Combining tables in mySQL to keep nulls [duplicate] - mysql

Question

This question already has answers here:
How can I do a FULL OUTER JOIN in MySQL?
(15 answers)
Closed 4 years ago.
I have two tables below. I am looking to find all the combinations of store and newid in sales2 which are not present in sales1 and combine them with sales1.
I am looking for the end result to be:
store newid amount total
123 | 123 | 12.3 | 1
456 | 123 | 45.6 | 2
789 | adsf | 78.9 | 3
321 | 123f | NULL | NULL
789 | 1654 | NULL | NULL
Original Tables
CREATE TABLE sales1 (
id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
store VARCHAR(30) NOT NULL,
newid VARCHAR(30),
amount DOUBLE,
total INT
);
CREATE TABLE sales2 (
id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
store VARCHAR(30) NOT NULL,
newid VARCHAR(30),
amount DOUBLE
);
INSERT INTO sales1 (store, newid, amount, total) VALUES
('123','123', 12.3, 1),('456','123', 45.6, 2),('789','adsf', 78.9, 3);
INSERT INTO sales2 (store, newid, amount) VALUES
('123','123', Null),('456','123', Null),('321','123f', Null),('789','1654', Null);

Answer 1

We can use two different SELECT queries for this. First query will fetch all the rows from sales1 table.
Second would fetch the remaining rows from sales2 table, which do not exist in the sales1 table. We will use LEFT JOIN from sales2 to sales1 and consider only those rows where sales1.id is NULL after the Join.
We will eventually use UNION ALL to combine the result-set from both the queries.
Query
SELECT
s1.store, s1.newid, s1.amount, s1.total
FROM sales1 s1
UNION ALL
SELECT
s2.store, s2.newid, s2.amount, NULL AS total
FROM sales2 s2
LEFT JOIN sales1 s1
ON s2.store = s1.store AND
s2.newid = s1.newid
WHERE s1.id IS NULL;
Result
| store | newid | amount | total |
| ----- | ----- | ------ | ----- |
| 123 | 123 | 12.3 | 1 |
| 456 | 123 | 45.6 | 2 |
| 789 | adsf | 78.9 | 3 |
| 321 | 123f | | |
| 789 | 1654 | | |
View on DB Fiddle