将列添加到现有表中，并将默认值添加到没有动态sql的另一列

Question

For Sql Server 2005 and 2008 I want to check if a column already exists on a given table and create it if it doesn't. This new column should have a default value of an ExistingColumn. Currently I need to use dynamic sql to fill the new column because sql server will complain of a syntax error.

Here is the current sql server code:

IF NOT EXISTS (SELECT TOP 1 1 FROM sys.columns WHERE [name] = N'NewColumn' AND OBJECT_ID = OBJECT_ID(N'ExistingTable'))
BEGIN
    ALTER TABLE [dbo].[ExistingTable] ADD [NewColumn] VARCHAR(50) NULL;

    exec sp_executesql  N'UPDATE [dbo].[ExistingTable] SET NewColumn = ExistingColumn'

    ALTER TABLE [dbo].[ExistingTable] ALTER COLUMN  [NewColumn] VARCHAR(50) NOT NULL 
END
GO

Is there any other way to solve this problem without resorting to dynamic sql?

Answer 1

SQL Server is parsing your statement before your ALTER runs, and saying "Hey, no such column." The parser doesn't understand IF and other branching and can't follow the sequence of events when you mix DDL and DML - or predict the sequence the events will take and what branching will happen at runtime.

Deferred name resolution allows you to access objects that don't exist yet, but not columns that don't exist yet on objects that do.

So, dynamic SQL seems like the way you'll have to do it.

Answer 2

Since you're creating the column regardless, you could do two separate batches.

    IF NOT EXISTS (SELECT TOP 1 1 FROM sys.columns WHERE [name] = N'NewColumn' AND OBJECT_ID = OBJECT_ID(N'ExistingTable'))
    BEGIN
        ALTER TABLE [dbo].[ExistingTable] ADD [NewColumn] VARCHAR(50) NULL;
    END
    GO
    IF EXISTS (SELECT TOP 1 1 FROM sys.columns WHERE [name] = N'NewColumn' AND OBJECT_ID = OBJECT_ID(N'ExistingTable'))
    BEGIN
        IF EXISTS (SELECT 1 FROM [dbo].[ExistingTable] WHERE NewColumn IS NULL)
        BEGIN
            UPDATE [dbo].[ExistingTable] SET NewColumn = ExistingColumn
            ALTER TABLE [dbo].[ExistingTable] ALTER COLUMN  [NewColumn] VARCHAR(50) NOT NULL 
        END
    END
    GO