PostgreSQL query returning multiple rows instead of one

Question

I have two tables: user and projects , with a one-to-many relationship between two.
projects table has field status with project statuses of the user.

status can be one of:

launched, confirm, staffed, overdue, complete, failed, ended

I want to categorize users in two categories:

1. users having projects in launched phase
2. users having projects other than launched status.

I am using the following query:

SELECT DISTINCT(u.*), CASE 
    WHEN p.status = 'LAUNCHED' THEN 1
    ELSE 2
    END as user_category
FROM users u
LEFT JOIN projects p ON p.user_id = u.id
WHERE (LOWER(u.username) like '%%%'
    OR LOWER(u.personal_intro) like '%%%'
    OR LOWER(u.location) like '%%%'
    OR u.account_status != 'DELETED'
AND system_role=10 AND u.account_status ='ACTIVE')
ORDER BY set_order, u.page_hits DESC
LIMIT 10
OFFSET 0

I am facing duplicate records for following scenario:

If user has projects with status launched as well as overdue , complete or failed , then that user is recorded two times as both the conditions in CASE are satisfying for that user.

Please suggest a query where a user that has any project in launched status gets his user_category set to 1 . The same user should not be repeated for user_category 2 .

Answer 1

The query is probably not doing what you think it does for a number of reasons

• There is DISTINCT and there is DISTINCT ON (col1, col2) .
  DISTINCT (u.*) is no different from DISTINCT u.* . The parentheses are just noise.

• AND binds before OR according to operator precedence . I suspect you want to use parentheses around the conditions OR 'ed together? Or do you need it the way it is? But you don't need parentheses around the whole WHERE clause in any case.

• Your expression LOWER(u.username) LIKE '%%%' doesn't make any sense. Every non-null string qualifies. Can be replaced with u.username IS NOT NULL . I suspect you want something different?

• Postgres is case sensitive in string handling. You write of status being 'launched' etc. but use 'LAUNCHED' in your query. Which is it?

• A couple of table qualifications are missing from the question making it ambiguous for the reader. I filled in as I saw fit.

Everything put together, it might work like this:

SELECT DISTINCT ON (u.set_order, u.page_hits, u.id)
       u.*
     , CASE WHEN p.status = 'LAUNCHED' THEN 1 ELSE 2 END AS user_category
FROM   users         u
LEFT   JOIN projects p ON p.user_id = u.id
WHERE  LOWER(u.username)       LIKE '%%%' -- ???
    OR LOWER(u.personal_intro) LIKE '%%%'
    OR LOWER(u.location)       LIKE '%%%'

    OR u.account_status != 'DELETED'      -- with original logic
   AND u.system_role = 10
   AND u.account_status = 'ACTIVE'
ORDER  BY u.set_order, u.page_hits DESC, u.id, user_category
LIMIT  10

Detailed explanation in this related question:

• Select first row in each GROUP BY group?

Two EXISTS semi-joins instead of the DISTINCT ON and CASE might be faster:

SELECT u.*
     , CASE WHEN EXISTS (
          SELECT FROM projects p
          WHERE p.user_id = u.id AND p.status = 'LAUNCHED')
       THEN 1 ELSE 2 END AS user_category
FROM   users u
WHERE
    (  LOWER(u.username)       LIKE '%%%' -- ???
    OR LOWER(u.personal_intro) LIKE '%%%'
    OR LOWER(u.location)       LIKE '%%%'
    OR u.account_status != 'DELETED'      -- with alternative logic?
    )
AND    u.system_role = 10  -- assuming it comes from users ???
AND    u.account_status = 'ACTIVE'
AND    EXISTS (SELECT 1 FROM projects p WHERE p.user_id = u.id)
ORDER  BY u.set_order, u.page_hits DESC
LIMIT  10;

Answer 2

You can use MIN() on your CASE result, and it seems dropping the DISTINCT would be a wise choice:

SELECT u.*, MIN(CASE 
WHEN p.status = 'LAUNCHED' THEN 1
ELSE 2
END) as user_category
...
GROUP BY <list all columns in the users table>
...

Since "launched" gives a 1, using MIN() will not only force a single result but will also give preference to "launched" over the other states.