mysql test IF variable IS NULL (or empty) - mysql

Question

we are selecting a row in mysql/mariadb and storing chosen columns in variables. After this, using IF we would like to test one of the variables and see if it has been set or is null, if it is then we assign it a value and continue on.
Using IS NULL does not seem to work on a non expression.
select id,history,active,jsonorder
INTO #id,#history,#active,#jsonorder
from myTable where uid = myUid
delimiter |
IF #jsonorder IS NULL THEN
#myNewVal="zzz";
ELSE
#myNewVal="yyy";
END IF|
insert into otherTable (colA) VALUES (#myNewVar);
What is the correct way to test if the select has provided a value into a variable such as #jsonorder?

Answer 1

We could use an expression in the SELECT list:
SELECT t.id
, t.history
, t.active
, IFNULL(t.order,1) AS `order`
INTO #id
, #history
, #active
, #order
FROM `myTable` t
WHERE t.uid = ...
Note that DELIMITER is not a SQL statement; it's command recognized by the mysql command line client (and some other clients).
documented here in MySQL Reference Manual: https://dev.mysql.com/doc/refman/8.0/en/mysql-commands.html
It's not clear in what context this script is being run; is this part of a MySQL stored program, or being called application code. What we are actually trying to achieve?
There are several other expressions we could use in the SELECT list, e.g.
CASE WHEN t.order IS NULL THEN 1 ELSE t.order END`
or
IF(t.order IS NULL,1,t.order)
et al.
FOLLOWUP
If we don't want to modify the original SQL statement; if we execute this:
SELECT t.id
, t.history
, t.active
, t.jsonorder
INTO #id
, #history
, #active
, #jsonorder
FROM `myTable` t
WHERE t.uid = ...
And then we want to perform an assignment to another user defined variable, based on a conditional test, we can achieve that in another SELECT or a SET statement.
For example:
SELECT #myNewVal := IF(#jsonorder IS NULL,'zzz','yyy') ;
ELSE 'yyy'
-or-
SELECT CASE
WHEN #jsonorder IS NULL
THEN 'zzz'
ELSE 'yyy'
END
INTO #myNewVal
-or-
SET #myNewVal := IF(#jsonorder IS NULL,'zzz','yyy')

Answer 2

The IF statement can only be used in Stored Routines. The IF function can be used virtually anywhere an expression can be put:
select id,history,active,jsonorder
INTO #id,#history,#active,#jsonorder
from myTable
where uid = myUid;
insert into otherTable (colA)
VALUES (IF(#jsonorder IS NULL, "zzz", "yyy"));
Where does myUid come from?
Is otherTable only one column wide? Or do all the other columns have defaults?
(I'm worried that you over-sanitized the question.)

Answer 3

This solve for cases where order is null, but not for when myUid doesnt exist in your table
SELECT id,history,active, COALESCE(order,1)
INTO #id,#history,#active,#order
FROM myTable
WHERE uid = myUid