SQL Select Column From Table Based on Another Select Statement

Question

I have a table containing column names from another table. I want to run an update statement to update some values from that table, based off of the other.

EX:

TableA 
ID|Column1|Column2
1 |  1.3  |  2.3
2 |  0    |   7
3 |  2.5  |  12.1

TableB
ID|ColumnName|MaxValue
1 | Column1  | NULL
2 | Column2  | NULL

Something along the lines of this:

So in this case, I would want to update MaxValue in TableB to be the max value from TableA where ColumnName is a colum in TableA.

Is this possible?

Answer 1

You can do it with a cursor and some dynamic sql. This isn't the best thing to do but if you needed a quick and dirty solution here you go:

 DECLARE @colName VARCHAR(50), @str VARCHAR(2000), @id int
 DECLARE c CURSOR FOR       
    SELECT id, columnName
    FROM tableB
OPEN c
FETCH NEXT FROM c INTO @id, @columnName
WHILE @@fetch_status = 0 
BEGIN
    SET @str = 'update tableB set MaxValue = ( select max(' + @colName + ') from 
                    tableA ) where id = ' + CONVERT(VARCHAR, @id)
    EXEC ( @str ) 
FETCH NEXT FROM c INTO @id, @columnName
END
CLOSE c
DEALLOCATE c

Answer 2

If you do not want to use dynamic SQL, you could always do something like this

Update TableB
Set MaxValue = MaxValues.MaxValue
From TableB
    Join 
    (
     Select MaxValue = Max(Column1)
         ,ColumnName = 'Column1'
     From TableA

     Union All 

     Select MaxValue = Max(Column2)
         ,ColumnName = 'Column2'
     From TableA

     -- Union All ... and so on for all columns

    ) MaxValues
    On TableB.ColumnName = MaxValues.ColumnName

Remember, if the TableA DDL changes, you must update this DML.