Question

When executing the following script, I get the error:

Msg 207, Level 16, State 1, Line 15 Invalid column name ‘b’.

Anyone can explain it please? Thanks.

DROP TABLE ttt;
CREATE TABLE ttt(a nvarchar)

IF NOT EXISTS ( SELECT  *
                FROM    sys.columns
                WHERE   object_id = OBJECT_ID('dbo.ttt')
                        AND name = 'b' )
    AND EXISTS ( SELECT *
                 FROM   sys.columns
                 WHERE  object_id = OBJECT_ID('dbo.ttt')
                        AND name = 'a' ) 
    BEGIN

        ALTER TABLE [dbo].ttt ADD b NVARCHAR

        UPDATE  [dbo].ttt
        SET     b = a

        ALTER TABLE [dbo].ttt DROP COLUMN a
    END

Answer 1

It’s trying to compile all of these statements before it executes the 1st:

 ALTER TABLE [dbo].ttt ADD b NVARCHAR

    UPDATE  [dbo].ttt
    SET     b = a

    ALTER TABLE [dbo].ttt DROP COLUMN a

(In fact, it tries to compile the entire batch, not just these statements, but the point still stands – at the point it’s trying to compile the UPDATE, the column does not exist)

When it’s trying to compile the UPDATE statement, it consults the table metadata and correctly finds that the column doesn’t exist.

Try EXECing the update statement.

    EXEC('UPDATE  [dbo].ttt
    SET     b = a');

And also, what Oded says about you probably wanting to specify a size for the column (otherwise, it defaults to the most pointless datatype ever – an nvarchar(1))

---

This script definitely runs without errors:

CREATE TABLE ttt(a nvarchar)

IF NOT EXISTS ( SELECT  *
                FROM    sys.columns
                WHERE   object_id = OBJECT_ID('dbo.ttt')
                        AND name = 'b' )
    AND EXISTS ( SELECT *
                 FROM   sys.columns
                 WHERE  object_id = OBJECT_ID('dbo.ttt')
                        AND name = 'a' ) 
    BEGIN

        ALTER TABLE [dbo].ttt ADD b NVARCHAR

        EXEC('UPDATE  [dbo].ttt
        SET     b = a');

        ALTER TABLE [dbo].ttt DROP COLUMN a
    END