coalesce on missing fields - sql

Question

any idea why COALESCE is not returning the string 'emptyfield' at all?
SELECT
userid,
string_agg(COALESCE(configvalue, 'emptyfield'), '|')
FROM
oc_preferences
WHERE
configkey IN(
'email',
'quota',
'lastLogin',
'displayName'
)
GROUP BY
userid;
i'm getting mixed field order and i'm also missing NULL values
480f0c81-8090aa8f|1 GB|John Smith|john.smith#e-mail.com|1551376267
9094f888-aa4ef8ef|peter.calo#domain.com|1 GB|Peter Calo
34555345-76867888|Mary Aston|2 GB
but i expected something like
480f0c81-8090aa8f|1 GB|John Smith|john.smith#e-mail.com|1551376267
9094f888-aa4ef8ef|1 GB|Peter Calo|peter.calo#domain.com|emptyfield
34555345-76867888|2 GB|Mary Aston|emptyfield |emptyfield
the table looks like this
SELECT *
FROM oc_preferences
ORDER by userid
FETCH first 5 rows only
userid|appid|configkey|configvalue
480f0c81-8090aa8f|avatar|generated|true
480f0c81-8090aa8f|files|quota|1 GB
480f0c81-8090aa8f|settings|email|john.smith#e-mail.com
480f0c81-8090aa8f|user_ldap|displayName|John Smith
480f0c81-8090aa8f|user_ldap|homePath|
live example: https://www.db-fiddle.com/f/2rNofuyfKv3j3YLyRctE7U/1

Answer 1

Applying a custom order before aggregating may help with the mixed field values. I assume that for every user that you will have all configkeys even if their values are empty in this table.
See fiddle below:
Schema (PostgreSQL v11)
CREATE TABLE oc_preferences (
userid VARCHAR(17),
appid VARCHAR(9),
configkey VARCHAR(11),
configvalue VARCHAR(21)
);
INSERT INTO oc_preferences
(userid, appid, configkey, configvalue)
VALUES
('480f0c81-8090aa8f', 'avatar', 'generated', 'true'),
('480f0c81-8090aa8f', 'files', 'quota', '1 GB'),
('480f0c81-8090aa8f', 'settings', 'email', 'john.smith#e-mail.com'),
('480f0c81-8090aa8f', 'user_ldap', 'displayName', 'John Smith'),
('480f0c81-8090aa8f', 'user_ldap', 'homePath', '');
Query #1
WITH oc_preferences_sorted AS (
SELECT
userid,configvalue,
CASE
WHEN configkey='quota' THEN 1
WHEN configkey='displayName' THEN 2
WHEN configkey='email' THEN 3
WHEN configkey='lastLogin' THEN 4
END as custom_order
FROM
oc_preferences
WHERE
configkey IN(
'email',
'quota',
'lastLogin',
'displayName'
)
ORDER BY 3
)
SELECT
userid,
string_agg(COALESCE(configvalue, 'emptyfield'), '|')
FROM
oc_preferences_sorted
GROUP BY
userid;
| userid | string_agg |
| ----------------- | ------------------------------------- |
| 480f0c81-8090aa8f | 1 GB|John Smith|john.smith#e-mail.com |
COALESCE will not return the string 'emptyfield' if a record with that configkey does not exist as there is nothing to aggregate as shown in the example above.
The following query creates all possible values and continues with your approach to create the concatenated field
WITH user_detaults AS (
SELECT
userid,
configkey
FROM
(SELECT DISTINCT userid from oc_preferences) users
INNER JOIN
(
SELECT 'quota' as configkey UNION ALL
SELECT 'displayName' as configkey UNION ALL
SELECT 'email' as configkey UNION ALL
SELECT 'lastLogin' as configkey
) keys ON 1=1
),
oc_preferences_sorted AS (
SELECT
u.userid,op.configvalue,
CASE
WHEN u.configkey='quota' THEN 1
WHEN u.configkey='displayName' THEN 2
WHEN u.configkey='email' THEN 3
WHEN u.configkey='lastLogin' THEN 4
END as custom_order
FROM
oc_preferences op
RIGHT JOIN
user_detaults u on u.userid = op.userid AND
u.configkey = op.configkey
ORDER BY 3
)
SELECT
userid,
string_agg(COALESCE(configvalue, 'emptyfield'), '|')
FROM
oc_preferences_sorted
GROUP BY
userid;
| userid | string_agg |
| ----------------- | ------------------------------------------------ |
| 480f0c81-8090aa8f | 1 GB|John Smith|john.smith#e-mail.com|emptyfield |
View on DB Fiddle
Recommended Edit
WITH user_detaults AS (
SELECT
userid,
configkey
FROM
(SELECT DISTINCT userid from oc_preferences) users
CROSS JOIN
(
SELECT 'quota' as configkey UNION ALL
SELECT 'displayName' as configkey UNION ALL
SELECT 'email' as configkey UNION ALL
SELECT 'lastLogin' as configkey
) keys
),
oc_preferences_sorted AS (
SELECT
u.userid,op.configvalue,
CASE
WHEN u.configkey='quota' THEN 1
WHEN u.configkey='displayName' THEN 2
WHEN u.configkey='email' THEN 3
WHEN u.configkey='lastLogin' THEN 4
END as custom_order
FROM
oc_preferences op
RIGHT JOIN
user_detaults u on u.userid = op.userid AND
u.configkey = op.configkey
)
SELECT
userid,
string_agg(COALESCE(configvalue, 'emptyfield') , '|' ORDER BY custom_order )
FROM
oc_preferences_sorted
GROUP BY
userid;
DB Fiddle