Create columns from row value pairs in SQL

Question

Is it possible to roll up multiple rows into columns with the column name as one of the returned values without multiple joins?

Example:

Table: user

ID	email	name
1	steve@gmail.com	steve
2	jan@gmail.com	Jan

Table: meta

ID	user_id	meta_key	meta_value
100	1	key_1	value_1
101	2	key_1	value_2
102	1	key_2	value_3
103	2	key_2	value_4

Desired query result:

ID	email	name	key_1	key_2
1	steve@gmail.com	steve	value_1	value_3
2	jan@gmail.com	jan	value_2	value_4

Currently, I use multiple left joins to generate this query, but as the list of keys grows, it would be great to prevent having to use more and more left joins.

SELECT u.*, meta1.meta_value AS 'key_1', meta2.meta_value AS 'key_2'
FROM users as u
LEFT JOIN meta as meta1 ON meta1.user_id = u.ID
LEFT JOIN meta as meta2 ON meta2.user_id = u.ID
WHERE 1=1
AND meta1.meta_key = 'key_1'
AND meta2.meta_key = 'key_2'

I'd love to just be able to do something like the following which is obviously invalid in its current form

SELECT u.*, meta.meta_value as meta.meta_key
FROM users as u
LEFT JOIN meta as meta ON meta.user_id = u.ID
WHERE 1=1
AND meta.meta_key IN ('key_1', 'key_2')

My current compromise is that if the list of keys grows large, I can just return a cell that concats the key/value pairs.

For reference, this is a WordPress database.

Answer 1

You can use:

• CASE statements to extract your specific key values
• MAX aggregation to remove your NULL values that your CASE statement will generate, hence grouping on the other selected column from the " user " table

SELECT u.*,
       MAX(CASE WHEN m.meta_key = 'key_1'
                THEN m.meta_value         END) AS key_1,
       MAX(CASE WHEN m.meta_key = 'key_2'
                THEN m.meta_value         END) AS key_2
FROM       user u
INNER JOIN meta m
        ON u.ID = m.user_id
GROUP BY u.ID,
         u.email,
         u.name

Check the demo here .