Question

I have data stored in a MySQL database according to the Entity-Attribute-Value pattern (EAV), specifically user profile values from Drupal 6. I would need an SQL query or view to get the data as a normal relational table. The tables have the following layout:

Table: users
user_id   username 
---------------------
1         steve
2         michelle


Table: profile_fields
field_id   field_name
------------------------
1          first_name
2          last_name


Table: profile_values
field_id   user_id   value
---------------------------
1          1         Steve
2          1         Smith
1          2         Michelle
2          2         Addams

And I would need to somehow get the following result from a query:

user_id    first_name    last_name
-----------------------------------
1          Steve         Smith
2          Michelle      Addams

I have understood this is impossible to do in a single SQL query in the general case. But this is not the general case, and I have two advantages:

• I know the content of the “profile_fields” table, and I am 100% sure that this data will not change for the time period that this query will be used.
• It doesn’t have to be in a single query – it can be a query, some PHP code to analyze the results and then another query.

Answer 1

This can be done in a sql query using columnar subqueries as follows:

SELECT 
  u.user_id, 
  (select value from profile_values f1 WHERE f1.field_id=1 and u.user_id=f1.user_id) AS first_name, 
  (select value from profile_values f2 WHERE f2.field_id=2 and u.user_id=f2.user_id) AS last_name
  FROM users u