Update one column value with another column value using SQL Query - sql

Question

I have a table which has three columns with integer values. In first two columns I have some values. Now I need to update the third column with the value which is minimum of other two columns.
I have tried using
update table set col3 = (SELECT CASE WHEN (col2 is null OR col1 < col2 )
THEN col1
ELSE col2
END AS col3
FROM table
WHERE col1 is not null or col2 is not null)`.
But I am getting an error like below:
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Answer 1

You can use LEAST in order to update:
UPDATE mytable
SET col3 = LEAST(COALESCE(col1, col2), COALESCE(col2, col1))
WHERE col1 IS NOT NULL OR col2 IS NOT NULL
You don't have to query your table table to get col2, col1 values. You can use them directly in the SET part of your UPDATE query.
Note: COALESCE is used to handle NULL values. If both col1, col2 are NULL then col3 is also set to NULL.
In SQL Server you can use:
UPDATE mytable
SET col3 = CASE
WHEN col2 IS NULL OR col1 < col2 THEN col1
ELSE col2
END
WHERE col1 IS NOT NULL OR col2 IS NOT NULL
In SQL Server 2012, or later, you can use IIF:
UPDATE mytable
SET col3 = IIF(col2 IS NULL OR col1 < col2, col1, col2)
WHERE col1 IS NOT NULL OR col2 IS NOT NULL
Demo here

Answer 2

You don't need to join to the table it self, you can use the data from the columns in the same query
update table
set col3 = CASE WHEN (col2 is null OR col1 < col2 )
THEN col1 ELSE col2 END
WHERE col1 is not null or col2 is not null
The problem with your query is that you are using a correlated query, that selects col1 and col2(which is basically fine but unnecessary) and you are not filtering the results(which col1,col2 to take?) so for each col3 you want to update, you have a 'bunch' of other values, when you can only have 1. If you want your query to work all you have to do is add a relation condition.