Additional select if one select result is not null - sql

Question

I am interested in the results found in table g, which shares a key, sample_name, with tables s and l. In this question the tables are
s - samples,
p - projects,
l - analyses, and
g, analysis g,
all within schema a.
In the interest of optimization, I only want to look for table g after having confirmed that l.analysis_g is NOT NULL.
Given: The only information that I start out with is the project names. The project table, p is linked with other tables by the samples table s. s is linked to every table. Table l contains types of analysis and each column is either NULL or 1.
In the example below I am trying a case but I realize this may be totally incorrect.
SELECT s.sample_name,
s.project_name,
g.*
FROM a.samples s
JOIN a.analyses l
ON s.sample_name = l.sample_name
JOIN a.analysis_g g
ON s.sample_name = g.sample_name
WHERE s.project_name IN (SELECT p.project_name
FROM a.projects p
WHERE p.project_name_other
IN ('PROJ_1',
'PROJ_2'))
;

Answer 1

Then perhaps in the where clause? It's still really hard to understand what you want . . .
SELECT s.sample_name,
s.project_name,
g.*
FROM a.samples s
JOIN a.analyses l
ON s.sample_name = l.sample_name
JOIN a.analysis_g g
ON s.sample_name = g.sample_name
WHERE s.project_name IN (SELECT p.project_name
FROM a.projects p
WHERE p.project_name_other
IN ('PROJ_1',
'PROJ_2'))
and l.analysis_g IS NOT NULL
;
As a side note, I think you could join p.project_name and avoid the where clause. AND I think you might want some inner joins -- but I'm not sure.
SELECT s.sample_name,
s.project_name,
g.*
FROM a.samples s
JOIN a.analyses l ON s.sample_name = l.sample_name
JOIN a.analysis_g g ON s.sample_name = g.sample_name
JOIN a.projects p ON s.project_name = p.project_name
WHERE p.project_name_other IN ('PROJ_1', 'PROJ_2')
and l.analysis_g IS NOT NULL

Answer 2

Again: Please show an example! We can't help if we have to guess, but I'll give it a try...
If l.analysis_g contains an ID from table g, then you can just use:
SELECT * FROM g
JOIN l on g.id = l.analysis_g
WHERE blah, blah, blah...
I removed your WHERE clause because you haven't provided enough information to allow anyone to help optimize it (if needed).