If null in one table select from Other - sql

Question

I have 2 tables inquiry and Application table I want to build a view that contains the user information based on data from these two tables.
Example
I Have Table Inquiry
FirstName, LastName, Address, email
I have table Application
FirstName, LastName, Address, email
Am querying the tables using the email field, This is what i want,
SELECT FirstName From InquiryTBL where email = #Email
If Null Select FirstName From ApplictionTBL where email = #email
This is kinda what I have been trying
SELECT
CASE
WHEN a.Email = null
THEN (SELECT FirstName from dbo.Inquiry_Tbl where email = #Email)
ELSE a.FirstName
END As [FirstName],

Answer 1

If email is in both tables, you can JOIN on that field and then use COALESCE to pull non-null data:
SELECT
Email = COALESCE( i.Email, a.Email)
, FirstName = COALESCE(i.FirstName, a.FirstName)
, LastName = COALESCE(i.LastName, a.LastNamej)
FROM InquiryTBL i
LEFT JOIN ApplicationTBL a
ON i.Email = a.Email

Answer 2

If the email is in both tables or in only one table, use a FULL OUTER JOIN on email inside a sub-select. Then you can filter that sub-select by the email address you need.
DECLARE #email varchar(20) = 'ted#excellent.com' ;
SELECT s1.FirsTName, s1.LastName, s1.Email
FROM (
SELECT COALESCE(i.FirstName, a.FirstName) AS FirstName
, COALESCE(i.LastName, a.LastName) AS LastName
, COALESCE(i.email, a.email) AS email
FROM InquiryTBL i
FULL OUTER JOIN ApplicationTBL a ON i.email = a.email
) s1
WHERE s1.Email = #email
;
https://dbfiddle.uk/?rdbms=sqlserver_2012&fiddle=62d0f0a1026150de6b4fd9be3d058dbb
NOTE: To pick the first non-null value, you can use either coalesce() or isnull(). coalesce() is ANSI-compliant, can take more than 2 arguments and can be used in most databases. isnull() is primarily a Microsoft T-SQL function and takes only 2 arguments. It can be quicker in some cases than coalesce(), but it's not as portable. Both will also pick the first non-null value in your selected order, so you can change which value you want first.