Using SELECT clause in ELSE part of SQL query? - mysql

Question

I have two tables.
Table number 1 (Lab_test): with integer columns named labtest_id (pk) and project_no. Second table (Project) has two columns, first column named project_id (int) and second column project_name (string). Also I have two parameters which are passed to the query.
I have to make a query which will extract labtest_id and proj_num. proj_num have to be obtained in following way: if Lab_test.project_no is not null, take this value, else go to referencing table Project and take project_name.
I tried with bunch of combinations using SQL CASE IF-THEN-ELSE statements, but no luck:
SELECT lab.labtest_id, proj_num,
FROM Lab_test lab
(CASE
WHEN lab.project_no IS NOT NULL THEN lab.project_no
ELSE (SELECT proj.project_name
FROM Project proj
WHERE proj.project_id = lab.project_no)
) AS proj_num
WHERE lab.status = 'DONE' AND lab.user_id = 436

Answer 1

You can use left join and coalesce() to replace the case when expression that you've used to check null
SELECT lab.labtest_id, coalesce(lab.project_no,proj.project_name) as proj_num
FROM Lab_test lab
left join Project proj on proj.p,roject_id = lab.project_no
where lab.status = 'DONE' AND lab.user_id = 436

Answer 2

if Lab_test.project_no is not null, take this value, else go to referencing table Project and take project_name.
I don't believe you. I think you have the logic backwards -- use the referenced table if it is available. Otherwise use the number.
That would suggest:
SELECT l.labtest_id, COALESCE(p.project_name, l.project_no) as proj_num
FROM Lab_test l LEFT JOIN
Project p
ON p.p,roject_id = l.project_no
WHERE l.status = 'DONE' AND l.user_id = 436;
If the project_no is a number, then you might have a type conversion problem. If that is an issue:
SELECT l.labtest_id,
COALESCE(p.project_name, CAST(l.project_no as CHAR)) as proj_num
FROM Lab_test l LEFT JOIN
Project p
ON p.p,roject_id = l.project_no
WHERE l.status = 'DONE' AND l.user_id = 436;