Unable to access column alias in WHERE clause - sql

Question

I am trying to filter rows from my Postgres database with below query. Everything works fine but when I try to check if latesttask column is null or has some value then it shows error:
error: column "latesttask" does not exist
SELECT *,
(
SELECT
JSON_BUILD_OBJECT(
'id', taskhistories.id, 'task', taskhistories.task,
'taskname', t.name, 'project', taskhistories.project,
'projectname', p.name, 'started_at',
taskhistories.started_at, 'stopped_at',
taskhistories.stopped_at
)
FROM
tasks AS t,
projects AS p,
latesttasks,
taskhistories
WHERE
taskhistories.user = users.id
AND latesttasks.task = t.id
AND latesttasks.project = p.id
AND taskhistories.id = latesttasks.taskhistory
AND (
LOWER(t.name) LIKE '%we%'
OR LOWER(p.name) LIKE '%we%'
)
) as latestttask
FROM
users
WHERE
(
latesttask IS NULL
AND (
LOWER(name) LIKE '%we%'
OR LOWER(email) LIKE '%we%'
)
OR latesttask IS NOT NULL
)

Answer 1

One "t" to many in your column alias latestttask.
But, more importantly, you cannot reference output column names in the WHERE clause anyway. There would have to be a column users.latesttask to make the WHERE clause work.
This would be a working equivalent with a LEFT JOIN to a LATERAL subquery:
SELECT *
FROM users u
LEFT JOIN LATERAL (
SELECT json_build_object(
'id', h.id, 'task', h.task,
'taskname', t.name, 'project', h.project,
'projectname', p.name, 'started_at', h.started_at,
'stopped_at', h.stopped_at) AS latesttask
FROM tasks t
JOIN latesttasks l ON l.task = t.id
JOIN projects p ON p.id = l.project
JOIN taskhistories h ON h.id = l.taskhistory
WHERE h.user = u.id
AND (lower(t.name) LIKE '%we%'
OR lower(p.name) LIKE '%we%')
) l ON true
WHERE l.latesttask IS NOT NULL
OR lower(u.name) LIKE '%we%'
OR lower(u.email) LIKE '%we%';

Answer 2

Please try with...
Select * from (SELECT
*,
(
SELECT
JSON_BUILD_OBJECT(
'id', taskhistories.id, 'task', taskhistories.task,
'taskname', t.name, 'project', taskhistories.project,
'projectname', p.name, 'started_at',
taskhistories.started_at, 'stopped_at',
taskhistories.stopped_at
)
FROM
tasks AS t,
projects AS p,
latesttasks,
taskhistories
WHERE
taskhistories.user = users.id
AND latesttasks.task = t.id
AND latesttasks.project = p.id
AND taskhistories.id = latesttasks.taskhistory
AND (
LOWER(t.name) LIKE '%we%'
OR LOWER(p.name) LIKE '%we%'
)
) as latesttask
FROM
users ) a
WHERE
(
latesttask IS NULL
AND (
LOWER(name) LIKE '%we%'
OR LOWER(email) LIKE '%we%'
)
OR latesttask IS NOT NULL
)