SQL - How to transpose?

Question

I have something similar to the following table:

================================================
| Id | UserId | FieldName     | FieldValue     |
=====+========+===============+================|
| 1  | 100    | Username      | John Doe       |
|----+--------+---------------+----------------|
| 2  | 100    | Password      | pass123!       |
|----+--------+---------------+----------------|
| 3  | 102    | Username      | Jane           |
|----+--------+---------------+----------------|
| 4  | 102    | Password      | $ecret         |
|----+--------+---------------+----------------|
| 5  | 102    | Email Address | [email protected] |
------------------------------------------------

I need a query that will give me a result like this:

==================================================
| UserId | Username  | Password | Email Address  |
=========+===========+===========================|
| 100    | John Doe  | pass123! |                |
|--------+-----------+----------+----------------|
| 102    | Jane      | $ecret   | [email protected] |
|--------+-----------+----------+----------------|

Note that the values in FieldName are not limited to Username, Password, and Email Address. They can be anything as they are user defined.

Is there a way to do this in SQL?

Answer 1

MySQL doesn't support ANSI PIVOT/UNPIVOT syntax, so that leave you to use:

  SELECT t.userid
         MAX(CASE WHEN t.fieldname = 'Username' THEN t.fieldvalue ELSE NULL END) AS Username,
         MAX(CASE WHEN t.fieldname = 'Password' THEN t.fieldvalue ELSE NULL END) AS Password,
         MAX(CASE WHEN t.fieldname = 'Email Address' THEN t.fieldvalue ELSE NULL END) AS Email
    FROM TABLE t
GROUP BY t.userid

As you can see, the CASE statements need to be defined per value. To make this dynamic, you'd need to use MySQL's Prepared Statement (dynamic SQL) syntax.