Preferential Select Query - mysql

Question

The issue that we are trying to tackle is best shown with the following illustrative example:
CREATE TABLE table_1
(
id INT UNSIGNED AUTO_INCREMENT,
colA INT,
colB VARCHAR(10),
PRIMARY KEY(id)
);
CREATE TABLE table_2
(
id INT UNSIGNED AUTO_INCREMENT,
colY INT,
colZ VARCHAR(10),
PRIMARY KEY(id)
);
INSERT INTO table_1(colA, colB) VALUES(1, 'NPD5A6V9EI'), (2, 'ISO4IK42YQ'), (4, 'J12QAN4O42'), (6,'V8YTZFHCU4');
INSERT INTO table_2(colY, colZ) VALUES(3, 'RBUNWLO753'), (4, 'X2BCEY7O8B'), (5, 'BNUS7R4225'), (6, '72NOWCTH5G');
We would like to select our result based on the value of colA in table_1 but if that does not return a result , we would like to return our result based on the value of colY in table_2. In other words SELECTing from table_2 is the backup for SELECTing from table_1. The query returns NULL only if neither table satisfies the condition.
A pseudo SQL query could be:
SELECT colB FROM table_1 where colA = 3 OR SELECT colZ FROM table_2 where colY = 3;
The query should return output based on the following I/O table:
I O
= =
1 NPD5A6V9EI -- From table_1
2 ISO4IK42YQ -- From table_1
3 RBUNWLO753 -- From table_2
4 J12QAN4O42 -- From table_1 (has precedence over table_2 entry)
5 BNUS7R4225 -- From table_2
6 V8YTZFHCU4 -- From table_1 (has precedence over table_2 entry)
9 NULL
Kindly suggest solutions that:
make use of the latest DB features (for posterity)
work with MySQL version 5.6.51 (for our application)

Answer 1

Write a subquery that generates all the I rows that you want.
Then left join this with the two tables, and use IFNULL to take the matching value from table_1 in preference to table_2.
SELECT ids.id AS I, IFNULL(t1.colB, t2.colZ) AS O
FROM (SELECT 1 AS id UNION ALL SELECT 2 UNION ALL SELECT 3 ... UNION ALL SELECT 9) AS ids
LEFT JOIN table_1 AS t1 ON t1.colA = ids.id
LEFT JOIN table_2 AS t2 ON t2.colY = ids.id
ORDER BY ids.id

Answer 2

I simply don't kn ow where you get your last row.
also with Myql 8 you can ise the window function ROW_NUMBER
the rest is self explantory, the sorting comes from colA and Col1, when there are teh same numbers the second column orderby2 comes and sorts first for the first table
CREATE TABLE table_1
(
id INT UNSIGNED AUTO_INCREMENT,
colA INT,
colB VARCHAR(10),
PRIMARY KEY(id)
);
CREATE TABLE table_2
(
id INT UNSIGNED AUTO_INCREMENT,
colY INT,
colZ VARCHAR(10),
PRIMARY KEY(id)
);
INSERT INTO table_1(colA, colB) VALUES(1, 'NPD5A6V9EI'), (2, 'ISO4IK42YQ'), (4, 'J12QAN4O42'), (6,'V8YTZFHCU4');
INSERT INTO table_2(colY, colZ) VALUES(3, 'RBUNWLO753'), (4, 'X2BCEY7O8B'), (5, 'BNUS7R4225'), (6, '72NOWCTH5G');
SELECT #i := #i +1 AS I,
colB AS O
FROM
(SELECT colA as orderby1,colB,1 ordberby2 froM table_1
UNION
SELECT colY, colZ,2 froM table_2 ) t1,(SELECT #i := 0) t2
ORDER BY orderby1,ordberby2
I | O
-: | :---------
1 | NPD5A6V9EI
2 | ISO4IK42YQ
3 | RBUNWLO753
4 | J12QAN4O42
5 | X2BCEY7O8B
6 | BNUS7R4225
7 | V8YTZFHCU4
8 | 72NOWCTH5G
db<>fiddle here