PRIMARY KEY (id)
  );
CREATE TABLE users_props (
      user_id int UNSIGNED NOT NULL,
      prop_id int UNSIGNED NOT NULL,
      value text, 
      PRIMARY KEY (user_id,prop_id)
  );
CREATE TABLE user (
      id INT UNSIGNED NOT NULL AUTO_INCREMENT,
      name varchar(255),
      PRIMARY KEY (id)
  );

INSERT INTO prop_set SET name="prop1";
INSERT INTO prop_set SET name="prop2";
INSERT INTO prop_set SET name="prop3";


INSERT INTO user SET name="user1";
INSERT INTO user SET name="user2";

INSERT INTO users_props set user_id=1,prop_id=1,value="prop1 user1";
INSERT INTO users_props set user_id=1,prop_id=2,value="prop2 user1";
INSERT INTO users_props set user_id=1,prop_id=3,value="prop3 user1";

INSERT INTO users_props set user_id=2,prop_id=1,value="prop1 user2";
INSERT INTO users_props set user_id=2,prop_id=2,value="prop2 user2";
INSERT INTO users_props set user_id=2,prop_id=3,value="prop3 user2";

now I run the select like following:

SELECT u.name,ps.name AS prop,up.value
      FROM USER u
        JOIN users_props up ON u.id=up.user_id
        JOIN prop_set ps ON ps.id=up.prop_id;

and get output:

|  NAME |  PROP |       VALUE |
-------------------------------
| user1 | prop1 | prop1 user1 |
| user2 | prop1 | prop1 user2 |
| user1 | prop2 | prop2 user1 |
| user2 | prop2 | prop2 user2 |
| user1 | prop3 | prop3 user1 |
| user2 | prop3 | prop3 user2 |

fiddle
Is there any way to make mysql to return the following output ?

| NAME  |   prop1     |     prop2   |    prop3    |
---------------------------------------------------
| user1 | prop1 user1 | prop2 user1 | prop3 user1 |
| user2 | prop1 user2 | prop2 user2 | prop3 user2 |

I looking for mysql – only solution.

Answer 1

The pivot syntax of MySQL uses an aggregate function with a CASE statement similar to this:

SELECT u.name,
  max(case when ps.name = 'prop1' then up.value else null end) Prop1,
  max(case when ps.name = 'prop2' then up.value else null end) Prop2,
  max(case when ps.name = 'prop3' then up.value else null end) Prop3
FROM user u 
JOIN users_props up 
  ON u.id=up.user_id 
JOIN prop_set ps 
  on ps.id=up.prop_id
GROUP BY u.name;

See SQL Fiddle with Demo

The above works great if you have known values to transpose into columns, but if the values are unknown, then you will want to look at using a prepared statement to generate dynamic SQL:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'max(case when ps.name = ''',
      name,
      ''' then up.value else null end) AS ''',
      name, ''''
    )
  ) INTO @sql
FROM  prop_set;

SET @sql = CONCAT('SELECT u.name, ', @sql, ' 
                  FROM user u 
                  JOIN users_props up 
                    ON u.id=up.user_id 
                  JOIN prop_set ps 
                    on ps.id=up.prop_id
                  GROUP BY u.name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

See SQL Fiddle with Demo

The result is same with both versions:

|  NAME |       PROP1 |       PROP2 |       PROP3 |
---------------------------------------------------
| user1 | prop1 user1 | prop2 user1 | prop3 user1 |
| user2 | prop1 user2 | prop2 user2 | prop3 user2 |