sql query to swap columns if 1 is greater than other

Question

I have 2 columns where there's a number in them if column 1 has a larger number than column 2 I want to swap the values between these 2 columns.

Is there an easy way to do this with a query?

start:

id | column 1 | column 2
------------------------
1   50         3
2   51         4

finish:

id | column 1 | column 2
------------------------
1   3          50
2   4          51

Answer 1

You can't just swap values. You need temporary variable First way:

 UPDATE table_name SET column1=column2, column2=@temp WHERE (@temp:=column1) > column2;

Second way: This way is better because it check is any value is NULL

UPDATE table_name SET column1=(@temp:=column1), column1= column2, column2 = @temp WHERE column1 > column2;

Second way is simple. It compare two cols and swap this cols if needs.

Answer 2

In an UPDATE statement MySql uses non standard SQL behavior when it comes to the use of values of the updated columns, because in a statement like this:

SET a = x, b = a

the value of a in the assignment b = a will not be the original value that is stored in the table, but the value of a after the assignment a = x .
So this:

update tablename
set column1 = column2,
    column2 = column1

that would work in other databases does not work in MySql.

There is a workaround that does not use variables or unnecessary joins:

update tablename
set column1 = column1 + column2,
    column2 = column1 - column2,
    column1 = column1 - column2
where column1 > column2;

which works since:

Single-table UPDATE assignments are generally evaluated from left to right.

(from 13.2.13 UPDATE Statement )

See the demo .
Results:

| id  | column1 | column2 |
| --- | ------- | ------- |
| 1   | 3       | 50      |
| 2   | 4       | 51      |

Answer 3

You have a primary key on the table, so a simple option is join :

update t join
       t t2
       on t.id = t2.id
    set t.x = t2.y, 
        t.y = t2.x
    where t.x > t.y;

Here is a db<>fiddle.

I prefer this method to using variables for several reasons:

1. This does not depend on the order of evaluations of the set clauses, which I don't think that MySQL guarantees.
2. I think this use of variables is deprecated in MySQL 8+
3. It does not depend on the ordering of the expressions, so it is more robust and readily adapts to more complicated expressions.