SQL UNION but fill in NULL value - sql

Question

I have two tables and a section of Table1 looks something like this
EmployeeID
FirstName
LastName
Gender
Age
A100
Bob
Odenkirk
Male
30
A101
Jon
Jones
NULL
36
Table2 looks similar but contains a duplicate entry with some varying amount of missing/available information, i.e.,
EmployeeID
FirstName
LastName
Gender
Age
A101
Jon
Jones
Male
NULL
A103
Angelina
Jolie
Female
40
I'm fairly new to SQL and I initially tried
SELECT *
FROM Table1
UNION
SELECT *
FROM Table2
But obviously, the A101 row has different NULL values so it doesn't get treated as duplicates and I get:
EmployeeID
FirstName
LastName
Gender
Age
A100
Bob
Odenkirk
Male
30
A101
Jon
Jones
NULL
36
A101
Jon
Jones
Male
NULL
A103
Angelina
Jolie
Female
40
Is there a general way (i.e. if the table is large and not sure which values might be missing from Table1 or Table2) to somehow "fill in" the NULL values and get the following target output:
EmployeeID
FirstName
LastName
Gender
Age
A100
Bob
Odenkirk
Male
36
A101
Jon
Jones
Male
30
A103
Angelina
Jolie
Female
40

Answer 1

Assuming you can never have "competing" values (i.e., both tables either have the same value or one of them has a null), you could union all the two queries, and then group by the EmployeeID and use max to get the present values and ignore nulls:
SELECT EmployeeID, MAX(FirstName), MAX(LastName), MAX(Gender), MAX(Age)
FROM (SELECT EmployeeID, FirstName, LastName, Gender, Age
FROM Table1
UNION ALL
SELECT EmployeeID, FirstName, LastName, Gender, Age
FROM Table2) T
GROUP BY EmployeeID

Answer 2

You could try to use FULL OUTER JOIN and COALESCE function
SELECT COALESCE(t1.EmployeeID, t2.EmployeeID),
COALESCE(t1.FirstName, t2.FirstName) AS FirstName,
COALESCE(t1.LastName, t2.LastName) AS LastName,
COALESCE(t1.Gender, t2.Gender) AS Gender,
COALESCE(t1.Age, t2.Age) AS Age,
FROM Table1 t1 FULL OUTER JOIN Table2 t2 ON t1.EmployeeID = t2.EmployeeID
If your DBMS does not support FULL OUTER JOIN, you could try to use UNION and COALESE function.
-- Retrieve all employees in Table1, then fill in NULL value using COALESCE function if it's exists in Table2
SELECT t1.EmployeeID,
COALESCE(t1.FirstName, t2.FirstName) AS FirstName,
COALESCE(t1.LastName, t2.LastName) AS LastName,
COALESCE(t1.Gender, t2.Gender) AS Gender,
COALESCE(t1.Age, t2.Age) AS Age,
FROM Table1 t1 LEFT JOIN Table2 t2 ON t1.EmployeeID = t2.EmployeeID
UNION ALL
-- Retrieve the rest employees, which is existed in Table 2, but is not existed in Table1
SELECT *
FROM Table2 t2
WHERE NOT EXISTS
(SELECT 1
FROM Table1 t1
WHERE t1.EmployeeID = t2.EmployeeID)