Conditional table updates - mysql

Question

Consider the following table.
myTable
+----+-----------+------------------------------------+
| Id | responseA | responseB |
+----+-----------+------------------------------------+
| 1 | | {"foo":"bar","lvl2":{"key":"val"}} |
+----+-----------+------------------------------------+
where:
Id, INT (11) PRIMARY
responseA, TEXT utf8_unicode_ci
responseB, TEXT utf8_unicode_ci
Let's say that I want to conditionally update the table with some outside data. The conditions are:
• if there's nothing in responseA, populate it with the outside data, otherwise
• if there is something in responseA, leave it as it is, and populate responseB with the outside data
I was pretty much convinced that I could just do this to get what I want:
UPDATE myTable
SET
responseA = IF(TRIM(responseA) = '','foo',TRIM(responseA)),
responseB = IF(TRIM(responseA) != '','foo',TRIM(responseB))
WHERE Id = 1
However, this updates both responseA and responseB to the same value - foo, making the table:
myTable
+----+-----------+-----------+
| Id | responseA | responseB |
+----+-----------+-----------+
| 1 | foo | foo |
+----+-----------+-----------+
I was expecting my table to look like this after the update:
myTable
+----+-----------+------------------------------------+
| Id | responseA | responseB |
+----+-----------+------------------------------------+
| 1 | foo | {"foo":"bar","lvl2":{"key":"val"}} |
+----+-----------+------------------------------------+
What am I misunderstanding, and how can I achieve this conditional update? Do the updates happen sequentially? If so, I guess that would explain why both of the fields are updated.

Answer 1

UPDATE TABLE
SET responseA = CASE WHEN responseA IS NULL
THEN #data
ELSE responseA
END,
responseB = CASE WHEN responseA IS NULL
THEN responseB
ELSE #data
END
;

Answer 2

here your changed query
UPDATE myTable
SET
responseB = IF(TRIM(responseA) != '','foo',TRIM(responseB)),
responseA = IF(TRIM(responseA) = '','foo',TRIM(responseA))
WHERE Id = 1

Answer 3

It seems the value of responseA is changed before the IF() for responseB is evaluated.
One possible solution is to do a simple UPDATE:
UPDATE mytable SET responseA = ? WHERE id = 1
Then adjust the columns in a trigger, where you have access to both the original and the new value of the columns:
CREATE TRIGGER t BEFORE UPDATE ON mytable
FOR EACH ROW BEGIN
IF TRIM(OLD.responseA) != '' THEN
SET NEW.responseB = NEW.responseA;
SET NEW.responseA = OLD.responseA;
END IF;
END
(I have not tested this.)
I am also assuming that your test for '' (empty string) instead of NULL is deliberate, and that you know that NULL is not the same as ''.

Answer 4

The key point in the UPDATE statement is that you should update first the column responseB, so that column responseA retains its original value which can be checked again when you try to update it:
UPDATE myTable
SET responseB = CASE WHEN TRIM(responseA) = '' THEN responseB ELSE 'foo' END,
responseA = CASE WHEN TRIM(responseA) = '' THEN 'foo' ELSE responseA END
WHERE Id = 1;