How do I go about checking the existence of records in all tables JOIN`ed in a statement? - sql

Question

I want ensure none of the tables included through INNER JOIN return 0 rows, resulting in an empty result-set due to the join being INNER. I want to be able to return information to the user about which table had no records, causing the query to return empty.
PS: Please ignore Foreign Keys for this question.
Example-query:
SELECT * FROM Person P
INNER JOIN [User] U ON U.PersonId = P.Id
INNER JOIN Email E ON E.UserId = U.Id
INNER JOIN Company C on C.Id = U.CompanyId
WHERE P.Id = 3
Let's say the user had no associated Email-records. I then want to be able to tell the user about this. Note: I only need to print the first failing step, I.E if the user has no Email and no Company, I only need to tell the user about him not having any Email.
How I would have solved it earlier:
I would perform the complete query in steps, building it up join by join, doing a whole lot of redundant querying. I really don't like this solution at all, which is why I'm asking for help.
-- Ensure Person exists
IF NOT EXISTS (
SELECT * FROM Person P
WHERE P.Id = 3
)
BEGIN
PRINT 'No associated Person was found';
RETURN 1;
END
-- Ensure User exists
IF NOT EXISTS (
SELECT * FROM Person P
INNER JOIN [User] U ON U.PersonId = P.Id
WHERE P.Id = 3
)
BEGIN
PRINT 'No associated User was found';
RETURN 1;
END
And so on. How can I write this more concise, in a way that solves the same problem but avoids repeating queries?
Thanks in advance.
Update
By looking at the answers, I realized my example was bad. Using LEFT JOIN is ok in this example, since the "join-chain" is not straight; Person joins User, then User joins in multiple directions, like this:
Person - User - Email
|
Company
I'll try to provide a different example:
SELECT * FROM a
INNER JOIN b ON b.a_id = a.id
INNER JOIN c ON c.b_id = b.id
INNER JOIN d ON d.c_id = c.id
INNER JOIN e ON e.d_id = d.id
INNER JOIN f ON f.e_id = e.id
INNER JOIN g ON g.f_id = f.id
The join-chain would then look like this:
a - b - c - d - e - f - g
If I were to use LEFT JOIN's to ensure existence of records, the IIF/CASE statements would get pretty terrible. Example:
SELECT
CASE WHEN b.id is null THEN 'b is null' END as b_is_null
CASE WHEN b.id is not null and c.id is null THEN 'b is null' END as c_is_null
..
CASE WHEN b.id is not null and c.id is not null and d.id is not null and e.id is not null and f.id is not null and g.id is null THEN 'g is null' AS g_is_null
FROM a
LEFT JOIN b ON b.a_id = a.id
LEFT JOIN c ON c.b_id = b.id
LEFT JOIN d ON d.c_id = c.id
LEFT JOIN e ON e.d_id = d.id
LEFT JOIN f ON f.e_id = e.id
LEFT JOIN g ON g.f_id = f.id
It can get really ugly. And these examples are with 1-character alias-names and 2-character property-names.
Keep in mind, I also want to check if the first table (Person/a, the one not joined) also returns rows.

Answer 1

You need to use left join. From your query, if a person doesn't have any user they obviously doesn't have any email or company. So all left join will be fine.
SELECT P.*,
IIF(U.PersonID IS NULL, 0, 1) AS isUserExists,
IIF(E.UserId IS NULL, 0, 1) AS isEmailExists,
IIF(C.Id IS NULL, 0, 1) AS isCompanyInformationExists
FROM Person P
LEFT JOIN [User] U ON U.PersonId = P.Id
LEFT JOIN Email E ON E.UserId = U.Id
LEFT JOIN Company C on C.Id = U.CompanyId
WHERE P.Id = 3
If U.PersonID IS NULL then no user existed, if E.UserId IS NULL then no email existed, if C.Id IS NULL then no company information existed for that person. By this you can use your messages as you need.

Answer 2

Use case statements like below and print your message.
select
case when u.PersonId is null and p.id is not null then 'user not exists'
WHEN u.PersonId IS NOT NULL AND p.id IS NOT NULL AND e.mailid is null THEN 'mail id not exist'
else 'No associated Person was found' end message
FROM Person P
left JOIN [User] U ON U.PersonId = P.Id
left JOIN Email E ON E.UserId = U.Id
left JOIN Company C on C.Id = U.CompanyId
WHERE P.Id = 3

Answer 3

DECLARE #Email VARCHAR(50)
DECLARE #Company VARCHAR(50)
SELECT #Email = E.Email, #Company = C.CompanyName FROM Person P
INNER JOIN [User] U ON U.PersonId = P.Id
LEFT JOIN Email E ON E.UserId = U.Id
LEFT JOIN Company C on C.Id = U.CompanyId
WHERE P.Id = 3
IF #Email IS NULL
Print 'NO Email'
IF #Company IS NULL
Print 'No Company'